sailboat hot water tank

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The Best Marine Water Heaters for Your Boat or RV

Written by J. Harvey / Fact checked by S. Numbers

A boat is a pricey investment just like your house. Therefore, it’s natural that you give it the same amount of attention. Equivalent to furnishing your house is getting the right and quality accessories for your boat, of which includes the best marine water heater.

Warm or hot water is essential when sailing as you utilize it for showering and washing up. Just make sure that you get the right heater size so that you can optimize the usage and performance. Unlike residential heaters, marine water heaters don’t consume a lot of energy.

There are numerous marine hot water heaters that you can find. You can avoid confusion by reading this comprehensive review so you’ll be oriented with the important features. Consequently, you can select the most suitable for your boat.

• Works with solar power
• Provides hot water in 5 minutes
• Easy installation

• Doubles the supply of hot water
• Has good-quality parts
• High-quality construction

• Durable 
• Has 2-year warranty

Table of Contents

1. Whale Premium Water Heater

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You’ll find this product from Whale which is a part of Attwood in several marine water heater reviews due to its dependability. It’s an efficient hot water supplier in my sailboat. I sometimes use solar power to make it work.

When it’s time to take a shower, I only need to wait for 5 minutes to get warm water. That’s why I call it an instant heater! Aside from quickly turning water hot, it can supply you with plenty of it. To prevent water from becoming scalding hot, I added an automatic thermostatic mixing valve to the outlet.

The installation is easy as I mounted it horizontally. It’s like doing plug and play as everything is a match. No tiresome process is required before reaping benefits from this 6-gallon heater that only requires 120V. It’s a relatively compact water heater. Thus, you don’t need a lot of space for it.

• Produces plenty of water supply
• Easy installation or horizontal mounting
• Compact so it take up a lot of space
• Durable because of its premium components
• Adding automatic thermostatic mixing valve to the outlet so that water won’t be scalding hot

Due to this quick marine water heater, the supply of hot water in my boat doubled. It’s lightweight with fiberglass sheets for insulation. This design allows me to remove the case to minimize the space that it occupies.

I was happy that I am able to connect it to the freshwater cooling system of the boat’s engine. Water heating can be done with 110V AC when the engine is not running. However, it’s recommended to fill and cap the pipe with antifreeze when you let it run with AC.

You’ll find black, white, and green wires that have good quality. If you have the same wires in your boat, it’s easy to connect them. Installation is manageable in my case. I figured out what to do after I saw the parts. It has a nice recirculation system that works in producing hot and cold water.

• Lightweight and space saver by removing the case
• Can be connected to the freshwater cooling system of the boat’s engine
• Has good quality parts
• Has a recirculation system for hot and cold water
• Durable due to its high-quality construction
• Needs antifreeze in the pipes when connected to 110V AC

With the 11 gallon capacity of this water heater, I have enough hot water onboard whenever I need it. I think that Whale is a part of Attwood company as the two-year warranty card is addressed to the latter.

It’s a great replacement for my previous heater even though it’s a tad bit larger. Luckily, the plates can be flattened against the case to make some room. This results in decreasing the depth. Although it’s another task, installation is fairly easy. Screwing down plates and horizontal mounting is not that difficult.

Operating this heater starts with 120 volts or 220 volts. It’s flexible when connecting to power sources as it has extra ports. Thus, there are options for you to plug it into the engine coolant or run off the heat engine. The second alternative works even though the boat is underway and it functions without the help of a generator.

• Provide enough supply of hot water onboard
• Fairly easy installation
• Has extra ports for different power sources
• Durable due to rugged construction
• Ignition is protection to prevent harm and accident
• Has plates that may be a tad bit larger than the designated place
• Water turns hot after 45 minutes

Since the exterior shell of this is composed of marine-grade aluminum, it can last for several years. As I’ve used it for a year with no issues, it’s much better than the other one. I didn’t have to make adjustments as its measurement is similar to my previous water heater.

This has a 6-gallon capacity that never fails to provide the necessary amount of hot water. Thanks to its large diameter and 1,500W heating element! There are other features that I like about it. A high-temperature limiter is present and it has an automatic reset. Also, the circuitry is ignition protected for safe operation.

The heat exchange barbs are found at the back while the fittings and connections are in the front. Installation is relatively easy so I didn’t have to spend a lot of time when going through all the steps. This heater can start working through AC power or engine heat when your vessel is underway.

• Composed of marine-grade aluminum to last for several years
• Provides enough hot water due to 1,500W heating element
• With temperature limiter and automatic reset feature
• The circuitry is ignition protected for safety
• Relatively easy installation
• Works with AC power and engine heat
• Capping off the heat exchanger with anti-freeze so it can supply the needed amount of hot water

Although this is a mini-tank water heater, it has a number of great features. First and foremost, it’s capable of giving off instant hot water as it can maintain the high temperature. It fascinates me because I don’t need to wait for even a single minute. The warmth is good enough for my outdoor showers. I also use it when washing dishes.

It stands out when it comes to the quantity of hot water. Five to seven gallons can be obtained at 98 degrees Fahrenheit.

Be aware that it may be too hot at first and may likely be so for the first gallon. Despite this minor drawback, it can serve two sinks without compromising performance. Also, the recovery rate is fast. With these abilities, I can say that it’s perfect for heating water from distant faucets.

Since this is tankless, the mounting options are not limited. You may place it on the wall, shelf, or floor. In any way you do it, it will always be easy because of its size. The same reason helps me avoid wasting water as I only get what I need. Just like any other RV marine electric tankless water heater, this saves energy as you only heat the water when necessary.

• Gives hot water in an instant even for two faucets at the same time
• Can provide five to seven gallons of hot water
• With a fast recovery rate
• Mounting options are not a limited and easy installation
• Separate warranties for the unit and the parts
• Makes you save water and energy
• The first gallon of water may be too hot

I read some marine hot water heater reviews which convinced me to buy this item. The efficacy of this equipment is not only for household use but onboard as well. Eventually, I wasn’t disappointed. I only need to count seconds to have hot water. Moreover, the performance remains incredible even with a distant faucet.

When onboard, I need hot water to complete menial tasks and quick washing. As such, this heater is a reliable companion. Even my boat bidet with a sprayer became functional because enough supply of warm water came out of it. What’s even better is that this mini tank can give you both hot and warm water.

I like that it’s compact and lightweight with a sleek design and a temperature display. The size of this tank reminds me of a toaster oven. It has pretty good insulation so no energy is wasted. Plug it into a 110-volt outlet and it will start working.

• Heats water in a matter of seconds
• Works on a distant faucet
• Gives water instantly so perfect for quick washing and simple tasks
• Can provide both hot and warm water
• Compact and lightweight with a sleek design and a temperature display
• Has good insulation to not waste energy
• Need to add couplers and tape to secure the connection

Water heaters made by Kuuma are known for their durability. They are made of aluminum to ensure that you can use them for many years. So, I didn’t think too hard before buying this one. My hunch was right as it turned out to be the perfect replacement for the previous heater in my 1985 O’Day 28 sailboat. I’m enthralled by how it perfectly fits into the cockpit locker due to its side mount feature.

Hot water when cruising the vast sea can be both a necessity and a luxury. Thus, I was prompted to get this heater with great abilities. There are two ways to heat water due to its dual-loop feature. It can be done through the boat’s engine and shore power. When the vessel is underway, you can connect it to AC power. You may even use raw power heating.

It also promotes safe operation, especially for gasoline-powered boats. The credit goes to the circuitry that is ignition protected. There’s a limit to high temperatures and an automatic reset. Hence, you won’t be surprised with scalding hot water. Quick recovery is another wonderful ability from its 1,500W heating element.

• Durable and lasts for many years
• Suits old model vessels
• Works with different power sources
• Promotes safety operation
• Quick recovery of hot water
• Uncomplicated installation
• Separately buying the necessary hardware

The need to have a huge supply of hot water made me buy this product. You can get 6 gallons or 23 liters from it. It has a thermostat that regulates the electric heating element while it can also produce cold water if you need it.

When heating water, it can reach a temperature of 140 degrees Fahrenheit. But with the temperature-pressure relief valve, the water can’t get too hot. A bilge pump is also added to stop electricity sparks. Since it can dispense both hot and cold water, its design includes waste management.

You can rely on its durability as the inner tank is composed of aluminum and the exterior is of marine-grade stainless steel. These materials guarantee long-term service and strength to face the rigors of the marine environment. That’s why the manufacturer has the confidence to give a two-year warranty to all users.

The double-walled construction is also great as it protects the freshwater supply from contamination. Thus, I don’t worry too much about the water that I keep in my boat.

It’s convenient to just focus on the same area when working on the tank since the fittings and heat exchanger barbs are all situated in the front. Therefore, both the installation and marine hot water heater troubleshooting are easy. Hold-down brackets complete the horizontal mounting of this heater and it’s done with no troubles.

• Contains a huge supply of hot or cold water
• Has parts for safe operation, waste management, and prevention of contamination
• Durable for long-term service
• Has a two-year warranty
• Easy installation and troubleshooting
• Hot water retention for 10 hours
• Very first water release is too hot

I like this product very much as it’s an effective marine tankless water heater. In just 35 minutes, the entire tank of 7 gallons is filled with hot water! Using it gives me so much ease and convenience. I don’t have to turn it on again and again as the water stays hot.

Installation is super easy since it’s substantially hardwired. I think everyone can do it with no difficulty at all. It can also produce cold water. You may mount it horizontally and vertically. But make sure to prepare enough space for it as it’s not exactly mini. Still, you have the freedom to put it on the wall or the floor.

I mounted it with PEX and once it’s done, hot water came out in just 9 seconds. Thus, it’s right to say that it heats up quickly. The temperature that it handles ranges from 65 to 145 degrees Fahrenheit. I can customize the temperature of hot water as there’s a thermostat swing.

• Can make 7 gallons of hot water
• Easy and convenient to use
• Super easy installation
• Heats up quickly and fast
• With a thermostat swing to customize the temperature
• Energy-efficient even though the faucet is far from the tank
• Prepare enough space before installation as it’s not mini

I bought this water heater to make sure that I always have hot water in my boat. Its 11-gallon capacity caught my attention among several other options in the market. It’s a product of the latest technology so it’s capable of bearing 1,500W.

It has a robust construction to withstand harsh marine environments and corrosion. So, I can rely on it in rendering the same performance through time. It simply means unrivaled durability. I can’t help but be astonished by its double-wall heat exchangers and integrated pressure relief valve. Both of these parts guarantee the security of water inside it and the preservation of heat.

Safety operation is also expected from this tank because of its protected ignition. The last thing that I want is to have a piece of equipment that acts up in my boat while sailing. Thankfully, this water heater gives me peace of mind. There’s a safety control to stop the build-up of unsafe temperatures as well.

• Has a large capacity to not run out of hot water
• Has a robust construction for durability
• With parts that keep and preserve hot water
• Safely operates with ignition protected and safety control
• Needs to be installed by two people as this is a large tank

I don’t go for a tankless water heater if I want more pressure and very hot water. The things that I’m looking for are provided by a hardy tank like this one. Since I sail for a couple of days, it serves me well in providing hot water that I use for showers and washing dishes. This is also an RV marine water heater as it’s mostly used by those who permanently live in RVs.

I was able to double up my hot water supply from 6 to 12 gallons. This tank can be powered by propane and 120 volts of electricity. Although it’s big, I’ve noticed that it consumes less propane and it’s quieter too. Due to the adequate supply of hot water, there’s no interruption when taking a shower as long as the power source doesn’t cease to work.

Before mounting this heater, I flushed it first to ensure that the inside is clean. I did some minor adjustments to the plumbing and it didn’t trouble me at all. It doesn’t take much time and effort to complete the installation. After the assembly, give the cold tap water 15 minutes to turn hot.

• A hardy water heater tank
• With a capacity of 12 gallons which means doubling up your hot water supply
• Can be powered by propane or electricity
• Consumes less propane and operates quietly
• Easy installation to produce hot water in 15 minutes
• Not prone to leaks
• Some minor adjustments on plumbing

When I go sailing, I see to it that I dock my boat before sunset. But, I sometimes spend the night on the waters and return home the next day. Hence, this 6-gallon water heater is just right for my needs. It can be operated by 120 volt AC or engine coolant heat exchange.

To have constant availability of hot water, there’s a thermostat that regulates the heating temperature at 140 degrees Fahrenheit. You may also get cold water from it because of its 1/2-inch NPT.

The inner tank is made of aluminum while the exterior is of marine galvanized steel. With these premium materials, I know that this tank will be with me for a long time. Durability of a water heater is important for me so I have the assurance that it can take care of my boat’s water supply.

Set it up by horizontal mounting and hold-down brackets that are already attached to the inner tank. I did it with ease by myself. You will have to thoroughly check the fittings and heat exchanger parts to avoid any issue. Fortunately, it’s uncomplicated as they are all placed in the front of the tank.

• Can be run by electricity or a boat’s engine
• Has a thermostat that regulates water temperature
• Durable as it’s made of premium materials
• Easy mounting or installation
• With valves for safety operations
• Thorough checking of the fittings during installation

Before buying a water heater, there are two main things that you have to consider. Of course, the size of the heater should match your boat’s size so you have enough space for it. The next thing you have to think about is the number of people onboard. Hold your horses as you can’t just buy a 12V marine water heater if you have a lot of people cruising with you! There are other aspects that you have to ponder as well.

Propane or Electric Powered

Most of the water vessels these days have an electrical system. So, getting an electric-powered water heater is not troublesome. You should know that the heater will always be running without limitations if it’s run by electricity.

When it comes to a propane-powered heater, it has a quick recovery rate even when the temperature is low. You’ll also save on running costs. Some think that electric-powered ones tend to last longer. Nevertheless, propane-powered heaters can keep up.

On the brighter side, you’ll find exceptional water heaters that can be powered by both propane and electricity. So, at any time, you can switch to the power source that suits you best.

Water Heater Size

The water heater should be the right size for your boat. It shouldn’t be too big or too small. If you ignore this matter, it may be bulky for your boat or won’t provide the necessary amount of hot water. If you don’t have ample space for it, go for the tankless type as it’s compact.

Everything you put in your boat is exposed to the marine environment. In this matter, you should consider your boating routine and preference. Experts recommend stainless steel that can withstand all kinds of environmental and weather conditions.

A brand is appealing if it becomes popular due to the satisfaction that it gives buyers. So, settle for a company that has a good reputation and you won’t be disappointed with the quality of their products.

Marine Hot Water Heater Troubleshooting

Be concerned about troubleshooting as it’s important to get the heater fixed in case something goes wrong. Research the product to see if you can handle the necessary troubleshooting steps.

A marine water heater heats water that is used for cooking, washing, or shower. It seems like the one that is used at home but the operating system differs. Hoses are made as connectors instead of pipes .

There are two ways to heat a marine water heater. You can let it run by AC power or marine engine heat. This equipment only needs minimal power as any excess can disrupt ignition. It usually comes as a built-in but selections are in various sizes, patterns, and durability.

Companies become competitive these days. Thus, it’s not only the popular ones that excel in making quality products. Aside from the commonly known Superstor marine water heater and Isotemp water heater, Whale, Kuuma, Bosch, Ecosmart, and Suburban are trustworthy brands in terms of creating marine water heaters.

Most marine water heaters are easy to install. You can follow the steps in the instructional manual but in some instances, you may not need it at all.

You can mount it by the brackets and other hardware that come along with it. Then turn it on and you’ll have water in no time.

Rheem and AO Smith are known as makers of quality water heaters. The former also specializes in crafting air conditioning and air cooling systems while the latter only focuses on water heaters. AO Smith has better customer service than Rheem, which is the primary difference between them.

After reading this review, you know the importance and value of the best marine water heater. It’s an investment with a load of benefits. Hot water is a necessity but it can be luxurious when you’re cruising with your boat. It can make a huge difference in your sailing experience due to the immense comfort that it brings.

“I am James Harvey – founder of Boating Basics Online. It is established with the drive to help out first-time boaters, which are those desiring to explore their way through the water. So if you are new to boating, start from here with me. “

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10 Best Boat Hot Water Heaters And Their Reviews For 2022

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So you have a boat, you might need a boat hot water heater for that.

You want to go sail out into the waters but are worried about having hot water to enable you to take a hot shower after a relaxing swim or to do some wash-up.

Just like in our homes, boat owners also feel the need to have hot water on their boats.

Hence will need to install a boat hot water heater unit to facilitate this.

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10 Best Boat Hot Water Heaters Reviewed

There are many water heaters in the market today, some of which have a modern touch while others are still in the bulky traditional designs.

In the boating industry, the water heaters used are usually either propane powered, use electricity, or make use of the waste from the engine to generate heat.

They are made from very high-quality corrosion-resistant material like aluminum steel or stainless steel, their unique design is to make them durable in any water conditions.

1. Whale Water Heater - Premium

The Whale Water Heater 120V is the ideal boat hot water heater.

It efficiently maintains an ample supply of hot water of up to 11 gallons.

With its double-walled heat exchanger system, the whale water heater utilizes the closed cooling system to heat water as the boat is running.

Also, while not in motion, the heater can be run electrically using a heating element that can be connected to the boat power supply or a generator.

To ensure its durability, the heater is made of sturdy materials that are resistant to corrosion.

You can choose a casing material of your choice for this heater, which can either be galvanized steel.

This is more substantial than most but has a longer lifespan; aluminum steel that is epoxy coated and blends well with the features of a boat or stainless steel, which gives it a modern look.

An additional feature to ensure durability is the anode that comes with the heater.

The whale water heater is especially useful for boats that sail in hard waters with high dissolved salt concentration or high currents.

The anodes help to elongate the life span of water tanks by corroding themselves instead of the containers.

They, therefore, need to be replaced as need be.

The heater is ‘ignition protected’.

This is a measure put in place to ensure safety when the heater is being used in the engine room.

It has a pressure relief valve and a drain valve that allow you to drain out water during the cleaning process.

Pros & Benefits:

• It is easy to install and quicker to heat water
• It can run off electric or heat power
• The heater comes with a two-year warranty
• It helps to prolong the lifespan of the water tanks by preventing corrosion

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2. American Standard Electric Water Heater

Second, on the list is the American Standard, a well-renowned water heater brand.

This brand seems to be a favorite among many boat owners.

This is a high-quality brand of water heater that is made to last long.

The heater can operate on gas or electricity.

The American Standard brand offers a wide range of models, each with different capacities to suit different needs.

The interior of the tanks on this heater is made from durable steel—couples with a fiberglass filler, and anodes that protect the tank from corrosion.

The anodes, however, have to be replaced as seen fit since they corrode with time.

Maintenance of the American Standard heater tank is easy since it has a hand hole allowance that allows for cleanup inside the container to remove the residues that build up with time.

The American Standard Water Heater is ideal for boat owners looking to heat a large volume of water on demand.

It has a large storage water capacity of up to 119 gallons of water with a heating element of up to 54Kw.

When it comes to warranty, the American Standard Water Heater is unbeatable.

The heater comes with a warranty of six years, while models with tanks lined with glass have a guarantee of twelve years, the same as the estimated lifespan of the heater.

• It helps to reduce the cost of energy by its ability to retain heat for longer periods
• Has an adjustable thermostat that enables you to regulate the temperature of the water as you feel fit
• The American Standard Water Heater comes with a preset temperature and pressure relief valve
• It is an easy to install point-of-use electric water heater: no long waits for the water to heat up
• The American Standard Water Heater has a superior thermal efficiency

3. Kuuma Water Heater

Kuuma Water Heaters have an excellent reputation among popular boat manufacturers in the industry.

This is a dual powered water heater meaning it can be powered either by electricity or by using engine power.

Therefore, the water heater can be used whether the boat is on the sea or at the docks.

The heater’s casing is made from high-quality aluminum that makes it resistant to corrosion.

Also, the casing makes it excellent at retaining heat.

The Kuuma Water Heater has 11 gallons, which makes it the best for boats that have high hot water requirements.

However, it also has models that have a small hot water capacity of 5 or 6 gallons or a large capacity of 20-26 gallons.

To ensure safety when in use, the heater comes with an ‘ignition protect’.

This is to prevent any hazardous occurrences when in use in the engine compartment.

• Kuuma Water Heaters are easy to install and maintain
• The heater comes with a two-year warranty on the heater
• It is highly effective when using either electricity or engine power
• The heater comes with hose barbs and pipes for domestic water

4. Bosch Tronic 3000 Water Heater

The Bosch Tronic 3000 is an electric mini-tank that is portable hence making it an excellent option for the outdoors as in a boat.

With its compact design and small size, the heater becomes easy to install by directly mounting it on a wall, under a sink or in a cabinet.

Bosch Tronicmodels come in different water capacities.

It is a cost-effective alternative since it operates without a tank.

The water capacity of these heaters could range anywhere from 2.5 to 7 gallons.

However, since the heater is small, you can only get enough hot water for one chore at a time.

The Bosch Tronic 3000 has a recovery rate of 6.8 GPH at about 90 F.

This rate varies depending on how cold the incoming water is.

The water pressure range is about 150 psi.

Although the heater is small, it has been equipped with features superior to other big models.

The Bosch Tronic 3000 is lined with glass that prevents it from corrosion over a long period of use.

The best feature of this model is that it is capable of supplying water to multiple sources simultaneously.

It’s also a good option if you are looking to conserve energy since it comes with a CFC- free form insolation.

It also comes with temperature control to enable you to adjust the water temperature to your liking.

The only downside of this heater is that it needs to be installed close to the hot water outlet.

• The heater comes with a two-year warranty on its parts and a six-year warranty on the tank
• Long lifespan: the tank is lined with glass making it corrosion resistant
• It has a fast recovery rate; hence no long waits between drawing water
• It is a point of use water heater which is fast and reliable: you get hot water as soon as you open the tap
• With this heater, warm water delays or cut out is at minimal

5. Atwood 94023 Water Heater

Atwood water heaters are electric heating units that are highly reliable.

It’s lightweight, therefore makes it easy to install and to service.

This heater is made for an aluminum alloy that makes the storage tank resistant to corrosion hence elongates its life span.

It’s a great feature, especially for boats that sail in harsh weather conditions.

For the Atwood 94023, the hot water capacity is at 10 gallons with a heating element of 1400Kw and operating on 120V.

To ensure that the freshwater is not contaminated, this heater is made using the double tube technique.

To heat the water, the heater uses a hot water coolant.

This is not a point-of-use heater; therefore, you will need to give the heater time to heat the water.

It is, therefore, essential to ensure that the connection between the engine and the water heater is made correctly to prevent damage to the engine.

• Due to its small size, this heater is ideal where there is minimal floor space
• It comes equipped with a factory set thermostat and with a temperature and pressure valve
• Keeps the water warm for a more extended period
• Atwood heaters are effective and resourceful
• The heater is compatible with most motors hence little to no adjustments are needed
• Atwood heaters have multiple safety protections such as temperature-pressure relief valve, thermostats, and a limit switch

6. Stiebel Eltron Tempra 24 Plus

This is yet another small electric heater that has no tank and is great for the outdoors.

This heater can be plugged in on a wall outlet, making it quite flexible and portable.

Its sleek design ensures that the heater can fit in places with minimal available space.

This heater has a flow rate of up to 4.68 gallons per minute, making it a higher performer than some gas heaters.

Since it doesn’t come with a tank, this water heater is considered the best in saving energy.

This is because water is only heated as required.

The Tempra 24 plus comes with a temperature control allowing you to adjust the water temperature to your liking.

A significant advantage of this heater is that two fixtures can use it at the same time.

One of its useful features is a thermostat that allows you to regulate water temperature as you use it.

Likewise, this heater comes with a high-tech flow control system.

This allows the regulation of water to be able to keep up with the demand.

When installed with a qualified technician, the product comes with a seven-year warranty.

• No venting is required
• The heater is small hence making it easy to install and maintain
• Temperature changes are easy to adjust
• Big on energy saving since it uses minimal energy
• Ensures an unlimited supply of hot water
• Since it has no tank, this water heater helps to save on space

7. Water Heater Propane, Camplux Outdoor Portable Camping Gas Tankless Hot Water Heaters, 2.11 GPM,...

This is the perfect water heater for the outdoors with a 10-liter capacity and a 2.64 gallons per minute model.

It comes packaged with a showerhead, a gas regulator, and the required adapters.

Camplux Heater is small, compact, and lightweight making it easy to be positioned anywhere.

The heater is hooked to a propane cylinder powered by D-cell batteries making it the best choice for off-grid locations.

Despite its small size, the heater’s maximum power output is about 68,000 BTUs.

It also has a stainless steel burner and a copper heat exchanger that generates thermal energy efficiently.

Camplux has an energy control point that allows you to save energy when the weather is reasonably warm.

It also has an automatic shut-off switch that automatically switches the heater off when the temperature exceeds the safe levels.

On the contrary, its anti-freezing technology helps to drain out residue water through the drain plug during cold days.

• Its thermal efficiency is very top-notch
• Can work effectively without mains electricity
• Comes with a showerhead and adapters on purchase
• Has a 20-minute automatic safety device

8. Isotemp Water Heaters

On this list, this is probably the most expensive choice, but it is worth it.

The Isotemp Water Heaters are more traditional, a bit heavier, and bulkier than the modern heaters.

It is the best choice if dealing with harsh water conditions.

These heaters are lined with a eutectic solution that enables water to remain warm for a more extended period.

Isotemp water heater is known to heat water to a temperature of up to 145F in an hour.

The unit can either be mounted horizontally or vertically and banded by two steadies, one on each side to help support.

This makes it robust when installed.

Additionally, the heater is made from stainless steel, which protects it from corrosion through hard water or harsh weather conditions.

The stainless steel casing is used for both the tubes and tanks, as well as any other part that might come into contact with water.

The Isotemp water heaters come in different sizes with a water storage capacity ranging from 4 to 6 gallons.

It has an immersion heating element of about 750W to 1000W and uses heat from the engine’s coolant to heat the water.

• All models come with a five-year warranty
• The polyurethane insulation helps preserves water temperature for long (up to 24 hours) hence the best when you want to use water without having to reheat
• Comes with a safety relief valve
• The water inlets and outlets in this product are carefully designed to minimize the mixture of hot and cold water

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The Raritan Water Heaters another famous brand in the boat water heater manufacturing industry.

These heaters are specifically designed for water machines.

At the price it comes in, this product works reasonably well.

Unlike the older model, the new and improved model of the tank is glass-lined with steel.

It, however, needs the support of anodes to prevent corrosion to the tank.

Raritan Water Heater fittings are galvanized while its tank is lined with a black foam sheet on the outside.

It comes with an adjustable thermostat that enables you to regulate the water temperature as you feel fit.

The heater has ignition and thermal overload protection to prevent any hazardous occurrences from happening.

It has a heating element of 120V with a water storage capacity of 20 gallons.

• The heater is quick at heating water
• The heater comes with a five-year warranty
• Raritan Water Heaters are environmental-friendly; they have a CFC-free foam insulation
• All models come with a five-year warranty on the tank and a two-year warranty on the heater
• It is simple to install and has a design that makes it compatible with many pressure systems

10. Whale Seaward Water Heater

The seaward heaters are made with stainless steel in several parts that are carefully joined together.

The plumbing pipes are galvanized while the tank is insulated with fiberglass, which protects it from corrosion.

This heater has a switch used to switch off the heater once the water temperature gets high.

It operates on a 12OV heat exchanger.

The Whale Seaward Water Heater uses a combination of both electric and heat exchangers to heat the water.

The seaward water heater has a water storage capacity of up to 11 gallons.

For safety, the heater has an ‘ignition protection’ to prevent any hazardous occurrences from happening when the engine is running.

• This water heater has a large drain valve to drain residue during cold days
• It has brackets welded on the tank to make it sturdy when mounted
• This water heater comes with a temperature relief valve

What To Consider When Choosing a Water Heater

Shopping for a water heater can be quite confusing and overwhelming.

There are many different types of brands and models available in the market, each with different specifications, build, capacity, and power.

A water heater can either make or break your boat ride experience.

A few things to keep in mind when planning to install or upgrade your water heater include:

With all the above considerations.

Choose a heater that fits your needs and is within your budget.

Although, it is best to note that with water heaters, the lowest cost is rarely the best value.

This is important as it helps to ensure the product works as intended.

The higher the warranty duration, the better.

How much available space you have on your boat to install the heater.

If working with little space, you might want to purchase a heater with no tank.

Heaters that use less energy are better since the energy supply on a boat is usually limited.

Water heaters can be quite hazardous if not correctly installed, especially when the engine is running.

It could result in a fire.

If not sure, ask a professional to install the heater for you.

Depending on the number of people, you anticipate being having on board regularly.

Choose a heater that can hold as much water as required.

Some heaters are best suited for commercial use, while others are made for small scale use.

Whether the boat is used for commercial use or personal use will determine the type of heater to purchase.

As much as most are highly durable.

You might choose to buy one made from stainless steel rather than one made from aluminum.

This is dependent on several factors, such as the type of water you sail on.

The prevalent weather condition in your region.

Final Thoughts

Nobody likes taking a cold shower.

Especially not on a cold day when you are out in the waters.

Therefore, having a boat hot water heater provides you with a different onboard experience, especially during the cold season.

Whether you are doing the installation by yourself or having a professional fix it for you, the process is quite easy.

A hot water heater installation ensures the storage and redistribution of freshwater within the system of the vessels.

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Hot Water Heater Installation Tips

Secure mounting, corrosion prevention, and smart plumbing are essential..

Although I spent most of my cruising years within 20 degrees of the equator, the landlubber luxury I missed most was not air conditioning (I’m a warm-blooded type), but a hot water shower.

We used a solar heated “sun shower” for most of our bathing needs. On a sunny day, the 1-gallon flexible plastic bag with one transparent side would deliver water that was plenty hot, but the drizzle of water usually left me unsatisfied. Friends used a manually pressurized garden sprayer which they had painted black. This added a bit more pressure, but like the collapsible foot-pump pressurized Helio shower we tested back in 2016, it was most effective in sunny warm climates. (Chapin now makes a black polyethylene industrial sprayer , in addition to their translucent garden sprayers.)

As I go about with the refit of Opal , the 1971 Yankee 30, my biggest challenge is deciding which of the comforts I missed during my life aboard Tosca I’d like to add. A below-decks hot water shower would be nice. Since a Lake Superior cruise is high on my bucket list and having spent the last two so-called “summers” in Marquette, MI, I’m pretty sure I won’t get the same performance out of my sun shower as we did in Fiji. 

What about a tankless propane heater?  You will see these on boats, but as we’ve found in the past, the propane systems are generally not safe on boats. The folks at Seaworthy published a safety alert regarding on-demand water propane water heaters a few years back which spells out the problems with these systems. Because of the associated risk, they are not compliant with the American Boat and Yacht Council ‘s standard. Although I think this risk could be manageable, it could invite insurance claim squabbles if there’s a fire, and I’m trying to reduce or eliminate my reliance on carbon-based fuels.

Since we last looked at boat heating options, advances in renewable energy and energy storage coupled with the explosion of interest in off-the-grid living has led to a variety of other options for moving heat where you need it. For those with a surplus of power (and bigger wallets). Two of the bigger names in marine heating, Webasto and Eberspaecher offer some dual water-air heaters with good reputations in the marine world. But when you include all that is required, these systems can be expensive.

If a “marine” 12-volt water heater like the ones we tested is out of reach, there are some less expensive 12-volt options aimed at RVers.  Computer nerds will no doubt enjoy the Everlander’s YouTube video on building their solar powered water heater , using a converted Rheem water tank. (Note that they have a massive rooftop solar array, pushing the limit for space on most monohulls.) I noticed that Bouge RV a company catering to the “van-life” crowd with 12-volt appliances recently introduced a portable hot water heater using propane cannisters. As a side note, I’ve been using a portable freezer-fridge from Bouge RV for 16 months without a hiccup. It’s not the most efficient system — but having freezer fridge that can move from boat to car in snap has changed the way I travel. (No more ice!)

Currently, I’m leaning toward one of the marine water heaters that Practical Sailor has tested in the past . My next step will be figuring out where to fit it. Here is tester and marine surveyor Frank Lanier ‘s assessments of the installation process:

T he physical installation of a water heater may seem pretty straightforward, but the devil is indeed in the details. It starts prior to purchase, with a search for adequate space that’s relatively near the engine and vertically as low as possible. Next is bonding in a well-reinforced surface to mount the water heater onto. The empty tanks are relatively light, but if you add 45 to 88 pounds of water, you can see why a sound support base is important in a rough seaway.

Plumbing can be a bit of a challenge, especially when it comes to trapping air bubbles that will cause the engine’s coolant pump to cavitate. Every diesel engine circulates coolant with a vane-type pump, and though its long-term reliability is legendary, this type of pump is not a positive-displacement pump. That means it is not self-bleeding, so if air bubbles are not bled from the system, the pump ingests the air and spins little or no circulating water. This is one of the reasons why remote surge or top-off tanks for coolant are always placed higher than the engine itself. It’s also why there’s also a butterfly bleed valve on the top of heat exchangers.

In situations where the heat exchanger in a water heater has an inverted U-shape, with the inlet and outlet lower than the high point in the loop, care must be taken to rid the coolant circuit of all air bubbles when refilling with water and antifreeze. Air always seeks the high point, and this is another reason why some manufacturers say that their water heaters should be mounted at or below engine level, nearly impossible on many sailboats.

Globalization has made plumbing more difficult, and unless you have a good source for metric bronze pipe fitting, it’s vital to purchase metric-to-NPT conversion couplings (usually available from heater vendors).

To keep pipe joints from leaking, use Teflon tape or pipe joint compound and remember that hose barb-to-hose connections are much easier to make drip-proof with a hose clamp than the same connection made on a threaded pipe stub. (See our test of tapes and thread sealants in the November 2021 issue. ) Care also needs to be taken when connecting stainless-to-stainless joints, in order to avoid galling the threads. Many European marine plumbing manufacturers have switched to high-quality, investment-cast stainless steel. Nevertheless, these fittings are not as malleable or corrosion resistant as silicone bronze, so be sure to not over-tighten or allow stray DC current to run through these fittings.

If you are shopping for a water heater, our in-depth report on water heaters is a good place to start. I’d love to hear about readers’ experiences with hot water systems. You can add them in the comments below, or send to me [email protected].

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15 comments.

Ralph’s point about clearing the air from the hot water heater’s plumbing pertains to the location of our SS heater tank on a Mason 43, which is a couple of feet above the engine. On these boats the engine is located under the cabin sole, an area that is already crowded with no room to add an insulated 11 gallon tank. We solved the problem about twenty-five years ago by adding a 12 volt pump into the run from the engine to the higher water tank. The small pump is low enough that it does not become air bound and a few minutes of running it clears air from the system. We then shut it off, throw two small ball valves to by pass it, and are good until we next lay the boat up for the Winter. Ralph is a sailor’s sailor with a great fund of knowledge. He kindly moderated several of the Safety At Sea Seminars that my wife and I coordinated for the Cruising Club of America before Bermuda races. These had upwards of three hundred attendees and were a challenge for anyone to moderate well.

To be clear, engines do not have vane type water pumps. Vane pumps are positive displacement pumps used in hydraulic systems.

So, is it incorrect to refer to those little radial legs on a rubber impeller as vanes?

Ok will share some insight from licensed marine engineer from the VLCC/LNGC industry at a high level, but also have an old classic boat with a 40hp westerbeak 1. You are both correct, vanes exist on some hydraulic pumps, rubber impeller pumps, and also the divisions on an open or closed type impeller for a centrifugal pump, are called vanes.. 2. A Positive displacement pump moves a specific volume of fluid for each full shaft rotation. Flow rate is proportional to shaft rpm. Many hydraulic pumps are vane type, with metal vanes the slide in and out of the center, within a non concentric casing, similar to the impeller on your raw water pump, only that those rubber vanes fold back in the non concentric casing, the next vane pushes the water out of the discharge port, as the space holding that pocket of water between the vanes is made smaller by the casing shape. (large ship ram type steering gears, have variable displacement hydraulic rotary piston pumps, with a tilting plate, to control the follow of oil to the system, so at constant rpm, the tilting plate controls the stroke of the pistons, and thus variable displacement -0-100% flow at constant rpm) Other hydraulic systems depending on size/design/service/fluid properties can use , gear pumps, vane pumps and screw pumps, ete. Larger systems may have regulating valves on the discharge, but always a relief valve to protect damaging the pump. Small raw water pumps, typically dont, and vanes are flexible, but operating with a blocked discharge on these will will result in damage to vanes over a short period of time, over heating, and eventual failure. (blockage on other types of pos dspl pumps will destroy pump, shear coupling, ete. Note the typical rubber impeller raw water pump moves water thought the raw water /fresh water heatexchanger and then overboard, likely via your exhaust system 2. There is a second pump that circulates the engine coolant fresh water, through the engine passages, lube oil cooler and the same heatexchanger that has raw water flowing through it(in separate passages), that is typically centrifugal, and is typically belt driven but can also be engine driven, and on big ships will be electric driven in most cases. Centrifugal pumps have very different properties than positive displacement, advantages disadvantages, but are typical for circulation of engine fresh water coolant for a number of reasons. The article above does not make clear some specific system/pump properties that is perhaps leading to some confusion a hot water loop for heading your potable water, which is engine engine coolant very typically driven by the fresh water coolant centrifugal pump on your engine. 3. Your lube oil pump, is typically a engine driven gear pump. (large ship main engines have 2 ea electric driven screw pumps) 4. you also have a fuel pump, and on non-common rail designs (newer environment tier III compliant) that is typically a variable positive displacement pump, with a rack that rotates the piston, which has passages machined into it to deliver a variable amount of fuel based on relative position of piston to cylinder liner ports (also called a jerk pump) 5. Centrifugal pumps can get air bound in the casing,(hence referance to head tank over pump in article) have limited suction lift capabilities if not primed, ete, where positive displacement are typically self priming. (but yes you can have air issues in your fuel system too…) Hope this helps! Sailboat sailor and past Dual License officer on merchant ships, remember it can be very hard for them to see you, even though you see them, my first cadet ship was over 1’000 long, coming out of English Channel in the TSS, very hard to pick up sailboats in white caps during the day, white sail/white hull/dark hull, I was shocked, as an experienced offshore sailor before my professional education at the Academy. So be proactive and assume they cant see you even with Class B AIS. All the best, sorry for any inacuracies above, was trying to be as general as possible, but may not be specific yours installation. time to check on the turkey:)

So Ralph, is there any way to provide hot water on a raw water cooled boat while underway other than heating some in a tea/coffee pot on the stove? I’m thinking of sailing to the Falklands in/on my Catalina 27 and then east around in the “Roaring Forties.” HaHa. Insincerely, Gale Carlson.

I had a raw water cooled motor on my Jenneau. I plumbed in a bypass from the raw water exhaust thru the water heater and back to the exhaust. The bypass had ball valves to that were opened when I wanted to heat water in the tank. It took about 3o minutes to heat the water in the 10 gallon tank. Those valves were then closed so that exhaust water did not flow thru thw water heater.

If the raw water leaving the running engine is warm to hot you can use insulated tubing to detour through a 4 gallon water heater’s hot water coil before entering the water lift muffler. Hot water heater is best if equipped with a thermostatic mixing valve on its hot water supply.

Knarly sailors hold a bucket under the exhaust to get hot water for the dishes. A little oil on the pan keeps it from rusting don’t you know, and keeps you regular.

Hi Guys, tpo nstay on the subject of HW spefically, the tank. Yup ties that sucker down. Our boat was a Beneteau 440 with a classic and highly successful installation. But to be failsafe, I installed two through hull type valves in both the raw water (coolant copper piping feeds to be able to isolate the HWT from the coolant source. That would negate loss of coolant if the HWT heat exchanger failed. Also, at Raritan’s suggestion, I changed the in-tank anode every couple of years. Never had a problem

The author of this article seems to have no plumbing experience at all nor even discussed how to fit piping nor dealing with air in recirculating lines. These are issues dealt with by domestic plumbers every day. So here’s some insight. First hardly anyone does threaded pipe or even soldered copper anymore. It’s all done with flexible PEX tubing. Look it up. Second, air is removed from lines by installing automatic bleaders in a high point in the line and not by limiting installation of the water heater above the heat source. Oh bye the way, make sure you install a pressure temp relief valve in the hot water tank. Without one you could have a real surprise 😂

Pex type plumbing is inappropriate in this application as we are talking about engine coolant, ethylene glycol at temperatures approaching 200f. Self bleederd also not acceptable in the engine coolant loop. Pex, Whale, SeaTech are all acceptable on the potable water side of the system. An alternative for raw water or fresh water cooled engines is diesel hot water on demand heaters from companies like Espar. They are popular with European builders. I have limited experience personally, I am sure the have there own pros and cons.

I have a Webasto diesel fueled water heater that is about 6″ cubed. I use that to heat and keep hot about 3 gallons of antifreeze solution. From there I use that working fluid in a loop to circulate the fluid to three ~8″ cubed air handlers to heat my various cabins. For domestic hot water the same fluid goes through a flat-plate liquid to liquid heat exchanger. I have a similar hot water loop that I recirculate shower water so I can take my usual 20 minute hot shower using only about a gallon of fresh water. The gallon of shower water is discarded after the shower and a final rinse in clean water is available if one so desires. Details can be seen at http://prestissimo.org/plumb_heating_system.htm

On my boat, i only heat the cold water; if i had the hot water, I would not need a heater (;-). Therefor, I installed a water heater (as opposed to hot water heater).

The best marine water heater is an investment with a load of benefits. Hot water is a necessity but it can be luxurious when I’m cruising with my boat. It can make a huge difference in my sailing experience due to the immense comfort that it brings.

I installed a Quick 20 litre HWH this spring, replacing a very tired, poorly insulated creature that I was not sure if original on my1981 Baltic 42 DP. I had to replace a platform right above the engine to support the HWH and while I was at it completely redid the sound proofing/insulation in the entire engine compartment. Plumbing was a cake walk. The Quick HWH holds hot water for almost 24 hours. It is easy to drain and winterize. The bracket that keeps the HWH in place is easy to adjust and holds! I have cruised Lake Superior for 25 years and used Sun Showers for many of them. My wife and I still keep a Sun Shower full while cruising but seldom need to tap into it. Ted Smith Duluth, MN

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Kuuma 120V Front Mount 6 Gallon Water Heater #11811

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Camco Fresh Water System Dewinterizer

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When you need hot water while enjoying your boat, you can find a dependable marine water heater from Wholesale Marine. We stock a variety of super reliable brands such as Camco , Kuuma , and Raritan Engineering . Water heater capacities range from 3-gallons to 15.8-gallon tanks and are perfect for supplying hot water for showers, laundry, doing dishes, and other onboard tasks that require hot water on demand. You can rely on Wholesale Marine for quality marine water heaters, replacement, even maintenance components such as bypasses, de-winterizing solutions, drain valves, blow-out plugs, and anode rods-everything you’ll need to keep your marine water heater operating at peak function.

Marine Water Heaters Provide Hot Water on Demand

Need a marine water heater but are tight on space? Consider a Kuuma 120V Front Mount 6 Gallon Water Heater .  While it may be small, you’ll find it offers the latest technology and has been manufactured with only the highest quality materials. Tough enough to withstand harsh marine conditions, this 6 gallon unit also conserves weight and footprint on board. If you're searching for a larger capacity marine water heater for showers and washing dishes, consider the Kuuma 120V 20 Gallon Water Heater - Vertical 4-Way Mount . Still compact but large-capacity, the aluminum tank maintains a constant temperature. Kuuma also offers marine water heaters in several capacities, including their uber popular 11-gallon model - a brand known for its reliability and long-life.

Wholesale Marine is well known for the best quality boat parts at competitive prices. This is why our happy customers return again and again.  If you need assistance in selecting what part you’ll need, simply give us a call and enjoy our courteous and efficient service. As boat enthusiasts too, and can help you make the right marine water heater selection. Be sure to ask about our Captain’s Loyalty program for additional discounts on everything you need to keep your boat in tip-top condition for your next adventure! Call us at (877) 388-2628; we’re here Monday through Friday, from 9:00 AM until 6:00 PM, EST.

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Marine Water Heaters Ultimate Guide: All You Need to Know for Efficient Heating Onboard

Marine water heaters are an essential appliance for any boat or recreational vehicle, providing hot water for various uses such as washing up, showering, and even cooking. These heaters are designed to be compact and lightweight, taking into consideration the limited space available onboard vessels. With numerous options available in the market, it can be challenging to choose the right marine water heater for your specific needs.

In this ultimate guide on marine water heaters, we will explore the different types of heaters available, their features, and the key factors to consider when making a purchase. By the time you finish reading, you will have a good understanding of marine water heaters and be better equipped to make an informed decision on which model is best suited for your boat or RV.

Types of Marine Water Heaters

Marine water heaters are an essential part of any boating experience, as they provide hot water for various purposes, such as cooking, bathing, and cleaning. There are three primary types of marine water heaters available on the market: electric water heaters, engine-connected heaters, and hydronic heaters. Each type offers different benefits and suits specific needs, depending on the boat's size and power capabilities.

Electric Water Heaters

Electric marine water heaters rely on an electric heating element to heat the water. These units are typically connected to the boat's electrical system, either 120V AC or 12V DC, depending on the model. They provide a reliable source of hot water with minimal maintenance requirements. Some popular electric marine water heaters include the  Whale 6-gallon Premium Water Heater  and the  Kuuma 6G 120V Rear Heat Exchange . Electric water heaters are ideal for boats with a stable electrical supply.

Engine-Connected Heaters

Engine-connected marine water heaters use the heat generated by the boat's engine to heat the water. These heaters often feature a double-walled heat exchanger system that transfers heat from the engine's cooling system to the water tank. A popular example of this type of heater is the  Whale Marine Water Heater - Premium . Engine-connected heaters are a highly efficient option for boat owners who frequently operate their engines, as they make use of the engine's waste heat.

Hydronic Heaters

Hydronic marine water heaters use a closed-loop system that circulates heated water or coolant throughout the boat's interior. These systems often have multiple heating zones, with individual temperature controls for each area of the boat. Popular manufacturers, like  Webasto , offer a range of models to suit different boat sizes and heating requirements. Hydronic heaters tend to be more fuel-efficient than electric heaters, as they use a smaller amount of energy to maintain the desired temperature. They are suitable for boaters who prioritize fuel efficiency and have the necessary space to install the heater and its required components.

Key Features to Consider

The capacity of a marine water heater is an important factor to consider, as it determines the amount of hot water available for use on your boat. Capacities typically range from 6 to 20 gallons, so choosing the right size depends on your boat's requirements and the number of people on board. A smaller capacity may be sufficient for smaller boats or short trips, while larger capacities are more suitable for bigger boats or extended trips.

Power Source

Marine water heaters can be powered by a variety of sources, including electricity, engine heat, and propane. Electric marine water heaters, like the  Kuuma 6G 120V , rely on an immersed heating element to warm up water, while engine-powered heaters connect to the boat's engine to utilize its heat. Propane heaters, on the other hand, use a propane burner to heat the water. Each type of power source has its own advantages and drawbacks, so choose one that best suits your boat's setup and your specific needs.

Materials and Construction

The materials used in the construction of a marine water heater can also impact its long-term durability and performance. Many marine water heaters feature stainless steel or galvanized steel tanks, as these materials are known for their corrosion resistance and longevity. For example, the  Whale F600 Water Heater  has a galvanized steel tank, while the  Kuuma 11G 120V  is made of stainless steel. It's important to select a water heater made from high-quality materials that can withstand harsh marine environments.

Safety Features

As with any appliance, safety is a crucial consideration when choosing a marine water heater. Look for water heaters with built-in safety features, such as overheat protection, pressure relief valves, and ignition protection. These features can help prevent malfunctions and potential accidents while onboard your boat.

Ease of Installation

Lastly, consider how easy it is to install a marine water heater on your boat. Some heaters, like the  Kuuma 6G 120V side mount front exchanger , offer more flexible installation options, making them suitable for tight spaces or corners. Others may have more specific requirements for placement, which can be a challenge in limited spaces. Ensure that the water heater you choose can be easily installed on your boat without requiring significant modifications.

Popular Marine Water Heater Brands

Several well-known brands in the marine industry offer high-quality water heaters suitable for various types of boats. In this section, we will discuss a few popular marine water heater brands to consider for your boating needs.

Kuuma:  Kuuma is a trusted name in marine water heaters, offering reliable products designed for marine applications. Their  6G 120V Side Mount Front Exchanger  and  11G 120V Front Exchanger SS  are popular models known for their durability and performance. Kuuma water heaters come in various sizes and configurations, making it easy to find the right one for your specific needs.

Whale:  Whale is another renowned brand in marine water heaters. They produce durable and efficient heaters suitable for boating enthusiasts. The  Whale F600 Water Heater , with a 6-gallon capacity and front heat exchanger, is one of their popular models. Whale water heaters are also available in various sizes and configurations to meet diverse boating requirements.

Raritan:  Raritan Engineering is known for its high-quality marine water heaters designed to perform in harsh environments. Their  1700 series  is engineered specifically for marine applications and offers reliable performance with dockside recovery rates of up to 18 gallons per hour. The adjustable thermostat in Raritan water heaters enables precise temperature control, making them an excellent choice for boaters.

In conclusion, Kuuma, Whale, and Raritan are some of the popular marine water heater brands to consider when upgrading or installing a new system on your boat. Each brand offers a range of products tailored to suit various needs, ensuring that you can find the right water heater for your specific situation. Keep in mind that researching and comparing different options is vital to making an informed decision and getting the most value for your investment.

Maintenance and Troubleshooting

Regular maintenance.

Proper maintenance is crucial to ensure the longevity and efficiency of marine water heaters. Regularly inspecting the water heater and its components can prevent potential issues and costly repairs. Some essential maintenance tasks include:

• Checking and replacing the water heater anode when necessary. The anode protects the tank from corrosion, and wrapping Teflon tape around the threads of the anode can help prevent leaks during reinstallation ( BoatUS ).
• Examining pipe joints and hose connections for leaks. Using Teflon tape or pipe joint compound can help prevent dripping, and hose clamps ensure tight seals on hose barb-to-hose connections ( Practical Sailor ).
• Ensuring proper and consistent supply of gas and electricity to the water heater. Test for stable gas supply by lighting a stovetop burner, and use a voltage meter to check that input to the heater is at least 10.5 volts ( RV Repair Club ).

Common Issues and Solutions

Marine water heaters may experience various problems, which can often be resolved with some basic troubleshooting:

Consulting the water heater's manual and seeking professional assistance when unsure about diagnosing or resolving an issue is recommended. Regular maintenance and diligent troubleshooting can help extend the life and enhance the performance of marine water heaters.

Environmental Considerations

When selecting a marine water heater, it is essential to consider the environmental factors that will impact its performance and longevity. Marine environments expose water heaters to high-humidity, aggressive water conditions, and other harsh factors that can affect their functionality and reliability.

One major concern in a marine environment is corrosion. It is crucial to choose water heaters that have corrosion-resistant components to ensure long-lasting performance. Many top-quality marine water heaters, such as those offered by  Torrid , include glass-lined inner tank vessels to mitigate the risk of corrosion and reduce bacterial buildup.

Eco-friendly operation is another important environmental consideration. Look for water heaters that meet stringent energy efficiency standards and provide optimal heat transfer without excessive energy consumption. Additionally, these heaters should have minimal noise output to limit noise pollution in the surrounding marine environment.

Furthermore, insulation plays a crucial role in maintaining the water temperature and reducing heat loss. Marine water heaters with good insulation, like the polyurethane foam used in  Sigmar heaters , can still have water heated at more than 80-degrees after 20 hours of shutting off the heating element.

Last but not least, proper installation is key in minimizing potential environmental risks. Ensuring that the water heater has a secure mount on a well-reinforced surface can help prevent potential damage and instability during rough seas, as mentioned by  Practical Sailor .

Considering these environmental factors will help guide boat owners in choosing a suitable marine water heater that is reliable, efficient, and durable in the challenging marine environment.

In this ultimate guide , we have discussed various aspects of marine water heaters, from the top-rated models to the factors that should be considered when making a purchase. The  5 best marine water heaters  we reviewed, such as Whale Premium Water Heaters and Kuuma Water Heaters, showcase the options available to boat owners.

When investing in a marine water heater, factors like size, capacity, heat exchanger type, and power source are crucial in making the right choice. For instance, smaller boats may benefit from a compact design like the  Cobalt Aquatics Neo-Therm , while larger vessels may need heaters with greater capacity.

An efficient marine water heater not only provides comfort but ensures a pleasant and warm experience during long sailing expeditions. Since various models are available on the market, carefully weighing your specific needs and preferences will lead to a suitable purchase that will cater to your boat's heating requirements.

Lastly, proper installation and maintenance of your marine water heater are key to reliability and longevity. Make sure to follow the manufacturer's guidelines and consult professionals when necessary. By taking all these factors into account, you can select and maintain the ideal marine water heater tailored to your unique needs.

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Boat Plumbing

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Boat plumbing is a lot easier for the do-it yourselfer than plumbing at home, mainly because it doesn't involve rigid pipes running inside solid walls.

Here is an overview of a typical on-board water system.

Because water is heavy, tanks should be mounted low in the boat. Where space is available, it is a relatively simple matter to add extra tanks. Rigid polyethylene tanks are available in hundreds of shapes and sizes, or you might use a flexible bladder tank--essentially a water bag.

Water tanks typically have three threaded ports, one for the outlet and one for the vent hose, both 1/2-inch, and one for the fill hose, usually 1 1/2-inch. Threaded hose barbs allow for hose connections. The inlet is connected to an on-deck fill. (Be sure the deck-fill has an O-ring to seal out seawater when it is closed.) The vent line leads to a vent fitting high in the boat — above the tank at every expected angle of heel. Be aware that if the vent is not also higher than the fill, it will overflow when you are filling the tank. The outlet connection leads directly to a pump or, in a multi-tank installation, to a manifold or Y-valve.

Use Teflon tape or thread sealant on all threaded fittings, and don't overtighten fittings in plastic tanks. Secure hoses with stainless steel hose clamps.

Supply piping for a boat water system must be non-toxic, non-contaminating, taste-free, and FDA approved for drinking water. If the system is pressurized or will carry hot water, the piping needs to be suitable. The traditional choice for water system plumbing has long been clear PVC reinforced with polyester braid. This same type hose can be used for tank fill and vent connections.

In recent years semi-rigid polyethylene (PE) tubing, long used in RV plumbing, has surged in popularity for boat plumbing. It has much to recommend it. With quick-connect fittings, a PE tubing water delivery system assembles with the simplicity of Tinkertoys. The opaque or at least semitranslucent nature of the PE tubing discourages algae growth that can be a problem with clear hose. PE tubing also comes in colors — typically red for hot water and blue for cold — which looks nice and might make plumbing failures easier to trace. Because the tubing is less flexible than PVC hose and it must be cut to the correct lengths, a PE plumbing system will be slightly more demanding to install. However, the primary negative to PE plumbing is the cost of the fittings, which at this writing run $4 to $8 each. On the positive side, the tubing is actually cheaper than reinforced clear PVC hose.

Drain hoses connected to through-hull fittings should be stronger than clear vinyl hose. For this use, select reinforced rubber hose, sometimes called heater hose. This is the same type of hose used on engine plumbing, and it typically has about three times the burst strength of reinforced vinyl hose. Double clamp all hoses connected to through-hull fittings.

Water pumps on a boat can be either electric or manual. An electric pump pressurizes the entire water system. Most electric pumps have a pressure switch that activates when the pressure drops below a set value--usually around 30 or 40 PSI. Opening any tap on the boat releases pressure and causes the pump to kick on and run until it rebuilds the pressure to the cut-out setting. The pump cycles on and off until the tap is closed. The inlet of an electric pump connects directly to the tank outlet (or multitank valve), and the outlet supplies water to all faucets and appliances.

Manual pumps — hand or foot operated — supply a single spigot connected directly to the outlet side of the pump. A regulating valve is not required; water flow is controlled by the operation of the pump. The primary advantage of manual pumps is that they dramatically reduce water waste, a major concern for boats that spend long periods away from water supplies.

Accumulator

Some water systems include an accumulator. Large accumulators have pressurized bladders in them, but most small ones are just empty tanks teed into the line downstream of the pump. When the pump runs, it tries to fill the tank from the bottom, compressing the air trapped inside the tank. The pressure from the tank allows small amounts of water to be drawn without the necessity of the pump running, thus reducing pump cycling.

A marine water heater is simply a small, insulated tank downstream of the pump. You must have a pressurized water system to operate a water heater. The pump draws water from the storage tank(s) and fills the water heater tank. Inside the water heater is an electrical heating element and usually a coiled tube called a heat exchanger. When AC power is available, the electrical element (controlled by a thermostat) heats the water. Away from the dock, the hot engine coolant is routed through the coiled tube to heat the water in the tank when the engine is running.

Water heaters have four threaded ports. The tank inlet connects via a tee-connector to the outlet hose from the pump. A check valve is required in this line or in the heater to prevent hot water from migrating back toward the pump. The outlet connection supplies heated water to the hot side of all faucets, also using tee-connectors. The other two ports are for the heat exchanger connection, which varies depending on engine installation. Use only metal fittings to plumb a water heater, never plastic. If a pressure-release valve isn't integral, the heater will have a fifth port for this essential component.

Faucets are the ultimate terminus for water system lines. Manual pumps require simple spigots, but in a pressure water system, boat faucets differ from those found ashore only in styling and that they may be fitted with hose barbs. Mixer faucets require two connections, one from the cold side to the supply line from the pump and the other from the hot side to the water-heater outlet.

Shower connections are identical to faucet connections. The only difference is that rather than delivering the water through a spigot, the water is delivered through a pipe or hose to the shower head.

A nice owner addition to almost any boat is a deck shower, easily installed by simply teeing into cold- and hot-water supply lines.

Sink drains typically connect with reinforced rubber hose to a through-hull fitting. On a sailboat, sinks are best located near the centerline of the boat so heeling doesn't put them below the waterline. Because head sinks are often well outboard, they may be plumbed to drain into the bowl of the toilet to avoid the risk of flooding. There are collateral benefits of running fresh water through the head.

Shower pans too often drain into the bilge to be pumped overboard by the bilge pump. However, this arrangement eventually leads to unpleasant bilge odors, and it risks jamming the bilge pump with hair. Shower pans should be isolated from the bilge and include a discharge pump, either automatic or connected to a switch. The through-hull discharge outlet must always remain above the water.

Since few boats carry sufficient fresh water to allow washing the decks with it, washdown pumps are not connected into the freshwater system. Nevertheless, a washdown pump is a great convenience for hosing the deck and knocking mud off the anchor chain.

The inlet fitting of a washdown pump is connected to a submerged through-hull fitting, and the outlet side is connected to a deck-mounted faucet or male hose connector. A dedicated through-hull is not required; if you are installing a deckwash pump; use a Y- or tee-connector to tie into an existing inlet line. Use heavy-duty rubber suction hose, wire reinforced to keep the hose from collapsing. Debris will damage or destroy a washdown pump, so it is essential to have a strainer in the intake line.

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Contributor, BoatUS Magazine

Don Casey has been one of the most consulted experts on boat care and upgrades for 30 years, and is one of the BoatUS Magazine's panel of experts. He and his wife cruise aboard their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.

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Marine Water Heaters Explained

Key facts voyagers need to know about these   valuable units

W ith a few exceptions, most electric marine water heaters are simply scaled-down versions of those used in homes and businesses the world over.  

The components of these units are relatively simple. An electric heating element is immersed in the water contained within the heater’s tank. It, in turn, is controlled by a thermostat which regulates the temperature of the water. When electricity is applied, the heating element warms the water within until the set point on the thermostat is reached, at which point the heating element turns off. 

Water heaters also typically include an important safety device, a pressure/temperature relief valve. This is designed to relieve pressure within the water heater’s tank in the event of a malfunction. If, for instance, the thermostat that controls the electric heating element malfunctions, and the water within the heater rises to the boiling point, creating steam and extreme pressure, the pressure relief valve will open and vent the pressure before the condition goes super critical. Of course, if the pressure relief valve is damaged, malfunctioning or capped, then it won’t be able to provide this all-too-necessary safety feature.

Even when a pressure relief valve is working properly it can pose a hazard. Several years ago, while I was managing a boat yard that was responsible for commissioning new vessels, a mechanic was injured by a water heater as he worked alongside it when the pressure relief valve unexpectedly vented, flooding the area with boiling water and steam. Fortunately, his injuries were not severe, however, a subsequent investigation revealed that the thermostat had been improperly wired, essentially bypassing its important control function, rendering the heating element continuously energized. For this reason, the discharge of a water heater’s pressure relief valve should be securely plumbed into the bilge or away from areas that may be occupied at any time. Flimsy hose or tubing simply slipped over the valve’s outlet pipe is inadequate, as it will be blown off by high pressure water and steam. It must be robust and securely clamped in place.  

If the vessel is equipped with a direct dockside water pressure system, one which enables a continuous supply of dockside water, the water heater’s pressure/temperature relief valve must be plumbed so that it vents directly overboard. If it’s not installed in this manner, and the valve leaks, the vessel could flood with an unlimited supply of dockside water. Plumbing a pressure/temperature relief valve directly overboard does have one drawback: If the valve leaks, it’s possible it will go unnoticed.

Heat exchangers One feature that marine water heaters, for those so equipped, do not share with those ashore is the heat exchanger. The heat exchanger, much like the one on your engine or generator, simply transfers heat generated elsewhere, in the engine’s closed cooling system or from a diesel-fired hydronic heating system, to the water inside the water heater’s tank. This arrangement is extremely common and has been used effectively for decades. Instead of sending all of the excess heat created by the engine out with the exhaust, some of it is reclaimed for washing dishes, taking showers, etc.

As well as the heat exchanger scenario works, there are a few caveats associated with its use. Utilizing engine coolant to make hot water means the water heater must become part of the engine’s closed or coolant-filled cooling system. While most engine manufacturers make provisions for a water heater connection, when pressed they aren’t keen on the idea because it takes complete control, design and implementation of the cooling system out of their hands; in some cases, they may require another heat exchanger and circulation pump close to the engine.

If the engine’s cooling system fails because of a design or manufacturing flaw, the engine manufacturer is usually responsible for repairs, provided the engine is under warranty. If, however, the plumbing between the engine and the water heater fails, the engine will almost certainly overheat, potentially causing permanent damage, while leaving the vessel powerless.

The engine manufacturer, however, bears no responsibility for a failure of this sort. Therefore, make certain the hose used to connect the engine’s cooling system to the water heater is especially rugged, chafe resistant and robust; hose that carries a J2006R “Marine Wet Exhaust” rating is ideally suited for this role, while ordinary “heater hose,” in spite of its name, may not be up to the task because of its propensity to crush, kink and chafe when routed off the engine and through the vessel.

Additionally, when a water heater is connected to an engine’s closed cooling system certain protocols, as well as the engine manufacturer’s instructions for such an arrangement, must be followed closely. Primary among these is the location or elevation of the water heater, or more specifically its heat exchanger. If the heat exchanger within the water heater, or any portion of the plumbing between the engine and the water heater, is located above the engine’s expansion tank cap, then a remote expansion tank must be plumbed into the system. This tank, with its own pressure cap, then becomes the primary fill point for the closed cooling system, rendering the original legacy fill cap dormant and unusable. In most cases, when a remote expansion tank is installed, it must use a pressure cap that the engine manufacturer calls for while the original cap located on the engine must be replaced with a higher-pressure rating cap. The reason for this cap swap is to ensure that only the highest cap in the system, the one that can vent air, opens and closes with temperature-induced pressure changes, while the cap on the engine’s expansion tank remains sealed at all times.

Hot water temperature Another peculiarity of the marine water heater when it’s interconnected with the engine’s closed cooling system is the temperature of the water it’s capable of producing. The traditional, built-in thermostat controls only the electric heating element, which is typically set somewhere between 120degrees and 140 degrees Fahrenheit.  It’s important to note that water heaters are normally set to temperatures above 131 degrees to prevent development of harmful bacteria, such as Legionella, the cause of Legionnaire’s disease, in the water supply. This is of particular concern with marine water heaters because they are turned on and off, which means they often contain stagnant, tepid water, which is an incubator for water-born bacteria.

Water at a temperature above 106 degrees is considered painful to some. Personally, I consider 109 degrees the perfect shower temperature, while my wife believes this is much too hot. She prefers something around 105 degrees, (our teenage children didn’t seem to care what the temperature of the water is as long as they used all of it). Anything under about 95 degrees begins to feel chilly. As you can see, there’s not much of a range between comfort and pain or potential injury. At a temperature of 131 degrees Fahrenheit (55 degrees Celsius), a child can be scalded in less than four seconds.

Therefore, it often comes as a surprise to boat owners that the coolant issuing from the engine, and going to the water heater may be as hot as 195 degrees, perhaps 180 degrees by the time it reaches the water heater. Without any external control, water exiting the water heater may be nearly this hot, which clearly presents a serious burn risk. Not long ago, while inspecting a vessel, I opened a galley faucet while underway, only to have a cloud of steam billow forth. It was a sobering experience.

There are two ways this potential safety hazard may be dealt with, both of which offer the same point-of-source approach. The first involves using a mixing or tempering valve. This adjustable device, installed at the water heater, controls the temperature of the water leaving the water heater by mixing it with cold water in order to lower its temperature to the desired level. Therefore, regardless of the temperature of the water in the water heater or the coolant running through the heat exchanger, the output temperature remains constant. The mixing valve has an added benefit, because it enables the water in the water heater to be maintained at a much higher temperature, it effectively provides the user with more hot water for one simple reason: not much of it is needed as it’s mixed with cold water to produce the desired 100 to 110 degrees temperature range.

Adding a mixing valve Mixing valves can be added to virtually any water heater as an aftermarket item, after which the water heater’s thermostat can be increased to maintain a higher electrically-heated pre-product water temperature. It’s important to note that not all mixing valves’ reaction times are quick enough to consider them as anti-scald protection. A variety of standards exist concerning the design and performance of these valves. For more info on this subject it’s worth a visit to www.watts.com/pdf/pg-mxv.pdf. If instantaneous anti-scald protection is desired, then install anti-scald faucets.

The other method utilizes a component known as a temperature compensation valve or TCV, which is plumbed, externally, to the water heater’s coolant heat exchanger while sensing output water temperature. This product takes a slightly different approach in that it controls the amount of coolant running through the water heater in such a way that it maintains a water output temperature of about 140 degrees, which does present a scalding risk.  

The beauty of either of these approaches is that they limit the temperature of the water leaving a water heater, regardless of whether the heating source is the electric element or the engine’s coolant. Quick reacting anti-scald valves are, of course, desirable, particularly for showers and sinks; however, it may not be practical or economical to equip every fixture aboard with one of these, particularly after a vessel is built. That’s where mixing valves and TCVs come in. If your water heater is connected to your engine’s cooling system you’re playing with fire unless you install one of these inexpensive devices.

Finally, many water heaters are installed using a check valve on the cold-water supply, which serves two purposes. One, it prevents water from running out of the water heater, potentially damaging the element. Unless it includes protection that prevents it from operating while dry, water heater elements must be submerged while operating. If exposed, their lifespan can be measured in seconds. Two, the check valve minimizes convective heat loss, and comingling of hot and cold water, which also hastens heat loss. If a check valve is installed, the water heater must also be equipped with an expansion tank, which allows water in the water heater to safely expand as it’s being heated. Without this component, the water heaters pressure/temperature relief valve is likely to (hopefully) open, thereby wasting hot water, and defeating the purpose of the check valve. ν

Steve D’Antonio is an ABYC-certified Master Technician and sits on that organization’s Engine and Powertrain, Electrical, and Hull Piping Project Technical Committees. He owns Steve D’Antonio Marine Consulting (stevedmarineconsulting.com) which offers marine systems consulting and pre-purchase services to boat owners, boat builders and others within the marine industry.  

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Get Free Hot Water from a Boat Water Heater

A boat water heater can usually be retro-fitted without too much difficulty, as long as your boat has a freshwater cooled engine. This type of marine engine, unlike a raw seawater cooled engine, has a separate closed system with its own header tank, which is then cooled by a freshwater/seawater heat exchanger.

The other requirement is for a pressurised domestic water system. A manually pumped system, laudable though it is, just won't do - but it's usually a straight forward affair to convert it to a pressurised system.

On a boat thus equipped, you can get that rare commodity; something for nothing - and you should take advantage of it.

Any boat that uses its engine fairly regularly produces hot water as a by-product of the combustion process, and it's free. Once you've shelled out for the equipment that is - which in this case is a boat water heater known as a marine calorifier.

This device is effectively an insulated heat exchanger, which plumbed into the engine cooling water system will transfer heat from the engine into the domestic water supply.

A typical hot and cold domestic water installation is shown in the illustration below.

How a Boat Water Heater System Works

Artwork by Andrew Simpson

This system incorporates both a pump and an accumulator. Most water pressure pumps maintain steady pressure by cycling on and off, producing a pulsed delivery.

The accumulator tank, containing water and air - usually but not always separated by a diaphragm - absorbs this pulsation and provides steady flow. Some pump manufacturers now produce microprocessor-controlled variable-speed pumps which eliminate cycling and make the accumulator tank redundant.

The mixer valve operates thermostatically, introducing a metered amount of cold water into the outlet flow, thus maintaining regulated temperature and conserving hot water.

The pipework from the engine should be as short as possible to minimise heat loss, and should be installed lower than the level of the engines header tank.

Calorifiers are well insulated and will continue to provide warm water for an hour or two after the engine has been turned off, but if you haven't run the engine for a much longer period, turning on the hot tap won't produce the desired result.

Immersion-Type Water Heaters

Electric immersion heaters, much like the larger versions we're accustomed to at home, require AC power, so unless you've got a generator aboard their use is restricted to when you're alongside and hooked-up to an onshore supply.

Immersion heaters are often combined with a calorifier in a single unit.

The stainless steel Isotherm units shown here are the immersion/calorifier type and are equipped with a thermostat.

In immersion heater mode, they draw around 6.5A at 120V. 

Hot Water from the Sun!

Intended for campers, but perfect for sailors cruising in warm, sunny climes.

Cheap and effective - every cruising sailor should have a couple of these on board!

Gas Water Heaters

Bulkhead mounted gas water heaters like the one shown here are no longer popular on sailboats, many skippers believing that they're just too plain dangerous.

If you do decide to invest in one of these however, make absolutely sure that it has an automatic low gas pressure cut-off switch and a flame failure device fitted, and it's properly serviced and regularly maintained.

I used to have one on a previous boat - this one in fact, on my Nicholson 32 'Jalingo' and lived to tell the tale.

But would I have another? No, definitely not.

In fact I'm not even going to tempt you with an example.

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Yachting Monthly

• Digital edition

How to install hot water onboard a yacht

• Toby Heppell
• May 5, 2023

Having hot water on board can make a long cruise a whole lot more pleasant. Duncan Kent explains how to install hot water onboard a yacht

Having pressurised hot water on your boat will make all the difference to comfortable living when on board. A hot shower at the start of the day can invigorate the crew for the day ahead, and washing dishes and clothes without boiling a kettle is so much easier.

To enjoy the luxury of hot water on your boat you will need both a pressurised freshwater system and a calorifier. Most modern production yachts come with at least a basic cold water system, incorporating a pump that activates by the drop in pressure when a tap is turned on. If yours doesn’t, then it’s a fairly simple and inexpensive modification to carry out. The hot water system then utilises the same pump and water supply, but is piped through a calorifier on its way to the hot tap.

The Calorifier

A calorifier is a heavily insulated copper or stainless steel water tank with an electric heating element, not unlike a domestic hot water tank. Copper tanks are marginally better than steel at retaining the heat and have excellent antibacterial properties.

The difference between a marine calorifier and a domestic tank is that it also contains a spiral copper tube through which water from the engine’s freshwater coolant pump is circulated. Some twin-coil calorifiers are also available for those who want to circulate hot water from both the propulsion engine and a diesel-fuelled, hot water heating system.

Hot and cold domestic water system, with a pump, calorifier, expansion tank, accumulator tank and multiple outlets

Electrical power

For running off electricity, a calorifier can either have a 220Vac element inside, for use when you’re on mains shore power, or a 12Vdc element to run off your domestic batteries. The latter will be much lower power (typically 300W), however, and will take a good deal longer to heat the water, but they are a great idea if you have excess solar power to use up during the summer months.

Some calorifiers use an inverter (DC to AC converter) to power the 220Vac element, but this merely adds increased inefficiency and consumes massive amounts (100A for every 1kW of heating element) of battery capacity. Though it is feasible, I would only suggest it to someone who has a lithium-ion domestic battery bank with oodles of solar power or a generator.

Brass screw connectors are required to connect to the calorifier, but otherwise plastic pipes with push-fit joints are more resistant to vibration and movement

Engine heating

Ideally, it requires an engine of 13hp (a two-cylinder diesel) or greater for a system to produce enough power for hot water without it having too detrimental an effect on the boat’s propulsion power. The motor will need to be run for around half an hour to make enough hot water for a shower or two, or an hour or so for all-day hot water.

The system requires feed and return pipes to be connected to the engine and uses the warm engine coolant to indirectly heat the fresh water contained in the tank. Although this method works best with indirect, heat exchanger-cooled engines, it will also work with a raw water-cooled engine, although less efficiently, and the resultant hot water will be at a slightly lower temperature. Raw-to-fresh water conversion kits are available for a wide range of engines, so now might be the time to upgrade.

Most indirect water-cooled marine diesels already provide attachment positions, which are usually blanked off if not connected. When required, these blanking plates are removed and replaced with spigots, to which heat-resistant, flexible hoses are connected between the engine and calorifier.

An expansion tank will prevent unnecessary activation of the pressure release valve

Regardless of the heating method employed, to avoid the risk of scalding, calorifiers have a thermostatic mixer valve on the fresh water exit pipe and a pressure relief valve (PRV) in case something goes wrong and the water boils. The former allows the water temperature to be set (within safe limits) by the user. The latter should be piped into the bilge to allow it to drain in an emergency.

Installing an expansion tank to the hot water pipe as it exits the calorifier will help relieve any excess pressure and can avoid the pressure relief valve from being activated unnecessarily. The tank should be sized to around 10% of the calorifier’s capacity and the pressure set to suit the system and occasionally maintained by connecting a bicycle pump to the valve on the tank.

Sizing up and saving water

Having a calorifier fitted effectively extends the capacity of your freshwater system, but the size of the calorifier tank you choose will be partly determined by the amount of space you have available. The most limiting factor will be the need for the calorifier to be installed with the engine water connectors below the level of the engine coolant header tank.

It is also better to mount the calorifier as close to the engine as possible, to reduce heat loss in the circulation pipes. This can often mean you have to settle for less hot water capacity than you would like, although by setting the temperature fairly high and mixing it with extra cold water will make it go further. For instance, a 15-litre calorifier can usually produce enough hot water for at least two showers plus general use like dishwashing during the day.

Clever planning can also make it go further, such as getting some of the crew to shower while you’re motoring into harbour or doing the dishes when the engine is on to charge the batteries.

When I’m off grid I don’t like wasting water waiting for the shower to heat up, so I keep a clean bucket nearby to run the shower into until it begins to warm up, using the water later for something else. Alternatively, you can put the plug in the sink if it’s nearby and run the shower into there until the hot water starts to come through.

Engine controls are to starboard, as is the deck shower

Installation

Although there are a few rectangular tanks available, most calorifiers tend to be cylindrical, which isn’t the best shape for squeezing into tight spots. They also have a correct orientation, so make sure you specify vertical or horizontal mounting.

The usual place to mount them is under a berth or the saloon seating. Wherever you choose it will need to be very firmly mounted using the supplied brackets as even a 15-litre model will be heavy when full, and will tear itself free in rough seas if it is poorly secured.

The favoured method of boat plumbing these days is to use plastic, push-fit piping and joints (like Hep2O or Speedfit), which are flexible enough to allow for some movement or vibration.

Under a berth or seating can be a good place to locate a bulky calorifier

The pipes and fittings are available from most plumbers’ merchants or DIY stores, although you may need to switch to copper screw-connectors when attaching to the tank. Take note of the inlet and outlet identification as the cold water inlet will usually incorporate a non-return valve to stop backflow of warm water and the outlet will usually have the mixer valve attached and already plumbed in.

The engine coolant hoses have to be graded as such and are usually made from canvas-reinforced rubber hose. They will get much hotter than the outlet pipes so need to be heat-resistant, pressure rated (usually 5 bar/70psi minimum) and unaffected by antifreeze in the coolant.

Try to install the calorifier as close to the engine as possible to save heat leakage from long pipes, which should ideally be insulated with foam tubing as in a domestic heating system.

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• Practical Boat Owner
• Digital edition

How to fit a marine hot water system to your boat

• September 27, 2021

Rupert Holmes looks at how to retrofit a calorifier tank to the fresh-water side of your boat's cooling system.

Today most new yachts come with pressurised hot water, but that wasn’t the case 20 or 30 years ago. Many boat owners are happy to rough it with a kettle as their only source of hot water. Others value the convenience of harnessing ‘free’ excess heat from the engine to warm a tank of water.

These are the parts you will need in order to retrofit a marine hot water system to your boat.

Estimated equipment prices ■ Calorifier – from £350 ■ Immersion heater element – from £68 ■ Water pump – from £49 ■ Expansion tank – from £62 ■ Food quality hot water hose – £2.78/m ■ Sink basin and shower mixer tap – from £94

Typical systems have a calorifier tank in which the water is heated by the fresh water side of the engine’s cooling system. This circulates through a coil of pipework within the body of the calorifier.

In addition a 240V immersion heater, typically of around 1kW, can be fitted for use with shorepower when in a marina.

Electric immersion heater element.

“The best calorifiers are made of copper,” says Ashley Bradley of ASAP Supplies, who offer a wide range of products for water systems.

“They retain heat for up to 24 hours and have better antibacterial properties.”

How big is a boat calorifier?

A typical 30-litre calorifier tank.

Sizing a calorifier tank is not an exact science and Bradley says it’s often determined by the available space, but where possible he advises always opting for a larger unit.

As a rule of thumb, a 30lt model is likely to be fine for 3-5 minute showers for a couple of people.

The calorifier needs a pressurised supply from the main water tanks. A mixer valve is also fitted to regulate water temperature and prevent it coming out of the taps scalding hot.

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What about a PRV – Pressure Relief Valve?

Often there’s a choice of buying a complete unit, or assembling components yourself. ASAP’s universal mount calorifiers, for instance, are supplied with all the essential fittings including mixer valve, pressure relief valve (PRV), immersion heater element, plus 13mm and 16mm barbs for connecting pipework.

On the other hand fittings can also be supplied separately, allowing a lower pressure PRV to be used, or for connecting to different types and sizes of existing pipework.

Today’s pressurised water pumps have built-in pulsation dampeners that help prevent the pump cycling too frequently. It’s still good practice, however, to fit an accumulator that will further smooth out the flow, preventing unwanted power draw and noise. This should be sized to be as large as you can sensibly fit.

What about an expansion tank?

Hot water expansion tank.

Given that water expands when heated, an expansion tank needs to be fitted on the hot water side of the system to prevent a build up of pressure. Bradley says this must be at least 8% the volume of the calorifier.

When used in this way, as an expansion reservoir on the hot water side, the pressure must be set just above the maximum pressure of the pump, and just below that of the pressure relief valve.

The same product can also be used as an accumulator on the cold feed, when the pressure must be set just below the pump’s cut-in pressure – 12-13psi, for instance, for a typical pump that activates at 15psi.

Bradley says one of the most common problems boat owners encounter is the pressure relief valve spurting water. Usually the cause is a lack of an expansion tank, or the expansion tank pressure being set incorrectly. The pressure can be changed using a bicycle pump fitted with a pressure gauge. There’s a diaphragm inside the expansion tank that simply needs to be pumped up to the required pressure.

Food quality hot water hose.

Hoses need to be of food quality, rated for use from -5˚C to +60˚C and be coloured red to avoid confusion when making repairs at a later date.

It’s also worth investing in thermostatic valves as these make mixer taps or showers work pretty much as they would at home.

What size engine take-off so I need?

Bradley says it’s important to check the size of the fitting for the take-off and return points on the engine for the hot water feed that circulates through the calorifier.

“These are typically a 16mm barb fitting,” he says, “but it’s not guaranteed, so you need to check and measure it on your own engine. Adapters that will work with different size fittings are available.”

Long pipe runs need an adequate pump size, as do multiple outlets. A typical Jabsco Par Max 2.9 pump, which is rated at 11lt per minute, will feed up to four outlets.

Ideally it makes sense to mount the calorifier as close to the engine as possible, as this will minimise pressure loss due to friction and heat loss in a long pipe run.

A common failure point is the pressure switch within the pump. These tend to have a shorter service life than the pump itself, but are usually user replaceable.

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How To Heat A Sailboat

Last Updated by

Daniel Wade

June 15, 2022

If you want to extend your boating season into the winter season, stay toasty warm, and dispel the dampness that can infiltrate your boat in cold weather, you need to get a reliable boat cabin heater.

Winter sailing is always a real challenge for most sailors given that the weather conditions are more adverse and calls for more preparedness. Fortunately, several types of boat cabin heaters can transform winter sailing into a more pleasant experience by keeping the interior of the sailboat warm and cozy. So if you do not want winter to spell the end of your sailing season, you’ve come to the right place: here’s how to heat a sailboat.

As we’ve noted, there are several ways to warm your cabin during the winter season or on a cold night. From hydronic systems, heating stoves, engine heaters, forced-air systems to reverse-cycle air condition systems and electric heaters, there are a lot of options for every budget. Although these systems may work differently, the basic idea is the same. They all either use fuel or another energy system to create heat that’s spread throughout the boat.

Let’s explore the most common ways to keep your sailboat warm during the cold season. But before going into that, let’s highlight some critical details.

Table of contents

How to Choose an Appropriate Boat Cabin Heater

The thermal outputs of these heating systems are generally measured in British Thermal Units (BTUs). The more BTUs a system produces, the warmer it can make your boat cabin. You should, however, remember that more BTUs will increase your electrical or fuel demands. In addition to BTUs, choosing the right heating system for your boat will depend largely on how well your boat is insulated, the kind of boat you have, how much time you spend sailing in cold weather, where you’re planning to sail the boat, and how much you’re willing to spend.

A Word of Warning

The dangers of poorly installing a heating system on your boat can never be underrated. Keep in mind that most of these systems will keep your cabin warm by burning oxygen, which can potentially fill the cabin with the deadly carbon monoxide that can quickly leave victims unconscious and may lead to death. As such, safety should be a priority.

How to Heat a Sailboat

Hydronic systems.

These systems are based on the same principles as household hot water heating systems. It revolves around a heated fluid running through a tube to radiators or fan units that warm the air. These tubes should run throughout the sailboat but the fan units or radiators can be divided based on the number of zones that the boat has.

In most cases, a hydronic system is smaller than a water heater but can also be used as a portable water heater for the boat, especially if you need hot water onboard. It’s generally installed in the boat’s engine room and can use a coolant to disperse the heat. It can use the fuel coming from the vessel’s main fuel tank or a special fuel tank.

The main advantage of using a hydronic system is that there is no moisture in the boat as there are no cold spots. An appropriate hydronic system should have a maximum output of 25,000 BTUs per hour while using 6 amps of power or 0.22 gallons of fuel.

Heating Stoves

Although they’ve been used for centuries, heating stoves remain one of the most popular ways to heat a sailboat. They can burn kerosene, diesel, and propane or even use solid fuels such as charcoal or wood. Some heating stove systems use the direct draft system whereby fresh air is pumped through a fan while others use the natural draft system whereby air is naturally pumped through the combustion process.

Either way, a heating stove should be located in the saloon to radiate heat and circulate hot air. They should be equipped with a damper to help in controlling the airflow, as well as an exhaust pipe to withdraw the exhaust gases arising from the combustion.

Keep in mind that most heating stoves do not use electricity. Nonetheless, a heating stove with an output ranging between 7,000 and 9,700 BTUs should be ideal, especially if it consumes about one pound of propane in about 5.5 hours.

Engine Heat

This revolves around using the engine’s heated coolant fluid to transfer the warmth into your boat’s cabin. The coolant lines should be designed to run from the engine into the heat exchanger. There should also be a fan that blows the cabin air into the heat exchanger so that the air is heated. The hot air can then be piped out into the boat’s interior through several hoses.

Using this system is advantageous in the sense that it’s quiet and depends on the engine’s closed-circuit cooling system, so it doesn’t require radiators or additional water pipes. Unfortunately, this system only works when the engine is running and can be quite expensive in terms of the extra engine maintenance and the fuel consumed.

Forced-Air Systems

This is a simple method that revolves around a heater burning fuel to heat air. This hot air is then forced through ducts via a series of vents to the boat’s cabin. Forced-air heaters are typically situated in engine rooms and generally have exhaust pipes to discharge combusted fuel.

The system can be turned on either manually or through its thermostat so that air and fuel are drawn into the combustion chamber to ignite the fuel and air blend under a controlled flame. This heated air is then forced into the duct and spread throughout the boat’s cabin.

Electric Heaters

This can be a perfect option if you occasionally go out on the water during cold days. Electric heaters function much like normal home ACs and usually run on 12-volt power. They’re typically mounted inside the cabin and run on electricity generated from the boat’s batteries that heat the coils. A fan draws air over the coils where it’s heated before another fan blows it into the boat’s interior.

The main downside of this system is that it has to use electricity, which means that you cannot venture very far from the shores. This is because you’ll need to recharge the batteries now and then and this will run them down very quickly.

Reverse-Cycle Air-Conditioning Systems

This works in the same way as a normal household heat pump by running in the reverse cycle. They’re typically very expensive but can be a great way to extend your sailing in the winter, especially if you want to circumnavigate the globe.

Another good option of heating your sailboat is heading south to sunny places such as Florida during winter. In most cases, the south is a bit warmer than the north during winter and you can keep going until you find the hotter climes of the south.

Installing a heating system is a great way of making the cold nights and winter season more pleasant and attractive. There are, of course, several ways to ensure that your boat is heated and warm, especially if you do not want to put the boat into storage and get back to the normal life on dry land just because it’s winter!

So if you still want to make winter a perfect sailing season , keep your cabin cozy with the above-described heating systems.

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winterizing hot water tank

• Thread starter javelin
• Start date Sep 26, 2012
• Catalina Owner Forums
• Catalina 30

I have a 1985 Catalina 30. Every year the mechanic has to crawl into the port hold to get at the water heater and reverse lines and horse around there for about 30 minutes on the hot water tank This is my thought: would it be possible to simply drill a hole in the top of the galvanized tank and suck out the remaining water instead of all the other trouble ? The wires and heater coils enter the tank from the stern but I don't know how far they go into the tank as I don't want to damage them. And afterwards put in some kind of plug. Is this a hairball idea or not ? Russ  

Don S/V ILLusion

Probably but rather than try that scheme, you can simply empty the tank by draining it once you install a by-pass kit sold anywhere including RV stores.  

Ralph Johnstone

Don't know your piping arrangement back there, but it would be a very simple matter to install a drain valve (bottom of the tank) which is in an "easily accessible" location and then blow out the whole system using compressed air. Far easier (and cheaper) than paying someone to crawl back there and suffer for a half hour. And when I say compressed air, I'm talking a little cheapy 12V tire inflater from a big box hardware store (around $30.00).  

Ralph Johnstone said: Don't know your piping arrangement back there, but it would be a very simple matter to install a drain valve (bottom of the tank) which is in an "easily accessible" location and then blow out the whole system using compressed air. Far easier (and cheaper) than paying someone to crawl back there and suffer for a half hour. And when I say compressed air, I'm talking a little cheapy 12V tire inflater from a big box hardware store (around $30.00). Click to expand

frogboy25 said: Don't know why you have to drain, I just run fresh water out of the system and then use 12 gallons of marine anti freeze run through system and out of all faucets. One gallon will do engine as well 1992 C 30 Click to expand

javelin said: This is my thought: would it be possible to simply drill a hole in the top of the galvanized tank and suck out the remaining water instead of all the other trouble ? The wires and heater coils enter the tank from the stern but I don't know how far they go into the tank as I don't want to damage them. And afterwards put in some kind of plug. Is this a hairball idea or not ? Russ Click to expand

Here is my setup. The valves at the cold in and hot out hoses would both be closed (the cold is closed in this pic...took it while preparing to winterize) and the one in the middle is on a hose connecting the hot and cold hoses. That would be opened when winterizing and closed (as shown) otherwise.  

Attachments

I open the drain on the heater and let it empty into the bilge. Then I flip the bypass valve for the water heater that I installed with a kit a couple of years ago. The kit is $29 at West Marine. The same kit at Camping World is $19. Next I drain my water tanks using the fresh water pump. Once there's no more water pumping through the system I add back in a couple of gallons of anti-freeze into the tanks and pressurize the system. Then I open each faucet and the deck shower one at a time until I see straight anti-freeze coming out. It really doesn't take very long to do. My hot water tank is under the aft berth right behind the steps so it's easy to get to. I'd recommend against putting antifreeze into your hot water heater. It takes a long time to get the residual taste out of the hot water because you're mixing fresh water into the heater rather than pumping it through as you are with the rest of the system. If you're at all sensitve to the taste or smell of the anti-freeze you'll notice it. For the engine I open the seacock after I'm blocked up, then take off the intake hose for the engine and put a funnel in. My wife starts the engine and I run three gallons of anti-freeze through and shut her down. A little more anti-freeze in the strainer and I'm done with the engine. My final step is dryer sheets in all the cabinets and drawers and Kanberra gel in the salon and head. Makes a big difference with boat smells. The last step is putting the cover on. :cry: Jim  

These are really great ideas. Thanks. Russ  

I only wish all hot water heaters had a drain valve! My Raritan WH does not. I only drain it as recommended and use a bypass kit to run antifreeze through the rest of my system. Little space causes me to turn a few wrenches as opposed to just turning a few valves but it works.  

use a shop vac to suck out the water instead  

melliott0352

I removed both water lines (inflow cold and out flow hot) and connected them together with a barbed connector. Having pumped out the fresh water prior with the on board water pump, then 3 gallons of antifreeze (-50 potable) in the water tank, ran hot and cold lines until all faucets ran pink. Opened the drain valve on the heater tank, and blew in the inlet with the foot pump that I use to inflate the dingy, dumping the water in the bilge. Like the bypass kit, on sale at camper world, thanks for the link! :dance: Will do that in the spring when I reconnect the lines, as I have to replace a connector that I broke taking it all apart anyway!:cussing:  

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