Marine water heaters

Marine water heaters provide hot water storage. A boats water system will include a hot water heater if your lucky. Marine Water heaters are not like the ones in your home. The marine environment is quite corrosive, plus the systems feeding the heater are also different necessitating a marine designed and built water heater.

Hot water heaters get their heat from;

  • Main Engine heat exchanger or
  • 110v powering the heating element

While the boat is under way the Main Engine heat exchanger, transfers heat through coils in the water heater. The coils heat the water inside the water heater.

Secondly a 110V heating element installed low down in the water heater can heat the water. 110v can be supplied by shore power or an onboard generator when away from the dock.

Hot water temperature control

isotemp mixing valve to regulate hot water temperatures
isotemp mixing valve to regulate hot water temperatures

Prime safety concern is the temperature of water delivered at the faucet. Scalding water is not what you need and without any safety temperature controls you need to mix in cold water at the taps.

Engine Hot Water can get to 180 degrees. To regulate this temperature you can either have a temp mixing valve on the tank or have an automatic temperature control at the faucet.

120v heating element are easier to control as they can be adjusted to produce the desired water temperature in the water supply.

The diagram right shows an Isotemp water heater with a thermostat mixing valve (highly recommended and should be compulsory). Here the temperature of the warm water can be set (between 38 and 65 °C / 100 and 150 °F ) to avoid scalding.

Water heater installation concerns

Water heater location; The heat exchanger for the water heater should be no higher than the exchanger for the Main Engine. If not a remote expansion tank must be added. The water Heater should also be close to the main engine so hose runs and heat loss is less. Follow the water heater manufacturer’s instructions.

Hot Water Hoses; it is highly recommended that hot water hoses should be of high quality; SAE J2006 (wet exhaust) hoses are recommended. Lower quality can rupture with the heat.

Winterizing If you have or are installing a hot water heater you must ad this to your list of winterization projects. Water left in the tank will freeze and potentially split hoses and tubes.

Engine flushing Some engine manufacturers recommend that cooling system be flushed periodically. To do this you must isolate heater from the engine.

Isotemp water heaters

One of the best water heaters on the market is Isotemp manufactured by Italian manufacturer Indel. The units are made from stainless steel for inner jacket and out casing plus tanks and tubes which are in contact with water. Stainless is the best material to combat the harsh corrosive environment.

Not only does isotemp have the best materials they also have safety features like the mixing thermostat and features to keep the cold water mixing with hot water hot. Some Isotemp models have dual heat exchangers for better performance

The heating element is a standard 750W or 1000W which is placed at the lowest point in the tank where the water is the coldest, so to make sure that water temperature equalizes.

Isotemp water heaters are not cheap but the construction, design and the safety valve makes them an excellent choice.

Isotemp water heaters; Basic, Slim and Square

Manufacturers comments

Isotemp Stainless Steel Marine Water Heaters are specifically designed for use in the demanding marine environment. Each unit is built using the highest quality components to ensure superior product performance. Isotemp marine water heaters are very rugged, designed to last for years of trouble-free service. Thanks to the Isotemp reputation for dependability, long life and handsome design, Isotemp marine water heaters add value to every installation.

Unlike common aluminum-core water heaters, the Isotemp marine water heater features brushed stainless steel cover as well as 316-grade acid resistant stainless steel inner tank that is backed by a 5-year limited warranty. Isotemp achieves the highest capacity by optimized positioning of the engine water heat exchanger as well as the electrical immersion heater. They are positioned in the lowest part of the tank where the water is coldest in order to ensure an equal heating of all the water in the tank. The water inlets and outlets are specially designed to minimize the mixture of cold and hot water.

Water is heated to 160+ degrees with just the electric heating element alone. With the engine heat exchanger, water can be heated to 190+ degrees. The innovative thermostat-controlled mixing valve mixes ships water with the hot water to deliver the temperature you want at the faucet. This mixing action speeds recovery time and extends the volume of hot water you get from a smaller tank.

Isotemp water heaters

Manufacturers comments


1.Rugged design

2.Water tank is 316-grade stainless steel

3.External cover is 304-grade stainless steel

4.Stainless steel mounting brackets encircle the, tank for deck or bulkhead installations.

5.Thermostat-controlled mixing valve comes standard on all Isotemp water heaters


Product Specifications:

  • Slim, Basic, and Square designs fit all spaces:
  • Extra long, corrugated coils for high heat exchanger efficiency
  • Special 7.0 bar/100 psi safety valve makes winterization easy
  • Ultra-thick insulation for lowest temperature loss
  • Nickle-plated copper immersion heating element for long life is
  • positioned to heat colder water at the bottom of the tank
  • Thermostat controlled mixing valve protects against accidental scalding.
  • Optional heating element available


  • Stainless Steel construction
  • Mixing valve for safety
  • Water in and outlets minimize mixture of cold and hot water
  • Heat element at bottom of tank ensures water temp equalization


  • Cost

Manufacturers Contact

indel webasto marine makes of Isotemp Water Heaters


There are other water heaters that are steel jacketed and also include alloy and glass lined tanks, none of these are as good as the Stainless Steel tanks.

Superstore hot water heater Made from Stainless Steel

Propane Heaters

Do not use one of these instant tank less water heaters unless you are very careful. If you do be aware that if the flame does not burn properly they will put out carbon monoxide CO which will kill you. If you are in a closed space, CO fumes will put you to sleep and you wont wake up. These units need to be vented overboard and not in a closed cabin, so please be aware of the venting issues. I believe Insurance companies do not like them.