Jon Walmsley explains how to heat water without a diesel engine

Dear PBO, In the Catamaran Refit article (PBO July 2023) in the section on hot water, the author, Jon Walmsley alludes to a ‘gizmo’ that diverts excess 12V power to a heating element thus utilising otherwise wasted power to heat the domestic water.

Is there any chance you could have a look at this concept and perhaps even produce an article on it?

So many boats these days have solar or wind to supplement their energy production and, in our case, the batteries are usually at 100% state-of-charge by lunchtime.

For the increasing number of electric boats, of course, engine-heated water is not an option.

a diagram showing how a solar hot water system works on a boat

How the solar hot water system works on Jon Walmsley’s catamaran Ciel Bleu

To be able to use the afternoon’s rays or wind to produce a tank full of hot water – or even just slow the cooling rate – would be fantastic.

Like many cruisers, I abhor the practice of running an engine or generator in a quiet anchorage just for battery charging or hot water, and even more so when this is done in the quiet of the evening which, for so many, seems to be the modus operandi these days. Whatever happened to the etiquette of conducting such things in the forenoon period?

Anything that PBO can do to bring this knowledge to a wider audience would, I’m sure, be greatly appreciated.

Ian Horne

Jon Walmsley responds: When I purchased Ciel Bleu, a Fountaine Pajot Maldives 32, high on the list of upgrades for extended cruising was a hot water system for the galley and the heads.

Ciel Bleu is powered by an outboard engine instead of an inboard diesel.

While I’m not a qualified electrical engineer, and every installation will be different, hopefully my system will help readers develop their own solutions.

Pressurised hot water from a tap is easily achieved on most cruising boats by using waste heat from the inboard diesel engine.

Engine coolant is passed through a coil inside an insulated hot water tank, known as a calorifier, where it indirectly heats water supplied by the vessel’s cold water tank.

The calorifier normally has a 240V immersion, so that water can also be heated by a shoreside mains supply.

This solar hot water system works very well in most circumstances. Usually, only a half hour of motoring is enough to raise the temperature of the water in the tank to a useful level.

A dual 12V and 240V twin immersion heater for a solar hot water system on a boat

A dual 12V and 240V twin immersion heater covers you for cruising and shorepower

Remember that a typical diesel engine is less than 50% efficient, which means every kilowatt of mechanical power made correspondingly produces more than a kilowatt of heat.

Apart from the prerequisite of an inboard engine, there is one major drawback.

Unless the boat is in a marina, the production of hot water is entirely dependent upon the engine being run on a regular basis.

Until recently, when cruising, this was not an issue. Even when the boat was not underway the engine was run, at least daily, to charge the batteries.

The problem is that with the widespread installation of wind and solar systems by the cruising community, there is no need to run the engine for battery charging.

A digital thermostat

A digital thermostat prevents overheating and shows the current water temperature

Using the engine solely to heat water is wasteful and defeats the purpose of fitting solar panels.

What if, when the batteries are fully charged by renewables, any spare solar or wind energy could be used to heat water?

The first thing required is an element in the hot water tank that will heat water with the low voltage output of the solar panels.

If you already have a calorifier with a 240V element in it, you can do one of the following:

  • Replace the element with a 12V version (simply putting 12V across a 240V element will not work). This is a good solution if you never have access to shore power.
  • Fit a dual 12V and 240V twin immersion heater
  • Add a separate 12V element to the tank using an Essex flange.

Whichever option you decide upon, you must make sure that a thermostat is fitted to each element.

The clever bit of the system is the voltage sensing relay. This measures the battery voltage and switches the solar energy to the 12V immersion at a preset level.

Similarly, when the battery voltage decreases to a preset level, the solar output is then reassigned to battery charging/load.

A digital voltage sensing relay (DVSR) allows you to configure the switching voltages depending on your preferences and the type of battery you have, ie lead acid or lithium.

Due to the large currents involved, the switching of solar output from the solar controller to the 12V immersion must be carried out by a relay of sufficient size to handle the maximum current of the solar system.

Similarly, for safety, circuit breakers and/or fuses of the appropriate size must be fitted to both sides of the solar controller.

Continues below…

Ciel Bleu has 400W of solar panels coupled to a 4kW lithium battery bank. There is a 300W 12V element in the calorifier.

When the solar hot water is switched off, the default is solar battery charging.

When switched on, the DVSR and the digital thermostat are powered up.

In practice, on sunny days there is sufficient spare solar to heat the water to temperatures in excess of 50°C.

A thermostatic mixing valve (TMV) reduces the water temperature to a more human-friendly 38°C and allows the hot water to go further.

It’s hard to find a downside to this system as the original method of heating the water is retained.

Enjoyed reading How a boat solar hot water system works?

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