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.

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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This feature appeared in the September 2020 edition of Practical Boat Owner. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a magazine subscription to Britain’s best-selling boating magazine.

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