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


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.

Enjoyed reading this?

A subscription to Yachting Monthly magazine costs around 40% less than the cover price.

Print and digital editions are available through Magazines Direct – where you can also find the latest deals.

YM is packed with information to help you get the most from your time on the water.

      • Take your seamanship to the next level with tips, advice and skills from our experts
      • Impartial in-depth reviews of the latest yachts and equipment
      • Cruising guides to help you reach those dream destinations

Follow us on Facebook, Twitter and Instagram.