How does a calorifier work? its a question I regularly get asked. The answer is below. As you read this article you’ll realise that you could get a tank of hot water on a regular basis just by running your boat’s engine or lighting your stove.

How does a calorifier work? Simply put, a calorifier is an indirectly heated vessel where hot water is stored. The heat comes from either an internal electrical heating coil or from an external source through independent heating coils fitted inside the calorifier body. They are very common in recreational vehicles and in many homes too. Many of my articles are based around subjects that relate to boats. However much of what exists in boats can also be found in recreational vehicles of all types. So this information should be relevant to people other than just boaters.

Calorifiers are usually manufactured from copper or stainless steel. Stainless vessels are stronger than copper vessels and less prone to accidental puncturing. Most calorifiers are offered with an electric immersion heater but should also be fitted with a pressure relief valve, also known as a PRV. The PRV is extremely important. When water is heated, it expands. If water is heated in a vessel without a PRV, the tank or connecting pipes could burst and cause serious scalding to anyone in the vicinity.

 As well as the immersion heater and PRV, a quality calorifier will be offered with a choice of one or two internal heat exchanger coils. My own calorifier, which incidentally is an insulated copper cylinder, has a one-kilowatt immersion heater and two heat exchanger coils. This is very important to us. Of the two heat exchanger coils, one is connected to the engine and the other to our solid fuel fired stove. How does a calorifier work? Extremely well – brilliant piece of kit!

The benefit of the above setup is that in winter we get free hot water from the stove, but also, whenever we go anywhere with the engine running, we get free hot water from the engine. In warmer weather where we would not be travelling or using the fire, we can use the immersion heater.

Our calorifier has a 55-litre capacity and we have found that setting the immersion temperature to 40C., is sufficient for all our needs. Here is a little trick for you. If you can keep a kettle on the top of your stovenwhen its lit, that water will heat to well over 40c. You can use that hotter water to increase the temperature of your washing up water -for free!

Our boat has a solid fuel stove that runs central heating and the calorifier. in the central heating circuit we have a pipe that goes to the calorifier via a zone valve. The valve is connected to a thermostat that is set to 42c. When the stove has heated the central heating water to 42 degrees celcius the zone valve opens and overcomes the electrical thermostat on the immersion heater. When that happens it completely bypasses the electrical heating and we’re on free hot water!

A word of advice before you buy a new calorifier. Calorifiers are generally supplied with a PRV, that is a pressure relief valve. Check the working pressure of your water pump. Just like water pumps, pressure relief valves come in different pressure ratings. If your pump pressure is 40 pounds per square inch (2.76 bars) you should choose a 3 bar (about 45psi) PRV for your calorifier. If your pump pressure is less, say 25 PSI, you could use a 2 bar PRV (about 30psi). If you have any doubts you can speak to the supplier.

If your PRV pressure rating is lower than your pump pressure you will simply lose water. So do get it right. When you’ve got everything fitted and commisioned you’ll never have to ask ‘how does a calorifier work’ because you’ll see the benefits of having one every day.

If you do not have space for a large calorifier you could buy a smaller one and adjust the immersion heater to a higher temperature. I would not normally recommend more than 60C on safety grounds.

Here is a link to an article I wrote about Safe Drinking Water Onboard on this same site.

Also, here for the curious is a link to Marine Heat Exchanges, which is exactly what a calorifier is but domesticated!