Immersion heaters- the right way

25th Oct 2016


The energy intensive world in which we live means that energy saving advice is part and parcel of daily conversations. At the Hot Water Association we hear a lot of suggestions ranging from the sensible to the absurd. My particular favourite is that it takes more electricity to turn lights off and on again than to leave them on.

This is completely untrue, and is probably resulting in lots of people wasting money on lighting. It doesn’t take any more electricity for a light bulb to turn itself back on. If it’s on, it’s using electricity, and if it’s off, it isn’t.

True or false there are a myriad of energy saving tips out there, and consumers are interested in them, yet it is surprising how many times we hear of consumers unwittingly wasting energy and costing themselves money, by using their immersion heater incorrectly.

The number of homes installers go into that have the immersion heater on 24/7 would surprise you. This is essentially like running an electric heater constantly – and it is normally the result of the occupiers not knowing whether their immersion heater is on or not.

This is where you, the engineer come in. The responsibility to ensure that the heating and hot water products you fit achieve what they are designed to achieve, in the most efficient and economical way rests with the entire supply chain, manufacturers, merchants and installers. Choosing products from suppliers who can clearly demonstrate compliance with the relevant standards and specifications is one way you can play your part.

Educating homeowners on how to correctly use heating and hot water products to achieve the greatest energy efficiency is another. 8 million home visits are made by installers every year, each one is an opportunity to ensure that your customers are fully informed.

There is no official way of using an immersion heater, generally if there is a boiler in the property then it’s more economical to use the boiler to heat the hot water cylinder. Household immersion heaters have a 3kW output whilst most boiler systems are capable of providing around 12kW to the cylinder so the cylinder will heat up much quicker as compared to using an immersion heater.

Gas is around 6 pence kw/hr, oil around 4 pence and electricity around 14 pence (standard rate) so it is very clear that electricity from the mains is more expensive than gas.

We’ve come up with some simple rules to ensure that immersions are being used correctly:

 Turn it off

An immersion heater is often used to accompany the primary source of heat - the boiler. Explain to the consumer that in this type of installation it is best to keep the immersion heater turned off unless absolutely needed. The immersion heater should be used as a backup only and not as the primary way of heating the cylinder. It should remain switched off at its own isolating switch unless needed.

Improved Control

Where the property uses electricity as it’s primary energy to heat water this would generally mean that a ‘direct’ cylinder is installed which uses 2 electric immersion heaters (primary and boost) to produce all the premises’ hot water requirements. Water heated in this way can be produced efficiently if the homeowner uses some simple control methods in order to minimise the use of electricity.


Often properties that use electric water heating can be fed by an off-peak tariff such as economy 7. Modern cylinders are highly efficient at retaining heat in water so we would always recommend that consumers use timers so their immersion heater switches on during cheaper off-peak hours- generally through the night.


The immersion heater thermostat will switch off when water in the hot water tank reaches the temperature. Setting the temperature at a lower level (above 55 °C to meet legionella requirements) will save money.

Take care

 While a high proportion of homeowners would recognise the need to have their boiler serviced annually, very few know that to keep their hot water cylinder and immersion heater running properly and efficiently, simple maintenance procedures need to be performed. Awareness will be higher amongst installers; however without communicating that knowledge to the homeowner the maintenance involved will most likely be overlooked. When installing a hot water cylinder, installers should take the time at handover to talk to consumers about the need for annual servicing.


There is also a very eco- friendly way of using an immersion heater and that is when a solar PV system is installed. Consumers can divert the excess electricity- via an additional product- to power the immersion heater and store the energy in the form of hot water in a cylinder, which can then be used to meet the hot water demands of the house later in the day.

In these instances the cylinder can be heated free of charge from the solar PV system. As of the end of May 2016 there were a total of 870,600 Feed-in Tariff commissioned installations (Monthly feed-in tariff commissioned installations by month report-DECC).

By fully explaining how to correctly use and maintain the hot water system, consumers will be more likely to appreciate its relevance and act accordingly. What’s more, installers can position themselves as trusted advisors, building a longer-lasting relationship and most likely securing future work later down the line or a referral to family and friends.

 As ever, HWA members will be pleased to offer advice on best practice. Member details can be found at