Home heating systems Immersion heaters

Immersion heater

If your boiler breaks down immersion heaters still generate hot water for your home

Homes that have immersion heaters will still get hot water even if their boiler breaks down. If you have an immersion heater, or are thinking about getting one, then you need to read on. 

Here, we tell you what you need to know about immersion heaters - including who needs them, the pros of cons of having an immersion heater and why it could be costing you more than necessary to heat your water. Plus find out how you could use an immersion heater to get free hot water.

What is an immersion heater?

An immersion heater is an electric water heater that sits inside a hot water cylinder. They act a bit like a kettle, using an electric resistance heater (which looks like a metal loop or coil) to heat the surrounding water. 

Immersion heaters are connected to their own power supply via a cable. They can be easily switched on and off, as there's no need to constantly heat the water in your hot water cylinder. Immersion heaters can either be used as a property's primary water heater, or can be used as a backup water heater for combi boilers.

Wondering whether there is a better option than an immersion heater? Check out our guide to home heating systems.

Who needs an immersion heater?

Most households will use a combined central heating/hot water boiler to heat their home and provide hot water. However, some households may choose to have an immersion heater too, as a backup. 

Other households, particularly those with older heating systems, use immersion heaters as their only source of hot water. Most immersion heaters are found in rented housing.

Save money by using your immersion heater at night

Economy 7 customers should use their immersion heaters at night

Immersion heater - pros

  • If your immersion heater has a thermostatic control, it will automatically turn off when it reaches the temperature you set on the thermostat.
  • Immersion heaters are not connected to your boiler. So if your boiler breaks down, you can still generate hot water for your home.
  • If your immersion heater has a well-insulated jacket, it can keep water hot for several hours after it switches off.
  • Time-of-use tariff customers can set timers so their immersion heater switches on during cheaper off-peak hours.
  • You can usually turn your immersion heater on or off by simply flicking the switch on the wall socket.

Immersion heater - cons

  • Heating water using electricity is more expensive than heating water with gas.
  • A typical immersion heater uses 3 kilowatts of electricity an hour, so it will cost the average house about 50p an hour to run.
  • Most households will need to run an immersion heater for at least a couple of hours a day to get the water hot enough - costing at least £360 a year.
  • An immersion heater needs a thermostatic control, otherwise it can heat the water to far too high a temperature.
  • Some heating engineers may recommend you leave your immersion heater on 24/7 - however, this can be hugely expensive unless it has a thermostatic control.
  • Be aware that you need to heat the water in your immersion heater to above 50°C to kill off bacteria.

Cheaper alternatives to an immersion heater


It's generally considered to be a lot cheaper to heat water using gas rather than electricity. For this reason, installing a gas boiler may well be cheaper than using an immersion heater every day.  If your boiler is quite old and inefficient, you might want to consider upgrading to a more efficient combi boiler - this will also provide your hot water and will save you money in the long run.

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Which? has conducted in-depth research on boilers so we can recommend the most reliable models that won't let you down. See our reviews of the best boilers.

It also pays to make sure you are on the best energy deal. Compare energy prices with Which? Switch. 

Renewable energy

If you don't have gas in your property and have a mainly south-facing roof, you could consider solar panels to heat your water. Solar heating systems currently qualify for financial help from the government under the Renewable Heating Incentive scheme - this gives you money in return for using renewable energy to heat your home. To find out more, see our Renewable Heat Incentive guide.

Finally, if you have an immersion heater as well as solar photovoltaic panels or a wind turbine, you can divert any electricity you've generated that isn't being used in your home to your immersion heater. This means you can heat your hot water free of charge. 

You will need to buy a device that does this, such as Immersun or Solar iBoost, which cost about £200 to £400 plus the cost of installation. Talk to an installation company to find out more.

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