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Home & garden.

Updated: 8 Sep 2021

Immersion heaters explained

We explain what immersion heaters are, who they're suitable for, how they work and their pros and cons.
Sarah Ingrams
Immersion heater 451072

Immersion heaters - sometimes known as megaflow boilers or unvented hot water systems - heat water using electricity, but can also be connected to solar panels. If you have an immersion heater, or are thinking about getting one, read on for what you need to know. 

Here, we explain who needs immersion heaters, the pros and 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.

Want to cut your energy bills? Use our independent switching site, Which? Switch, to compare energy prices

What is an immersion heater?

An immersion heater is an electric water heater that sits inside a hot-water cylinder. It acts 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 as a backup water heater for combi boilers.

Want to know which boilers are the most reliable? We reveal the best boilers.

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 in flats with no gas supply, use immersion heaters as their only source of hot water. 

Pros of immersion heaters

  • 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 aren't connected to your boiler. So if your boiler breaks down, you can still generate hot water for your home.
  • If your immersion heater is well insulated, it can keep water hot for several hours after it switches off.
  • Time-of-use tariff customers (such as Economy 7) can set timers so their immersion heater switches on during cheaper off-peak hours.
  • They are easy to operate: you can usually turn your immersion heater on or off by simply flicking the switch on the wall socket.

Cons of immersion heaters

  • Heating water using electricity is more expensive than heating water with gas.
  • A typical immersion heater uses three 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.
  • 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.
  • You need to heat the water in your immersion heater to above 50°C to kill off bacteria.

Cheaper immersion heater alternatives


It's generally considered to be a lot cheaper to heat water using gas rather than electricity. For this reason, and if you're able to, 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.

Which? conducts in-depth research on boilers to reveal the most and least reliable brands. Check our reviews of the best boilers to make sure you buy a brand that won't leave you in the cold.

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 water heating systems currently qualify for financial help from the government under the Renewable Heating Incentive (RHI) scheme - this gives you money in return for using renewable energy to heat your home. To find out more, see our guide to the Renewable Heat Incentive.

Finally, if you have an immersion heater as well as solar 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.

Want to cut your energy bills? Use our independent switching site, Which? Switch, to compare energy prices