Immersion Heaters: Benefits, Costs & Free Hot Water

Our complete guide to immersion heaters including problems, benefits, costs and how they can be used to provide your home with free hot water.


If you’re replacing your old boiler with a new Combi, you might be thinking about taking the opportunity to get rid of a hot water tank and immersion heater. It’s a great way to free up space in your home, but have you considered the potential benefits of hanging on to it?

What is an Immersion Heater?


An immersion heater uses electricity to heat water as a kitchen kettle does. Older central heating systems often rely on a hot water tank with an immersion heater inside it which works to either store the hot water produced by a boiler or as the primary way of heating water.

How Does an Immersion Heater Work?

1. The immersion heater has a metal element which sits within the tank where it is immersed in water (hence the name).
2. The immersion heater is connected to the mains electric via a cable and has its own on/off switch. When the heater is turned on, electricity heats the metal element and this heat passes to the surrounding water.
3. Depending on how much water the tank holds and how powerful the immersion heater is, this heating process could take 1-2 hours to reach the desired temperature*. Note: You need to heat water to at least 50°C to kill any potential bacteria.

Immersion Heater Problems

One of the main issues with immersion heaters (and why many homes are now opting for a Combi boiler replacement) is that you have to heat a whole tank of water when you may only need a small amount. This not only means having to wait for hot water or planning ahead, but also uses unnecessary electricity. When you turn the immersion heater off the water will cool down again.

To combat this unfortunately many households end up leaving their immersion heater on all the time so hot water is always on hand. There is a myth that the immersion heater uses more energy to heat up from cold than it does to maintain the heat. It’s always advisable to insulate the hot water tank to keep the water hot for as long as possible, but fitting a thermostat and timer alongside the immersion can help to keep energy use under control.

It is true, however, that electricity is an expensive way to heat water.

Immersion Heater Costs

The average cost of electricity is about 13.33p per kWh** in the UK. This means it will cost around 40p to power a 3kW immersion heater for an hour.

Based on this figure we’ve estimated some running costs to give you an idea of what an immersion heater could cost to run.

Size of Immersion Heater Approximate Running Cost Per Hour Approximate Cost Per Week (Running 2 hours a day) Approximate Cost Per Month (Running 2 hours a day over 28 days) Approximate Cost Per Year (Running 2 hours a day over 52 weeks)
3kW 40p £5.60 £22.40 £291.20
6kW 80p £11.20 £44.80 £582.40

Combi Boiler Benefits

  • Combi boilers produce hot water on demand and so don’t need to store any water. This means you could get rid of your hot water tank to free up space.
  • You will always have a fresh supply of instant hot water whenever it’s needed.
  • As the water is fresh there’s no risk of the bacteria associated with stored water.
  • You’ll only heat as much water as you use so won’t be wasting unnecessary energy.
  • The price of gas is far cheaper than electricity at just 3.63p per kWh.
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    So it’s clear that in most cases a gas Combi boiler is the better choice, so why would a household with a Combi boiler choose to keep their immersion heater?

    Benefits of Keeping an Immersion Heater

    Backup Hot Water

    The main benefit of keeping an immersion heater is as a backup supply of hot water. Immersion heaters have their own electricity supply and don’t rely on boilers to work. This means that if your boiler breaks down you will still have access to hot water.

    Easy to Manage Energy Use

    Your immersion heater can be turned on and off at the flick of a switch so you can keep its energy use under control when it’s not needed. An immersion heater with thermostatic control will automatically turn itself off when the water reaches your desired temperature and turn it back on if the temperature cools too much.

    Saving Money with an Immersion Heater

    If you install a immersion heater timer you can set it to automatically heat up at specific times of the day or night. If your energy tariff is cheaper at night this would be the best time to have it working. The average Economy 7 tariff for electricity is 8p per kWh which means to run a 3kW immersion heater for an hour will cost around 24p. Potential savings (based on a 3kW immersion heater) are illustrated in the table below.

    Average Price of Electricity (per kWh) Approximate Hourly Running Cost Approximate Weekly Running Cost (2 hours per day) Approximate Monthly Running Cost (2 hours a day over 28 days) Approximate Annual Running Cost (2 hours a day over 52 weeks)
    Standard Tariff (Day) 13.33p 40p £5.60 £22.40 £291.20
    Economy 7 Tariff(Night) 8p 24p £3.36 £13.44 £174.72
    Potential Saving 5.33p 16p £2.24 £8.93 £116.478

    Immersion Heater + Solar = Free Hot Water!

    Homeowners who have or are considering solar panels have the biggest opportunity to benefit from an immersion heater. Solar water heating systems (also known as solar thermal) convert the sun’s energy into usable electricity for your home; this free electricity can be used to power your immersion heater which means free hot water! And, if you choose to install a solar water heating system, you could be entitled to the government’s Renewable Heat Incentive (RHI) scheme. This means the government would pay you for the energy your solar panels generate. To find out more about solar water heating for your home and to get free installation quotes visit Solar Guide.

    Buying or Replacing an Immersion Heater

    The cost of buying an immersion heater will vary depending on the type and size of the heater. Copper immersion heaters are the cheapest at under £20 but are not suitable for hard water, stainless steel tanks, unvented cylinders or thermal store units. Incoloy or titanium are recommended in hard water areas and are usually a little more expensive, but still under the £30 mark. This doesn’t include the cost of the hot water tank or the installation which will vary from installer to installer.

    The price of the immersion heater will also increase in line with the size. You can work out what size of immersion heater you need if you know how many litres of water your tank holds and the temperature rise*** that is required. When you have these figures there is a simple calculation you can do to work out how much power is needed to heat your tank of water in 1 hour: Volume of tank x 4 x temperature rise / 3412 = Power (kW) required

    For example, if the cold water in your 120 litre tank is 15°C and you would like it heated to 60°C, the calculation would be: 120 x 4 x 45 / 3412 = 6.3kW

    So you would need at least a 6kW immersion heater to heat a full tank of water in 1 hour or a 3kW immersion heater to heat the same amount in 2 hours.

    Replacing an immersion heater can take a qualified engineer up to half a day and involves either partially or fully draining the tank of water so they can access the immersion heater. The job involves both electricity and plumbing so we highly recommend that you get quotes from a professional engineer in your area.

    Immersion Heaters: To Keep or Not to Keep?

    It seems immersion heaters still have a place in the modern home as both a backup for your Combi boiler and as a potential source of free hot water with solar technology. You can keep its energy supply switched off when it’s not needed so you won’t be wasting energy by having it. So if you can live comfortably with your hot water tank it may well be worth holding on to it.

    However, if you’re currently using an immersion heater as your only source of hot water you should be considering a Combi boiler replacement. An all-in-one gas Combi boiler is not only cheaper to run but also delivers the convenience of a fresh supply of hot water on demand.


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    *If the water entering the tank has already been heated by a boiler the immersion heater will work to maintain the desired temperature.
    **According to figures from the Energy Saving Trust reviewed in March 2018.
    ***The temperature rise is the difference between the temperature of your cold water and the temperature it needs to reach (measured in centigrade).