What is an immersion heater?

Immersion heaters are a common part of properties up and down the country, as they can service both small apartments and large properties, but what are they? 

Here, we explore immersion heaters in more detail, looking at how they work, their pros and cons, and how they match up against gas boilers. 

But first, let’s take a look at how immersion heaters work… 

How do immersion heaters work?

Immersion heaters heat water via a metal element inside a hot water cylinder. Essentially, it's like a giant kettle, using an electric resistance heater (a metal loop or coil) to heat the surrounding water.

Immersion heaters are usually connected to the electricity via a switch, so you can switch them on and off as needed. They can also be fitted to timers so you can control when they come on and off automatically.

Which homes use immersion heaters?

Immersion heaters are used in a wide range of homes, from small flats to those with multiple bathrooms. In smaller properties that don’t have a gas supply, immersion heaters can be used to heat water stored in a hot water cylinder via the mains electric. 

In larger homes and properties with a greater demand for hot water, an immersion heater will ensure you have a backup supply of warm water if your boiler breaks down.  

Because they use electricity to heat water, immersion heaters are also used in homes connected to heating systems like heat pumps. If your home relies on a non-traditional heating system like this, you could explore using an immersion heater to meet your hot water needs.

Are immersion heaters energy efficient?

Immersion heaters run solely on electricity, so they do not use gas. This means that no exhaust fumes are created during the heating process, making immersion heaters extremely energy-efficient compared to standard boilers.

You can also fit timers and thermostatic controls to your immersion heater. These help you reduce the energy wasted heating hot water that you may not need and give you greater control over how hot the immersion heater gets. 

You can also fit your hot water cylinder that's heated by an immersion heater with a ‘jacket’ that insulates the appliance and improves heat retention. 

If you’re thinking of swapping your current combi boiler for a heating system including an immersion heater, however, there are some things you need to consider: 

• Combi boilers produce hot water on demand, so there’s no need to waste electricity storing hot water you might not need
• As the hot water from a combi boiler is fresh, there’s no risk of bacteria building up
• The price of gas is cheaper than electricity 

How much does an immersion heater cost to run?

According to research from Which?, most immersion heaters use 3kW of electricity per hour. This means that, based on the January 2024 energy price cap (£0.29 per hour), an immersion heater costs £0.87 for every hour of use.

Take an example case of a 3kW, 180-litre immersion heater that runs through a full heating cycle twice daily, once in the morning and once in the evening, so that your household can shower and do the washing up. An immersion heater of this capacity typically takes around 3 hours to come up to temperature, so it would need to be running for a total of 6 hours each day.

6 x £0.87 = £5.22 ✝

It’s important to note that while this may represent a saving compared to heating your home all day, using your immersion heater in this way means you may not be able to access water as and when you need it, which can be impractical. 

The actual price you can expect to pay when using your immersion heater will vary depending on:

• The size of the tank
• The amount of water you use
• The temperature the water is heated to
• How well insulated your hot water tank is
• When it’s programmed to turn on (off-peak electricity tariffs may mean running your immersion heater at night is cheaper)
• Whether you have temperature sensors installed to automatically turn the heater off once it reaches a set temperature (leaving it running at all times will, of course, be more expensive)

If your immersion heater is connected to a home solar panel system, you can heat water without relying on energy from the National Grid, which will make it more cost-effective. Of course, the process of installing solar panels has costs of its own, so you’ll need to factor in more than just the cost savings per unit of energy to work out whether such a hybrid heating system is right for you.

✝ This price is an estimate based on the current energy price cap and does not include daily standing charges or additional fees from your energy supplier.

How to maximise the efficiency of your immersion heater

To ensure you have hot water when you need it without overspending on energy bills, you should:

1. Insulate the hot water tank to improve its heat retention - You can do this using foam tubing around any pipes and an immersion heater blanket around the cylinder itself
2. Use a thermostatic control - These will automatically turn the heat off once the intended temperature is reached
3. Use an immersion heater timer - Turn the heater on and off when you need it, or if you're on an economy 7 energy tariff, you can set the immersion heater to turn on during off-peak hours
4. Reduce the temperature - Your immersion heater might be providing water that’s hotter than you actually need, meaning energy is being wasted. Provided it’s still reaching a temperature of at least 50ºC to kill off any bacteria, try reducing the shut-off threshold of your heater by 1 or 2 degrees.

What are the pros and cons of immersion heaters?

Whether you're thinking of investing in an immersion heater or just researching alternatives to traditional central heating systems, it’s useful to know the pros and cons of these appliances.

Pros of immersion heaters

• Immersion heaters are standalone heat sources, so if your central heating boiler breaks down, you'll still have access to hot water
• If your hot water cylinder is insulated, the water will keep warm while the heater is turned off
• If you're on an economy 7 energy tariff, you can use a timer to set the heater to come on when rates are cheaper
• You can use your immersion heater with renewable energy sources, like solar panels
• Easy to use, typically immersion heaters only feature an on-and-off switch and straightforward thermostat controls

Cons of immersion heaters

• Immersion heaters only use electricity, which is more expensive than gas per unit
• The immersion heater must heat water to a minimum temperature of 50°C to kill any bacteria present in the water
• Unless your heater has a thermostatic control and timer, it may be expensive to run
• Immersion heaters can’t supply radiators
• They do not provide hot water on demand like a combi boiler
• Immersion heaters cannot heat small amounts of water, making the process of accessing hot water time-consuming
• Even with insulation, the water in the heater can quickly cool down

Immersion heaters vs gas boilers: which should you choose?

While immersion heaters can be helpful, particularly as a substitute if your central heating boiler breaks down, they are not as effective as gas boilers. As they work separately from boilers, an immersion heater will not be able to heat radiators independently, which is why people use them as part of a combined central heating system.

For more information on the different types of boilers, check out our guide: Which type of boiler is right for your home?