Immersion Heater – What is it, and how does it work?

James Elston
Written by James Elston
Updated on 4th December 2023
Posted on 30th November 2023
Topic: Boiler Guides
Immersion Heater – What is it, and how does it work

What is an immersion heater and are they any good for domestic heating?

water temperature

On average across the UK, over half the money spent on energy bills goes towards heating and hot water. With this in mind, it’s important to know all of your options when it comes to choosing your hot water system.

Need a new boiler – Get a fixed price in 20 seconds online now here

If you are considering whether or not an immersion heater would be right for you instead of a new replacement boiler, and are interested to learn more about what an immersion heater is and how it works, then read on.

Get an online fixed price in 20 seconds:

What kind of fuel does your boiler use?

Immersion heaters and boilers: Are they worth it?

Immersion heaters are still quite popular amongst many households and offer a great backup to boilers should they break down.

However, there are a lot of questions about immersion heaters and whether or not they are worth the cost. If you are using it to help an old boiler, why not see how much a new boiler costs instead as it may be cheaper in the long run?

In this article, we are going to cover all of these questions, from what an immersion heater actually is and how it works, to how to keep the costs down. We’ll also look at the main differences between boilers and immersion heaters.

f you’re thinking about an immersion heater because a new boiler installation costs too much, then why not check out boilers on finance where you can spread the cost of a boiler.

What is an immersion heater and how does it work?

Immersion heater

An immersion heater is a system that provides hot water to your home and is powered by electricity.

It uses a heating element that is immersed in a hot water cylinder that is filled with water, once it reaches the desired temperature set on a thermostat, it automatically clicks off and will remain hot for the following few hours.

Let’s dive further into exactly how an immersion heater works as well as the best practices to keep the costs down should you wish to use one.

if you are considering an immersion heater doe the price rise in gas, depending on how old your boiler is, it may be a better option to invest in a new hydrogen ready combi boiler to 20%. These energy-efficient A-rated boilers can save you up to £800 every year if you change from a G-rated boiler.

Not only will this save you money, the fact that these gas boilers can run on up to 20% hydrogen fuel and natural gas means that they will be able to survive the gas boiler ban in 2025 which you can read about here.

How immersion heaters work

An immersion heater works similarly to that of a kettle. Within the hot water cylinder, which is the main body that stores the water, is an electric resistance heater element. This element is “immersed” in the water, which is where it gets its name from. Some systems such as Megalfo boiler systems have immersion heaters built-in.

Immersion heater in hot water cylinders

The hot water cylinder is filled with water, however, it can take a while for it to heat up. Approximately, it takes about one to two hours to heat an average immersion heater from cold water, however, this varies if the water has already been heated.

clock time

Depending on the size of your hot water cylinder, you may have multiple electric elements that heat the water.

They do so by passing a powerful electric current through a coil-like element that heats up, which in turn then heats the water in contact with it.

Do immersion heaters automatically turn off?

Immersion heaters tend to use a thermostat to monitor the temperature. Once the water has reached the set temperature, then it can automatically switch off which helps to reduce costs.

This also means that you don’t need to remember to switch it off and you are not paying for hot water when you don’t need it.

Are immersion heaters plugged into the mains electricity?

In addition to this, immersion heaters are connected to the property’s main power supply, which means that they can be switched on and off on-demand. This can be ideal for those with second homes when they don’t need hot water all of the time.

New boilers such as combi boilers only produce hot water when needed too, so if you have an older boiler this may be a cheaper option.

Typically, an immersion heater will be used as the main source of hot water. However, it is also quite common for them to be used as a backup system for boilers. If the boiler breaks down then having an immersion heater will still enable you to access hot water.

Get an online fixed price in 20 seconds:

What kind of fuel does your boiler use?

Is an immersion heater expensive to run?

Immersion heater costs

Unlike gas boilers, immersion heaters use electricity to heat the water. Electricity is certainly more expensive than gas, oil, and solar.

According to the energy saving trust, the standard rate for electricity is 16.36 pence per kWh, whereas it is only 4.17 pence per kWh for gas and 4.81 pence per kWh for oil. This is almost four times the price and the reason why air source heat pumps use more electricity in winter makes them a less viable option than gas boilers.

If you have an old boiler and are thinking about using an immersion heater, then you should also consider the fact that a new gas boiler that is A-rated and over 90% energy-efficient can save you up to £800 on your energy bills every year.

How long does it take for immersion heaters to heat water?

In addition to this, it does take a few hours to heat the full tank of water, and it is important to heat the water to a minimum of 50 degrees celsius to ensure that the bacteria in the water have been killed off.

Also, any of the water that you don’t use whilst it is hot is wasted and will need to be heated up again for when you do want it.

insulated hot water cylinder

Nevertheless, whilst this may be the case, you can optimise how you use an immersion heater to keep costs down with a thermostat or forward planning.

Invest in a well-insulated hot water cylinder

The hot water cylinder is what stores the hot water for you to use. Immersion heaters are commonly used in system boiler hot water tanks or conventional boiler hot water tanks. Once the water has heated up to the required temperature, then a well-insulated hot water cylinder will remain hot for hours, even when it is turned off.

This is a great way to keep costs low, as you do not need to leave it on all day to reheat the unused water.

Immersion heaters with a thermostat

Immersion heaters can be set using a thermostat. The beauty of this is that it will automatically switch off when it reaches the desired temperature. This saves you both money and hassle and enables you to get on with your day without worrying about your immersion heater being left on.


Another benefit of using a thermostat is that you can set it to specific times. This allows you to make the most of off-peak prices.

According to the energy saving trust, across England, Scotland, and Wales the average off-peak rate for electricity is 9.76 pence kWh compared to 20.03 pence kWh on-peak.

So it can certainly make a difference to your bills. By using a thermostat along with a well-insulated hot water cylinder, then you can ensure your water remains hot for when you need it most.

Connect your immersion heater to a renewable energy source like solar panels

An environmentally friendly and cost-effective way to run your immersion heater is to link it to a renewable energy source such as solar panels. Of course, this has a pretty substantial cost upfront, however, there are government initiatives that incentivise this cost.

In January 2020, the government introduced the Smart Export Guarantee (SEG), which pays households for any excess electricity that their solar panels have generated and then distributes this across the national energy grid.

In addition to this, there was also the domestic Renewable Heat Incentive (RHI) which helped homeowners to access financial support to help cover upfront costs associated with renewable heating systems, however, this scheme ended on 31 March 2022.

Get an online fixed price in 20 seconds:

What kind of fuel does your boiler use?

Is it better to leave the immersion heater on?

The answer to this question depends on your hot water needs. If you need a steady supply of hot water throughout the day, then you can leave your immersion heater on. However, this is a costly and inefficient way to use your immersion heater and it might be worth considering a new combi boiler as a better alternative.

energy efficiency

There is no need to leave your immersion heater on all day. By using an automatic thermostat you can set the times that you want hot water. As we identified above, this can be a great way to optimise your costs and leverage off-peak rates.

In addition to this, a well-insulated immersion heater can remain hot for a few hours. So even when it is off, you will still be able to take a hot shower and wash the dishes.

However, this will work better for some than others. For some people having set times for showering fits perfectly into their routine, but this isn’t the case for everyone.

What is the difference between a boiler and an immersion heater?

Despite partly doing the same job, there are differences between the types of boilers and immersion heaters. Depending on your home, what you have access to, and your needs, you can identify the best way to generate hot water for your household.

worceter 4000 boiler


Unlike an immersion heater, a boiler can provide both hot water and central heating for the house. They usually run on either gas or oil and have pipes entering the boiler from underneath.

There are typically three pipes, one feeding through the gas, one that enables cold water to enter the boiler to be heated up, and another to allow the hot water to be pumped around the house.

Generally, gas and oil boilers work in the same way, with the only significant difference being that gas is supplied by a national network of pipes, whereas oil is supplied by a tank that is kept on the property.

Boilers are generally a lot more common than immersion heaters, and there are several different types of boilers.

Here we have briefly outlined the main different types:

  • Regular (or conventional/traditional/heat-only) – The original boiler type. A regular or conventional boiler can provide central heating directly to radiators throughout the house and along work with a cylinder to provide hot water.
  • System – Similar to a regular boiler, system boilers also provide central heating and, when paired with a cylinder, hot water. The main difference between the two is that a system boiler is connected up to the mains, whereas a regular boiler requires a cistern, often stored in the attic.
  • Combi (or combination) – Providing both hot water and central heating throughout your home, a combi boiler has no need for a cylinder and is much more compact, and therefore easier to store in smaller homes. However, since both the heating and hot water come from the same system, if it breaks down then you are left without either. If your boiler is not working, check out some of the best combi boilers here.
viessmann hot water tank

Immersion heater

Unlike a boiler, an immersion heater only supplies hot water to a home. Immersion heaters only run on electricity, however, this could be supplied through renewable energy sources such as solar panels.

As we identified above, immersion heaters are heated via an electric resistance heater that is immersed in the water, similar to that of a kettle but on a larger scale.

It’s not uncommon for some households to have both a boiler and an immersion heater. This means that if your boiler was to ever break down, you would still have a supply of hot running water.

Immersion heaters are always separate from boilers, which can be quite beneficial for those with old boiler systems such as back boilers that might be unreliable.

It is also not uncommon for flats and second homes to use immersion heaters if this is more appropriate for their needs.

Get an online fixed price in 20 seconds:

What kind of fuel does your boiler use?

Are immersion heaters gas or electric?

Immersion heaters are run on electricity and not gas or oil. With this being the case, it is not uncommon for new flats and even second homes to use immersion heaters for their hot water supply.

Depending on your circumstances, you may want to use an immersion heater. For example, if your home is a little more remote and you don’t have access to the main gas network, then an immersion heater is a great option.

Similarly, some new flats don’t have a gas supply and some households don’t have room to store their own oil, in which case they rely on an immersion heater for their hot water.

Can immersion heaters explode?

Whilst it is rare for an immersion heater to explode, it is still possible. Immersion heaters are equipped with various safety mechanisms to prevent this from happening, however, malfunctions are still possible.

If it does explode then it can certainly cause damage, so as with anything, preventative measures are always worth taking. If you check and your boiler is leaking we suggest that you get a suitably qualified plumber to fix the issue.

pressure release valve

There are some signs to be mindful of that could suggest that the temperature and pressure have risen too high. Fortunately, all immersion heaters and water heaters, in general, do have warning signs that can act as signals to call your emergency plumber.

Temperature and pressure relief valve

The temperature and pressure relief valve regulate both the temperature and pressure within the tank. It works by opening when the internal temperature or pressure is too high, this then relieves the pressure in the boiler if it’s too high and lowers the temperature.

If the relief valve is working properly, then it will do this automatically and will close when the problem is fixed.

However, if the relief valve remains open, this could be a sign that there is a consistent problem with the temperature or pressure.

Similarly, if the valve is leaking it could be a sign that your immersion heater cannot handle the internal pressure. A build-up of minerals, sediment, and rust can all contribute to this.

Sounds – popping, knocking, or clicking

Monitor your immersion heating system for peculiar sounds. If it is not maintained then there is potential for sediment build-up. The knocking sound occurs when water is trapped between the sediment and the heating element.

However, if your tank is maintained and flushed out, you should be able to avoid this build-up of sediment.

Brown water

Similarly to the peculiar sounds, if the hot water runs brown or discoloured it could imply that there is a build-up of sediment or that there is rust in the immersion heater tank. If the tank has serious erosion and rust it might mean that the immersion heater needs to be replaced.

gas safe plumber

If you detect any of the above warning signs, it is worthwhile reaching out to your plumber to err on the side of caution.

It is recommended to conduct your own general maintenance examination every six months and if there is anything that doesn’t seem quite right, it is worth contacting a plumber.

Who fixes immersion heaters?

Generally, a plumber can resolve the majority of issues with immersion heaters, especially those who are electrically qualified. On occasion you may need an electrician and plumber, however, the first point of call would usually be your plumber.

Depending on the problem and your location, the costs will completely vary. However, it is always worth using a fully qualified and trained Gas Safe registered heating engineer if you have a gas boiler. They will also be able to tell you whether or not an electrician would be required for the job.

How long do immersion heaters last?

How long do immersion heaters last

Immersion heaters are designed to last a long time. However, as with anything, they are susceptible to wear and tear. Naturally, the more you use your immersion heater the more wear it will endure.

For this reason, immersion heaters are ideal for second homes. Nevertheless, most households will use their immersion heater daily, so with this in mind, there are a few things that you can do to support longevity for your immersion heater.

  • Invest in annual maintenance – During these annual maintenance checks your immersion heater can be flushed and drained. This helps to remove and reduce the levels of sediment build up and rust within the immersion heater. This is also the perfect opportunity to have a professional plumber cast their eye over the full device, to make sure that it doesn’t have any other faults.
  • Observance – This might seem obvious, but it is certainly important to remain observant of your immersion heater. If there are any peculiar sounds or leaks, then call your plumber immediately. Trust your own judgement when something seems slightly off.

How to fit an immersion heater?

When fitting an immersion heater, you should think about the price of immersion heaters and if you’re replacing it the costs involved then. Here’s how to fit an immersion heater:

  • Check that everything is turned off, including the power.
  • Wrap your immersion heater around the water tank.
  • Remember to insert the fibre seal before thoroughly flushing the immersion heater into its slot. To ensure a tight seal, use a jointing paste.
  • Install your immersion heater. You have the option of connecting it to your ring main or a radial circuit. Follow the wire installation instructions in the product documentation and ensure that all electrical connections are tight.
  • Always follow the rules and regulations for electrical work in the UK, or engage a local electrician to do it for you.
  • Set the temperature you want

Immersion heater or new gas boiler?

So there you have it. When it comes to an immersion heater, it really does depend on your own household needs. For some flats, second homes, and those that live more remotely, immersion heaters can be a great option. However, they are a more costly way to supply hot water to your home on the grounds that they run on electricity.

Hopefully, you can use this article for guidance to weigh up whether or not an immersion heater would be the best solution for your home and your needs or whether working out the price of a new boiler is worth it in the long run.

If you’re looking for someone to install a new boiler, you can simply use our online boiler quote calculator tool and get a fixed price quote in under 20 seconds for an energy-efficient boiler perfect for your home.


What is a small immersion heater?

A small immersion heater is just a tinier electric heating element that is placed in the liquid being heated.

It is less expensive, more effective, smaller, and easier to transport to the area you need it most—and it heats you just as well—than a conventional space heater.

Is a new heater worth it?

Yes, but first consider your alternative heating options. Although new heaters are expensive, running expenses will be lower since they are more efficient than older models in a variety of ways. The effectiveness of an old heater may have been impacted by the heating core’s surrounding insulation degrading over time. But we advise looking at our new boilers and replacement service which you can find out more about here if you want a device that heats your house the most effective.

Get an online fixed price in 20 seconds:

What kind of fuel does your boiler use?
Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 30th November 2023
Topic: Boiler Guides

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

Buy a new boiler in minutes

Instant online quotes. Nationwide next-day fitting

Get my fixed price
Get a great price on a Which? Best Buy boiler online in 20 seconds
  • Over 15,000 boiler installed
  • Rated Excellent on Trustpilot
  • Fair, fixed and final prices
  • No pushy salespeople
  • Next-day installation
  • 0% deposit interest-free finance
Next-day fitting available until 2pmGet a quote now >