Immersion heaters – what are they and how do they work? Re March 25, 2021

Immersion heaters – what are they and how do they work?

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What is an immersion heater, and how does it work?

When choosing the correct heating system for your home, for example, a standard boiler, renewable energy, or an electric immersion heater, there are a few factors that should be taken into consideration before the final decision is made.  

You should consider the ease of use, the type of property you have, and the cost of running these different types of heaters.  

What is the difference? Well, as standard boilers use gas or oil to create the heat that powers your hot water and central heating, renewable energy such as solar panels rely on the sun’s light for its energy, and an immersion heater runs on electricity. But deciding which one is best for you is the important part.

This blog aims to help you decide whether or not an immersion heater is the best source of heating for your home. 

What are immersion heaters and what do they do?

If you’ve never seen an immersion heater before, you’re probably going to be asking yourself, ‘what is an immersion heater and what’s its purpose?’  

In simple terms, immersion heaters are devices that provide hot water for your home, powered by electricity. However, an immersion heater could be your knight in shining armour if your central heating fails, as it can still provide your home with hot water. Hot water immersion heaters can also be used as your main source of hot water or as a back-up for your traditional gas boiler, as it is separate from your central heating boiler and radiators.  

Those who still have older gas boilers or oil boilers in their property often choose to install an electric immersion heater as a safety blanket, so that they always have a supply of hot water. You will also find that immersion heater tanks are the primary source of hot water in newly built homes that are not supplied by the main gas network or homes that are off the grid as a result of their remote locations.  

Immersion heaters are located within a large hot water cylinder, also known as an electric resistance heater that heats the surrounding water. They are connected to the main power supply and can be switched on and off on-demand, meaning you do not have to have it switched on and heating the water constantly. 

How do immersion heaters work?

When turned on, electricity passes through the metallic element, heating the surrounding water. This is where they get their nickname ‘the giant kettle’. The immersion heater sits inside a hot water cylinder and is powered by a strong electric current which works to heat the water until it reaches optimum temperature. It is important to note that to kill any potential pathogen (any nasty bacteria inside the tank) the immersion heater needs to heat the water to at least 50 degrees so that the water is safe to use.  

Your immersion heater will automatically turn back on and get back to work if the temperature of the water reaches below a certain level, however, for the times that you’re not at home, or during peak electricity hours where the water does not have to be kept warm, you can turn this feature off to save you some extra coin.  

The heating process can take anywhere from 1.5 to 3 hours, depending on the size of the cylinder being heated and the power of the immersion heater.  

‘But how does an immersion heater actually work?’, I hear you ask. Well, the storage cylinder is filled with cold water which enters through the bottom of the tank. The cold water then rises to the top as it is heated. As more cold water enters the bottom of the tank and the pressure in the tank builds up, the required pressure is reached for the hot water to leave the immersion water heater and find its way to your baths, showers, and sinks.  

In addition to using electricity from the mains supply to activate the immersion heater, it is also possible to connect them to solar panels and other renewable energy sources, which can help to reduce the energy costs.

When and how to use immersion heaters.

If you feel that your home requires a constant and steady supply of hot water, you may choose to keep your immersion heater on at all times. However, this can be quite costly and inefficient as it uses a lot of electricity to keep the water hot. 

What is the most effective way of using an immersion heater? Well, there are a few ways of doing this to ensure that those energy bills don’t start creeping up.  

You could set your immersion heater thermostat to a lower temperature so not so much electricity has to be used for the water to reach optimum temperature. However, it is important to remember that it needs to reach a minimum temperature of 50 degrees to remove any bacteria. Setting the thermostat will mean that the immersion heater will stop working once it reaches the right temperature and only power up when it drops below a certain level of heat.  

Another way of doing this is by using an immersion heater timer to only allow your immersion heater to turn on during off-peak hours. The only issue with this is that you will need to pre-plan when you’re going to need hot water for your home, otherwise, you may have to wait a while for the tank to heat up before being able to take a shower, have a bath or even do the washing up.  

This leads us on to the next section of the blog, the pros and cons of using immersion heating.

Pros and Cons of using an immersion heater.

Although immersion heaters can be a great way of providing homes with a reliable supply of hot water, there are some downfalls that you may want to consider before installing one in your home.  

Pre-planning when to turn your immersion heater on. 
With immersion heaters, unfortunately, you can’t simply heat a small amount of water for perhaps, let’s say, one load of washing up. You will need to wait for a full tank of water to be heated. Not only is this inefficient because once the immersion heater is off, the water will lose its heat and you would have wasted electricity for nothing, but you could also be waiting up to three hours to use the hot water.  

For this reason, combi boilers are often the preferred choice of boilers as they are much more efficient and convenient for homes that have a demand for instant hot water and central heating.  

If you’re interested in learning more about combi boilers, we have another blog to cover this. Click here to read about how combi boilers work.

For a combi quote in 90 seconds, click here.

They are not always cost-effective. 
Unless you have an immersion heater thermostat installed, you will have less control over the temperature your immersion heater needs to reach. 

To avoid the issue of wasting electricity, many immersion heater owners leave them switched on. However, this is one of the most expensive ways of heating water.  

Now let’s get rid of that negativity. Aside from these factors to consider, immersion heaters have some great advantages that could benefit you and your home.  

Now, you’re probably sitting there wondering if there are any benefits at all to having immersion heating in your home. Well, don’t worry, there are. 

They’ve got your back.
Immersion heaters do not require a gas supply to function, which means that if for any reason your central heating boiler fails, you can rely on your immersion heater back to help supply your home with hot water. Great for that extra peace of mind when it gets absolutely baltic outside! 

No gas supply? No drama.
Immersion heaters are a great hot water supply for homes that are off-grid and have no gas supply, for example, flats or homes in remote locations.  

Free hot water with solar panels!
Have you ever considered solar panels? Or have you already got solar panels and are thinking about buying an immersion heater?

Well, if so, you could benefit massively. Solar water heating systems (solar thermals) convert the sun’s energy into usable energy. Yes, you’re right, that 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. 

Easy to manage your energy usage. 
You have full control of your immersion heater, meaning that you can turn it on and off when you please so that you can limit the amount of energy being used. If you have an immersion heater thermostat installed, it will automatically turn itself off when the water reaches the desired temperature. Once the water cools below a certain temperature, it will then turn itself back on and get back to work.  

You can also install an immersion heater timer, which you can set to automatically turn on at certain times of the day which could save you money, For example, if your energy tariff is cheaper throughout the night, this would be the most cost-effective time for it to be on.  

Now we know a little more about what immersion heaters are and how they work, let’s talk about costs.  

How much do immersion heaters cost?

The cost will depend on the size and the type of immersion heater you choose, although they are relatively inexpensive especially in comparison to other types of heaters. However, it is important to note that you will need to purchase and accommodate extra room in your home for an additional hot water storage cylinder. In addition to this, you will need to hire a Gas Safe Engineer to carry out the installation.  

Two types of immersion heaters are more commonly used. These are the:  

Copper immersion heater

These usually range between £20 and £30. 
It is important to note that this immersion heater is not appropriate for hard water, stainless steel tanks, unvented cylinder,s or thermal store units.

Titanium immersion heater

These usually range between £30- £40.
These are used mainly for areas that suffer from hard water.

The costs of having immersion heating

It is difficult to say exactly how much it is going to cost to run an immersion heater, as they use electricity to heat water, which is considerably more expensive than gas. On top of this, you’re adding the cost of heating the whole hot water cylinder and often not using all the water, resulting in wasted energy and wasted money.  

For a 3kW immersion heater, you are looking at a running cost of around 40p per hour, meaning that you will be spending close to £300 on heating your water annually, based on heating the cylinder for only 2 hours a day. However, it is important to take into consideration that if your home has a greater demand for hot water, then you could be spending significantly more. The average cost of electricity is about 13.33p per kWh, in the UK. This means it will cost you around 40p to power a 3kW immersion heater for an hour. See the table below for a breakdown of the cost for both a 3kW and a 6kW immersion heater.

Size of Immersion Heater Approximate Running Cost Per HourApproximate Cost Per Week (2 hours a day)Approximate Cost Per Month (2 hours a day for 28 days)Approximate Cost Per Year (2 hours a day over 52 weeks)

However, as mentioned, installing an immersion heater timer and set it to specific times of day where energy use is lower, for example, during the night, can help save your money on your energy bills. 

For example, the average Economy 7 tariff for electricity is 8p per kWh, which means it will cost 24p to run a 3kW immersion heater for an hour. See the table below for more information on this (based on a 3kW immersion heater).  

 Average Price of Electricity (per kWh)Approximate Running Cost Per HourApproximate Cost Per Week (2 hours a day)Approximate Cost Per Month (2 hours a day for 28 days)Approximate Cost Per Year (2 hours a day over 52 weeks)
Standard Tariff (Day)13.33p40p£5.60£22.40£291.20
Economy 7 Tariff (Night)8p24p£3.36£13.44£174.72
Potential Saving5.33p16p£2.24£8.93£116.47

Replacing your immersion heater can also be costly. It can take a Gas Safe Engineer up to half a day and in some cases a day to complete as it involves either partially or fully draining the current water tank so that they can access the immersion heater, so bear this in mind.

How do I work out what size immersion heater I need?

Now, I bet you didn’t know that there was more than one option for size when it comes to an immersion heater, well there is and it’s important that you get it right.  

The size of the immersion heater can also have an impact on the price. It is also important to know that buying an incorrect size heater will not serve its purpose and your water may not ever reach the desired temperature.  

Don’t panic, it’s not too hard to figure out what size you need. Start by finding out how many litres of water the cylinder stores and your required temperature increase.  

Once you have that information, you can determine how much power is required to heat your water in 1 hour using the following equation:  

Volume of tank x 4 x temperature increase / 3412 = Power (kW) required 

For example:  

If you have a 100-litre storage cylinder and want to increase the temperature of the water from 10°C to 60°C, the calculation would be 100 x 4 x 50 / 3412 = 5.8kW 

Therefore, to get your water heated to the desired temperature you would require a 5.8kW immersion heater. 

Alternatives to an immersion heater

We know that’s a lot to take in, so we’ve decided to scale it back a bit. 

If you’ve got to this stage in the blog and figure that an immersion heater may not be the right choice for you, then we’ve come up with an alternative.  

Combi boilers are ideal for homes with a high demand for hot water, and they only heat the water that you use, meaning less energy is wasted. Gas is also much cheaper may of heating water than electricity, making a combi boiler much more cost-effective.  

Combi boilers are supplied with cold water directly from the mains. They have the central heating and water heater built into one unit meaning you don’t have to wait for the tank to heat up and it saves you space in your home. In addition to this, combi boilers use fresh water, meaning there are no risks of nasty bacteria.  

Like what you read? We have an article on the cheapest combi boilers on the market that might help you find the perfect combi boiler for you.

Is an immersion heater right for me?

Immersion heaters have a lot of beneficial features such as supplying hot water in times of need, compatibility with solar panels to help you save money on your energy bills, ideal for homes with no gas supply. However, ultimately, they are expensive to run and you could get the same, if not better results with a combi or system boiler.