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How Does an Immersion Heater Work and Cost?

Immersion Heater KettleImmersion heaters also referred to as megaflow boilers or unvented hot water systems are essentially like giant kettles – using electricity and a metal element they heat the water within a large storage cylinder.

So, who exactly are immersion heaters suitable for? Should you install one? And what are the alternatives? 

In this guide we will answer some of the most common questions and concerns about immersion heaters, so you can make the right decision on whether they are the best option for your property. 

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First up, immersion heaters – what actually are they? 

What Is An Immersion Heater

An immersion heater uses a large hot water storage cylinder and is immersed within the water within the tank, hence the name. This is an electric resistance heater that heats the surrounding water. 

Immersion heaters 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. 

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How do immersion heaters work?

Immersion Heater Element

When turned on, electricity passes through the coil, heating the metallic element, which in turn heats the surrounding water. For this reason, they are often compared to the everyday kitchen kettle. 


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Once turned on, the heating process can take anywhere from 1.5 to 3 hours depending on the size of the cylinder and the power of the immersion heater. Ideally, the water needs to be heated to at least 50°C to kill any potential pathogens. 

Common Immersion Heater Problems and Drawbacks

Immersion Heater Problems

In times gone by, immersion heaters were one of the staple methods of providing homes with a reliable supply of hot water on demand. But, they do have their drawbacks. Plenty of drawbacks.

You may be waiting hours for hot water

However, the major issue with them is that you cannot simply heat a small amount of water to use immediately, instead, you must wait for a full tank of water to be heated. This is also extremely inefficient since once turned off, the water will lose the heat and all the electricity you just used will have been for nothing. 

Immersion heaters, over time, have largely been replaced by more efficient "combi boiler" systems

For this reason, today they have largely been replaced with the much more efficient and convenient combination or combi boilers. These are similar in that they provide properties with hot water, but also deliver hot water to the radiators, helping to keep homes warm too. 

They can be expensive without thermostatic control 

Immersion Thermostat

Unfortunately, many immersion heater owners try to negate this problem by keeping it constantly switched on, so they have a constant supply of hot water. This results in a huge waste of electricity, which also happens to be one of the most expensive ways to heat water. 

It is possible to improve efficiency by installing an insulating jacket, as well as a thermostatic control and timer. Yet still, compared to other methods of heating water such as gas combi boilers, they are expensive to run. 


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Immersion heater problems super snappy summary: 

  • Heating water with electricity is far more expensive than gas 
  • Need to wait at least a few hours for the water to heat up – inconvenient and expensive 
  • Must be heated to at least 50°C to kill any potential pathogens (evil things)
  • Requires thermostatic control, otherwise, it is extremely expensive 

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Benefits of immersion heaters - yep, it's not all doom and gloom

Immersion Heater Positives

With all the negative aspects of immersion heaters being inefficient and expensive to operate, you are probably wondering if there are any benefits to having one. 

Here are some of the major ones: 

Backup hot water - they've got your back

One of the main advantages of immersion heaters is that they do not require a gas supply to function. Since they have their own electricity supply cable, they can be used whenever your electricity supply is available. 

If your central heating boiler breaks down, in an emergency you can still get a supply of hot water by using your immersion heater. 

No gas supply? No problemo for the immersion

Immersion heaters are a viable hot water supply option for properties that have no gas supply, such as flats and other properties in remote locations. 

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In control of energy use

Many people like the idea of being in total control of their heating and having the ability to switch it on and off when they want. 

Ideally, a thermostat and control system should be installed in conjunction with an immersion heater, this will ensure the heater switches off when it reaches the desired temperature and switches back on when it becomes too cool. 

Learn all about combi boilers in our what is a combi boiler guide. 

Saving money with an immersion heater

Immersion Heater Jacket

You can easily set your thermostat timer to reap the rewards of certain energy tariffs. For example, customers on the Economy 7 tariff can set their timer to only switch the heater on during off-peak hours. 

You can also insulate your immersion heater; in which case your water will stay hot for several hours after it has been turned off. Cosy, cosy.

A super snappy summary of the immersion heater's benefits

  • With a thermostat installed, immersion heaters can be efficient by turning off once the desired temperature has been reached 
  • Timer controls allow you turn the immersion heater on and fit it around your routine and energy tariffs 
  • Provides backup hot water if your boiler fails
  • Insulated well, hot water can stay warm for hours after operation 
  • Easy on/off switch provides full control over energy consumption 

Are immersion heaters expensive to run? 

Immersion heaters use electricity to heat water, which is considerably more expensive than gas. 

For example, 1 kW of gas will cost you around 4p, while 1 kW of electricity will cost around 15p. That means a 6 kW immersion heater would cost 90p an hour to operate, substantially more than operating a gas combi boiler.

Additionally, you will typically need to wait several hours for the water within the storage tank to reach the desired temperature, which again means it costs a considerable amount of money over the year. 

For instance, say you operated your 6 kW immersion heater for just 2 hours a day, this would cost around £1.80 a day or £12.60 a week. 

Even worse, some homes opt to keep their immersion heaters on full time to attempt to avoid having to wait, racking up the costs even further.

As mentioned previously, thermostatic control and insulation can both help to improve efficiency. Making use of your immersion heater during off-peak hours can also cost you less, for example, those on the Energy 7 Tariff (Nighttime) can expect to pay just 8p per kW, as opposed to 15p during the day, which is almost half the cost. 

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Immersion heater costs

Heating Immersion Cylinder OptionsThe cost of purchasing the immersion heater is dependant on the size and type of immersion heater you choose, but they are still relatively inexpensive, especially when compared to other types of heaters. 

Although, it is worth remembering that you will need to purchase the hot water storage cylinder separately and you will need to hire an engineer to carry out the boiler installation

The two major types of immersion cylinders, along with costs and applications are as follows: 

  • Copper immersion heaters
    These are between £20 - £30, but are not appropriate for hard water, stainless steel tanks, unvented cylinders, or thermal store units.

  • Titanium immersion heaters
    These cost between £30 - £40 and are used for areas which suffer from hard water. 

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Immersion heater size calculator 

The size of the immersion heater also impacts the price, but it is also important to know since an incorrect heater will not serve its purpose and your water may not ever reach the desired temperature. 


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

Simply find out how many litres of water the cylinder stores and your required temperature increase. 

Once you have the following numbers to hand, 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. 

Should you keep your immersion heater on all the time? 

Many people claim that it is cheaper to keep your hot water heated than to let it cool and reheat it, but this is a myth. Additionally, many people like to keep their heater on all day as they want a constant supply of hot water available, yet this will inevitably cost you more. 

...people claim that it is cheaper to keep your hot water heated than to let it cool and reheat it, but this is a myth

The best option is to use an immersion heater with thermostatic controls and timers. This will turn the heater off once it reaches the required temperature and turn back on should it become too cool. 

You can also time the immersion heater to come on during times when your energy tariff will be cheaper, such as the evenings. 

Ensuring your immersion heater is adequately insulated will also help the water to retain its heat for as long as possible.

Is an immersion heater right for you? 

Ultimately an immersion heater is expensive to run due to the fact it uses electricity as its fuel source, which is considerably more expensive than natural gas. 

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For this reason, if possible, a better alternative is to have a boiler replacement instead. Not only will this be cheaper to run, but it will also provide your home with a constant supply of hot water and keep your radiators heated. 

Yet, if you want a backup option in the event that your gas boiler or gas supply fails, then an immersion heater may be a wise option.  

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