Home  >  Advice Centre   >   How Does an Immersion Heater Work, Cost, Benefits, and Disadvantages?

How Does an Immersion Heater Work, Cost, Benefits, and Disadvantages?


Immersion heaters, also known as megaflow boilers or unvented hot water systems, aren’t new to the domestic heating industry. They have been around long before combi boilers took centre stage in the heating industry. In fact, at one time, they were the most popular hot water heating solution in the UK.

But what exactly is an immersion heater, and how does it work? Well, lucky for you, you have come to the right article that will teach you all you need to know about immersion heaters. This guide will cover everything, including how an immersion heater works, its advantages, disadvantages, as well as its associated costs.

What is an Immersion Heater?

The immersion heater is essentially a metal coil immersed into a hot water cylinder. It operates like a giant kettle and uses electricity to heat the coil. The loop or coil then heats the hot water stored in your cylinder to the desired temperature. It is, however, possible to power the immersion heater using solar energy provided you have solar panels installed. In either case, you can switch the immersion heater on and off whenever you want.

Immersion heaters are mainly found in homes that aren’t connected to the gas mains. These heaters either serve as the primary hot water source or a backup one in the oil or gas boilers are old and unreliable.

How does an Immersion Heater Work?

As earlier said, the immersion heater is a metal coil or loop inserted into the hot water cylinder in your home. The heating element is usually made of a material with high electric resistance and is connected to the mains electricity using a cable. Once the heater is switched on, electricity flows through the heating element and is met by the high electrical resistance of the heating element.

The resistance to the flow of electricity results in the metal element heating up. And since the heating element is inserted in the cold water, it passes the heat to the surrounding water. The hot water in the storage cylinder usually rises to the top while the cold sink to the bottom during the heating process. The process continues until the water in the storage tank reaches the desired temperature, which usually takes 1-3 hours.

It is vital to ensure that the water in the storage cylinder must be heated to at least 50°C to make it safe for usage. The reason is that a temperature of 50 degrees Celsius is enough to kill any potential bacteria in the stored water.

Once you open a hot water outlet, cold water flows into the bottom of the hot water cylinder creating a pressure that forces the hot water into your home’s hot water system. The hot water then makes its way into your showers, bath, and hot water faucets.

What happens when the water in the storage cylinder cools down?

Once the water in your storage cylinder has reached the desired temperature, the immersion heater automatically switches off like a regular electrical kettle. However, the stored hot water will begin to lose its heat at a rate directly proportional to the cylinder’s level of insulation. If the heater’s main switch is on, it will automatically reheat the water once it falls below the desired temperature.

You can, however, opt to switch off the main switch, especially if you won’t be requiring heating. Switching off your immersion heater helps you minimize electricity usage and cut down on your annual heating costs.

Types of Immersion Heaters

There are three types of immersion heaters: over-the-side, flanged, and screw plug. But how do they differ?

  • Over-the-side immersion heaters: These are usually installed through the top of the tank. The heating element either rests on the side or the bottom of the cylinder. Over-the-side immersion heaters come in different shapes and sizes but are easy to remove and clean.
  • Flanged immersion heaters: These heaters consist of tubular elements welded into a flange on the side of the hot water tank or nozzle.
  • Screw-plug immersion heaters: They consist of a heating element screwed directly to the hot water storage tank wall.

Benefits of an Immersion Heater

Immersion heaters weren’t a popular heating solution for no reason. They rose to fame thanks to the many advantages outlined below.

Reliable hot water supply backup

One main benefit of installing an immersion heater is that you can always rely on it should your central heating boiler malfunction. And since these heaters only need electricity to operate, it can be a great backup option regardless of the type of boiler you currently have.

Excellent hot water solution where there is no gas supply

Many UK households are not connected to the gas mains, meaning they have to turn to alternative heating solutions for their central heating and hot water needs. And since the immersion heater requires electricity, it becomes an excellent hot water alternative for flats and homes located in remote areas.

Easy to control your energy use

Since you have total control of your home’s immersion heater, you can turn it on and off whenever you please. This way, you get to monitor the amount of energy being used to heat the water in your hot water system.

To better control your immersion heater, consider installing an immersion heater thermostat. When fitted with a thermostat, the immersion heater will automatically switch off once the hot water reaches the desired temperature. It will also turn itself on when the hot water temperature falls below a certain level.

You can also install a timer, especially if you live in a Zone 7 district.  The timer allows you to set the times of the day that you want the heater to come on. As such, if you are using cheaper electricity tariffs at night, you can set your boiler to come on at this time as it would be cheaper to heat the water in your storage cylinder.

Low upfront costs

Immersion heaters are perhaps the cheapest hot water heating solution to install, provided you have a hot water storage cylinder on your property. All you need is around £30 to purchase an immersion heater, although you will have to pay separate installation charges. Moreover, the installation process is fast and less complicated than a gas boiler.

Free hot water when paired with solar

An immersion heater gives you free access to hot water, provided solar panels are installed on your property. This is because you can use the energy generated by your solar panels to power your immersion heater. Additionally, you stand to benefit significantly from the government’s Renewable Heat Incentive (RHI) scheme if you have a solar water heating system. In other words, you get to enjoy free hot water and get paid by the government for the solar energy your panels generate.

Disadvantages of an Immersion Heater

Despite its reliability, cost-effectiveness, and environmental friendliness, the immersion heater’s shortcomings saw many people abandon it for modern heating solutions. Some of the drawbacks to immersion heaters include:

Lengthy waiting process for hot water

The main disadvantage of an immersion heater is that you can’t use it to heat a small amount of water. Each time you need hot water, you have to wait until all the water in the storage tank has been heated to the desired temperature. This is a time-consuming process and one that will unnecessarily waste electricity.

Moreover, it is highly inefficient in that once it is turned off, the stored hot water will start to lose heat, and you will have to reheat it in a few hours. This simply means that the electricity you used to heat the water in the first place would go to waste.

Expensive to run without a thermostat

There is no doubt that electricity is the most expensive method of heating hot water. This means that if you leave your immersion heater running at all times, you will end up paying a massive electricity bill. You should, therefore, invest in thermostats, timers, and insulating jackets if you want to keep your annual electricity bill low.

Requires pre-planning

Each time you need hot water in your home, you have to heat an entire tank of water, a process that can take anywhere between one and three hours. To avoid wasting electricity by heating the water over and over again, you need to come up with a heating schedule that takes into account your home’s hot water needs. You will have to align activities such as laundry and bathing to times when the water in the storage cylinder will be hot.

Immersion Heater Costs

If you consider acquiring an immersion heater to act as a primary hot water source or a backup one, you are probably wondering how much it will cost you to buy and run it.  Usually, the initial purchasing cost of an immersion heater varies depending on the size of the heater and the type of heating element.

There are two versions of heating elements used in immersion heaters. The first one is copper, which usually costs between £20 and £30. It is important to note that copper immersion heaters, although cheap, aren’t suitable for use in hard water areas, stainless steel tanks, and unvented hot water cylinders.

The second type of immersion heater heating element is Incoloy or titanium, which costs between £30 and £40. Although more expensive than copper, Incoloy or titanium heating elements are durable and work well with hard water.

But, how much it will cost you to operate an immersion heater?. Well, given that the current cost of electricity in the country is 13.33p per kWh, it will cost you around 40p to run a 3kW immersion heater for an hour. This means that if you operate the heater for two hours daily, you will be spending £5.60 weekly and £291.20 yearly to run the immersion heater.

What are the Common Immersion Heater Problems?

After seeing the advantages and disadvantages of immersion heaters, you might be contemplating going all out and acquiring one for your hot water needs. Well, before you make that decision, first familiarize yourself with the common immersion heater problems you are bound to experience along the way.

Damaged immersion component

As the heating component of the immersion heater is in contact with water, it starts to rust as the heater approaches the end of its lifespan. Rust damages the immersion component, and you will find your immersion heater running into constant problems such as turning itself on and off and taking ages to provide hot water.

The only remedy for a damaged immersion component is to replace it with a new one. And since this is a tricky electrical task, you are better off leaving it to a professional technician.

Tripped circuit breaker

Like any other electrical appliance, the immersion heater sometimes experiences circuit breaker trips due to overloading. When this happens, your immersion heater will not be able to operate or provide hot water to your home.

To fix the issue of a tripped circuit breaker, check whether the immersion heater’s circuit breaker is on. If it is on, but tripped, switch it off, wait a few minutes, then switch it back. This is usually enough to resolve the tripping issue. Still, if the circuit breaker trips again, you might be dealing with a more severe problem, and you need to contact a certified technician.

Limescale buildup

Excessive limescale buildup is a common issue in regions with hard water. Limescale accumulates around the heater’s heating element and results in your immersion heater breaking down faster than expected. Some common problems associated with limescale accumulation are overheating, reduced energy efficiency and expensive maintenance costs.

To avoid limescale buildup, consider introducing a domestic water softener or a magnetic filter in your immersion heater to protect the heating component. If, however, your heater has already accumulated limescale, consider taking it apart and flushing out the scale using a high-pressure water stream.

Insulation issues

An insulation breakdown is perhaps the worst immersion heater problem you can experience. It usually disrupts the electricity running through the heating element, causing a system overload. And the overloaded system can trip your main circuit breaker, damaged wires, and blow fuses.

Insulation problems aren’t something you can usually handle on your own. As such, it is recommended that you call a certified professional whenever you suspect you have an insulation issue. They will then analyze your heating system and inform you of any parts that need maintenance or replacement before your immersion heater can be up and running again.

How do I Calculate What Size Immersion Heater I Need?

Immersion heaters come in various sizes, and it might seem not very easy to determine the correct size heater for your property. Size is an essential factor when choosing an immersion heater as it is tied to the price. Moreover, getting the wrong size immersion heater may mean that your hot water won’t be heated to the desired temperatures.

However, it would be best if you didn’t panic as there is a simple formula for calculating the amount of power required to heat the cold water in your storage cylinder within an hour. All you need is the volume of your storage cylinder and the required temperature increase.

Once you have the mentioned data, apply the following formula to determine the size immersion heater, one that will satisfactorily fulfil your hot water needs.

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

To put this into perspective, let’s assume you have a 100-litre storage cylinder that holds water at 20°C, and you need to heat this water to 60°C. In this case, the required temperature increase would be 40°C, and the calculation will be: 100 x 4 x 40 / 3412 = 4.7kW.

This means a 5kW immersion heater will be able to heat the water in your storage tank to the desired temperature within a single hour. A 2.5kW immersion heater, on the other hand, will be able to achieve the same feat in two hours.

Should I buy an Immersion Heater?

Absolutely yes. Despite their many flaws, immersion heaters remain an excellent heating choice, especially if you aren’t connected to the gas mains. By installing an immersion heater, you get to enjoy an efficient yet environmentally friendly heating solution at an affordable cost. Moreover, if you connect it with solar panels, you will benefit from the free hot water supply.

There is, however, one catch. As immersion heaters are susceptible to problems such as limescale accumulation, you have to foot an expensive maintenance bill.  That said, an immersion heater, though not as efficient as a combi boiler, is still an excellent primary or backup hot water source, provided you maintain it properly.

  • Buy Now Pay Later Schemes!
    • Interest Free Credit
    • Buy Now Pay Later
    • Spread Payments
    Finance Options
  • Free Boiler Quotes

    No-obligation to buy!

    • From the comfort of your home
    • You choose the best company
    • Gas Safe trusted companies
    Get Quote NOW
Need a New Boiler

Get FREE quotes from tradesmen in your area today. What are you looking for?