A Buyer’s Complete Guide to Immersion Heaters

Oct-2020 / 6 min read

Reading Time: 6 minutes

When you think about a central heating system’s different elements, an immersion heater might not even make it on the list. The reality is that most homes in the UK depend on their boiler for hot water. Whether they have a hot water cylinder or not, boilers always do the trick – until they don’t.

When the boiler breaks, it can be a real headache to fix. UK residents who need a steady source of hot water, or those who have unreliable boilers, look to immersion heaters. If this is your first time purchasing or using an immersion heater, Plumbingforce outlines everything you need to know. These are the things we will discuss in this blog post: 

  1. What Is An Immersion Heater?
  2. How Does An Immersion Heater Work?
  3. Different Types of Immersion Heaters
    • Over-The-Side
    • Flanged
    • Screw Plug
  4. How Do You Install An Immersion Heater?
  5. How To Replace Your Immersion Heater
  6. How Do You Use An Immersion Heater?
    • How Do I Turn My Immersion Heater On?
    • How Long Does It Take To Heat Water?
    • Ideal Temperature For Your Immersion Heating
  7. Benefits of Having Immersion Heating

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What Is An Immersion Heater? 

An immersion heater takes the same principles of how an electric kettle heats water – just on a different scale. Instead of boiling enough water for a cuppa or two, it heats the hot water cylinder and provides you with hot water that can last for a couple of hours. 

Besides using it for day to day needs, there are times where it really shines—for example, having the immersion heater during the cold season. With out-of-town guests and increased hot water usage during the holidays, your boiler might not be able to heat the water fast enough. Using an immersion heater can be a great way to have a steady source of hot water.

How Does An Immersion Heater Work?

An immersion heater has an attachment that gets submerged in the hot water cylinder. A strong current runs through the metal coils, and it heats the surrounding water. Unlike your radiators that depend on your boiler, your immersion heater is hooked up separately. It has its own power supply and can be switched on and off independently without affecting your central heating. 

An immersion heater is relatively flexible. You can use it with a vented or unvented cylinder. If you want to learn more about unvented cylinders, also known as megaflo, you can read more about it in our dedicated blog post: Everything You Need To Know About A Megaflo Cylinder.

Different Types of Immersion Heaters

No matter what type of immersion heater you get, the principals are the same. It is just a matter of finding the most suitable one for the job. There are 3 different types of immersion heaters available:


Over-the-side immersion heaters are meant to be installed from the top, but the heating happens along the side or bottom of the tank. It is shaped like a wand or a whisk extension of your mixer. This model is mostly used for industrial applications, and it isn’t commonly found in residential homes. 


Unlike the over-the-side immersion heaters which have the metal coils at the end, the flanged immersion heater has metal coils all the way, from top to bottom. The tubes are attached to a flange and are shaped like a circle. This model is something you are more likely to find in your house. 

Screw Plug

A screw plug immersion heater might not look too different from a flanged one. Both models are both great for circular tanks, and the metal coil runs from top to bottom. The main difference is how they are installed. The top part of a screw plug is well, a screw. You can see the thread up near the top of the heater. Again, this is something you are likely to find in your house.

Besides the different types, you can also pick between an on-peak and off-peak immersion heater. They generally look and work the same way, but the main difference is your energy bills. On-peak immersion heaters run during the day when the electricity is expensive. 

In contrast, the off-peak immersion heaters run during the night. The low energy demands mean cheaper electricity and lower energy bills. An off-peak heater is an “out of sight, out of mind” option whose only con is that you would have to wait before your next top-up. It can be an excellent choice for people who aren’t at home all day or use hot water sparingly. 

How Do You Install An Immersion Heater?

If you are looking to install an immersion heater for your new hot water tank, it is possible to do it on your own. However, with your plumber and heating engineers fixing up the other parts of the house, why don’t you let them take over this one? 

The hot water tank has a very thin copper wall to help with the heating. Being too rough with the installation can damage this wall and introduce wear and tear to your brand new water tank. Not so ideal, isn’t it?

Plumbingforce can get a heating engineer and plumber on site within the same day as your call to help you install your electric immersion heater. 

Here are the steps you need to take to install a new immersion heater:

  1. Make sure that everything is disconnected, especially electricity.
  2. Wind up your immersion heater on top of the water tank.
  3. Before you fully flush the immersion heater in its slot, remember to add the fibre seal. Use jointing paste to ensure a tight seal. 
  4. Wire your immersion heater. You can opt to connect it to your ring main or hook it up to a radial circuit. Follow the wire placements on the product manual and make sure all the electrical connections are tight. 
  5. Set your desired temperature. 

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How To Replace Your Immersion Heater

The steps for installing and replacing your immersion heater is very similar. However, the key difference is working with a new vs used immersion water tank. A new installation is very straightforward since the tank is empty and it isn’t fully connected. However, replacing means working around your existing system. There are more added steps. 

Here is how you can replace your immersion heater.

  1. Make sure your immersion heater is not plugged. Remove all electrical connections. 
  2. Turn off the cold water supply. You can check the diagram of your hot water cylinder so you can identify the cold water feed. Follow the pipe and turn off the valve. If you have a faulty valve, you may need to disconnect the mains and drain your system.
  3. Open all the hot water taps, or use it generously, so the immersion tank is not full. It can take 10-15 mins, depending on how many taps you have open.
    1. Make sure it doesn’t drip. If there is a bit of water, you may have a faulty valve – again disconnect it at the mains.
  4. Connect the drain point with a hose and drain the tank. 
  5. Unwind your immersion heater gently. Again, you do not want to damage the copper walls.
  6. Insert the new immersion heater to the tank and seal it. Make sure it is tight, but not so much that you go past the thread on the screw plug. 
  7. Refill your tank, remember to close off all drain points, which also includes your taps. 
  8. Wire your new immersion heater, and make sure it is set at your desired temperature. 

There is a lot more back and forth between your hot water tank storage and the rest of your house. If you want to do away with the hassle, Plumbingforce has local plumbers who can help you get it done in no-time. 

How Do You Use An Immersion Heater?

With your immersion heating in place, it’s time to put it to work. Here are common questions people have about their immersion heaters.

How Do I Turn My Immersion Heater On?

Your immersion heater is usually installed with a thermostat, timer, and switch. Switch your immersion heater on 2 hours before you need to use your hot water, and turn it off later. The switch doesn’t have to be in the loft or basement with your hot water tank. It can be wired near your other central heating controls in the main living area. 

Now, you don’t always have to turn your immersion heater on manually. If you have an off-peak immersion heater, which is the more eco-friendly option, you can keep it running. An off-peak immersion heater will use cheaper electricity overnight to heat your water. When you have an on-peak immersion heater, you have to be conscious about when you need your hot water, and how much you have. 

How Long Does It Take To Heat Water?

The answer varies from the brand, model, and age of your immersion heater to your holding tank’s size. A safe estimate would be 2 hours to heat water for your whole tank. 

Ideal Temperature For Your Immersion Heating

When it comes to setting the temperature for your immersion heating, it is up to personal preference. If we were to give a number, 55°C is a good place to start. It is between the ideal temperature between hard water and soft water. 

However, if you want to follow the recommendation for your water type, hard water areas should have the temperature set at 50°C while soft water areas can go up to 60°C. Still, you can try different temperature settings on your heater until you find the one you like best. 

Benefits of Having Immersion Heating

Using an immersion heater means you can have hot water readily available. It is a great addition for homes with old and unreliable boilers. In between your many boiler repairs, you don’t have to say good-bye to the comforts of your home. It can also be really helpful in homes that have fuel-burning central heating systems. When you are running on empty, you can still have hot water. 

Immersion heaters are also getting more traction with the Green Homes grants. The UK government is pushing for it more so that more homes can have reliable heating and hot water without increasing the carbon footprint. Like other eco appliances in the market, an immersion heater can run off solar so if you live in the sunnier-side of the UK, your immersion heater and work with your solar panels.

Besides being suitable with sustainable energy sources, it can be energy-efficient as well. Heating water with an off-peak immersion heater can give you the same convenience without any additional work or hassle. 

Plumbingforce can help you set this up. All you have to do is call us at 0330 173 7900 or book us online! Besides installing immersion heaters, we also take care of blockages, boiler repairs, boiler maintenance, and emergency call-outs for all your plumbing

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