What Is an Immersion Heater? A Comprehensive Guide

How Does an Immersion Heater Work

An immersion heater uses an electric heating element that is submerged in a hot water cylinder filled with a fluid such as water to heat it up. The heater is typically made of a metal sheath with a heating coil inside it and is designed to be submerged in the liquid.

These heaters are often used in homes and businesses to heat water for various purposes, such as for domestic hot water, industrial processes, or to maintain a temperature in storage tanks or aquariums. They are also used in laboratories for heating chemicals and solutions.

How Does an Immersion Heater Work?

An immersion heater works by converting electrical energy into heat energy using a heating element, which is typically made of metal and has a resistance to the flow of electrical current. When electricity is passed through the heating element, it generates heat, which is then transferred to the surrounding water.

When the immersion heater is switched on, an electrical current flow through the heating element, which heats up the metal and the surrounding fluid. The heat is transferred from the metal to the fluid through a process called conduction. The fluid then absorbs the heat, and its temperature rises.

The temperature of the fluid can be controlled using a thermostat, which is typically integrated into the immersion heater. The thermostat is set to a particular temperature, and when the fluid reaches that temperature, the thermostat switches off the heating element, preventing the fluid from overheating.

What Immersion Heater Made Of?

The heating element of an immersion heater is encased in a protective sheath made of a material such as copper, stainless steel, titanium, or Incoloy. The sheath protects the heating element from the fluid and prevents it from directly contacting the liquid.

What Are the Types of Immersion Heaters?

Immersion heaters come in various types to tackle a wide range of heating needs and requirements for different water reservoirs. The following are those types.

Screw Plug or Threaded Immersion Heaters

Screw Plug or Threaded Immersion Heaters

A screw plug or threaded immersion heater is designed to be inserted into a tank or vessel through a threaded opening. The heater consists of a heating element encased in a protective sheath, with a threaded fitting on one end that allows it to be screwed into the tank or vessel.

These heaters can be customised to meet the specific needs of different applications, with thermostats and other controls to regulate the temperature. They can be designed with different lengths, wattages, and voltages to match the tank or vessel heating requirements.

Over-the-side Immersion Heaters

An over-the-side immersion heater is designed specifically to be installed on the side of a tank or through the top of the vessel without the need for a threaded opening. The main body of the heater hangs over the side of the tank, with the heating element submerged in the fluid.

Overall, these are a perfect option to heat up the water in the vessels where you can’t insert submersible heaters. For your information, the rest of the features are similar to the threaded heater; these can be customised in various lengths, wattages, and voltages to match the tank or vessel heating requirements.

Flanged Immersion Heaters

A flanged immersion heater is installed into a tank or vessel through a flange opening, unlike the threaded one that uses a thread and over-the-side heaters inserted by the side. The heater consists of a heating element encased in a protective sheath, with a flanged fitting on one end that allows it to be bolted or welded onto the tank or vessel.

Furthermore, the process of installing and maintaining these heaters is uncomplicated. They are formed into a hairpin shape and then welded onto a flange. During installation, the heater is affixed to the corresponding flange that has been welded to the side of the tank, which establishes a dependable and secure connection.

Pros and Cons of Immersion Heater

Pros and Cons of Immersion Heater


  • You Always Get Hot Water – The boiler may break down anytime during the winter. In such a case, if you have installed an immersion heater, you can get hot water.
  • They Are Compatible with Solar Panels – You can combine the immersion heater with solar panels to utilise renewable energy while taking the cost to almost zero.
  • Keep Water Hot for Several Hours – The immersion heater can be combined with insulated tanks to keep the water hot for hours without inconvenience.
  • Have Automatically Turning Off Feature – If you pair the immersion heater with insulated tanks, you get another feature; these heaters can turn on/off automatically to keep the water always at an optimal temperature while being efficient.
  • Ideal for Homes Not Connected to Gas – Immersion heaters use electricity for heating purposes. If there is no gas in your home or area, these are ideal options for you.


  • Not Ideal for Heating a Small Amount of Water – Although immersion heaters use electricity to heat water, they cannot heat small amounts of water.
  • Known for Wasting Electrical Energy – Water quickly loses heat when the immersion heater is turned off, wasting a lot of energy; you must use the water quickly to save it.
  • Take a Reasonable Amount of Time for Heating – The immersion heaters typically take 2-3 hours to heat up the water. This is a slow process, making them time-consuming.
  • Bad for Energy Efficiency if Used Without a thermostat – As I mentioned earlier, an immersion heater can save energy. But it’s only possible with a thermostat.

Running Cost of An Immersion Heater

When determining the cost of an immersion heater, many factors are there that must be considered in the first spot. As mentioned earlier, immersion heaters are available in various types and materials, and each combination will supervise the final cost. The following is the cost table for immersion heaters.

Immersion Heater Material Operation CostInstallation Cost
Copper £25 – £120£105 – £120
Incoloy  £17 – £30£106 – £130
Titanium  £25 – £120£115 – £220

Note: When installing the immersion heater, you may have to bear a few additional charges as well. Those may include the labour cost or the cost of repairing the radiator or thermostat when installing the heater. Ensure to take all these factors into account when determining the cost.

Should I Go for the Immersion Heater?

Based on our discussion up to this point, it appears that whether or not you should go for an immersion heater depends on your individual circumstances. An immersion heater can be a useful backup option if you have an old or regular boiler. Additionally, if you’re not connected to a gas network, an immersion heater can provide hot water.

It is worth mentioning that excessive reliance on an immersion heater can lead to high expenses since it necessitates a substantial amount of energy to operate at its maximum capacity. In the long-term, investing in a more efficient combi boiler is advised. In short, you should consider your specific needs and circumstances before deciding whether or not to invest in an immersion heater.