Glossary · Central heating, radiators and hot water
What is an immersion heater?
An immersion heater is an electric heating element fitted into a hot water cylinder. It heats the stored water directly from the mains electricity supply, separately from the boiler. Many homes use it as a back-up if the boiler fails, or to heat water on a cheaper overnight electricity tariff.
Key points
- It is an element in the cylinder that heats stored water with electricity, so it works whether or not the boiler is on.
- Energy Saving Trust says electricity is around four times more expensive than gas per unit, so cheap-rate tariffs matter.
- Gledhill's manual for one unvented cylinder lists 3 kW immersion heaters with thermal cut-outs, and says heaters without cut-outs must not be fitted.
- Energy Saving Trust says an old cylinder jacket should be at least 80mm thick, because a poorly insulated cylinder leaks heat.
- GOV.UK says all electrical work in a dwelling must be carried out in line with Part P of the Building Regulations.
How an immersion heater works
Ideal describes an immersion heater as essentially a kettle element sticking into the cylinder. It means the water is warmed with electricity instead of gas.[2]Ideal Heating, Should You Turn Your Boiler Off at Night? The boiler, by contrast, heats cylinder water by circulating hot water around it.[2]Ideal Heating, Should You Turn Your Boiler Off at Night? Because the two are separate, an immersion heater has nothing to do with your central heating.[2]Ideal Heating, Should You Turn Your Boiler Off at Night?
A thermostat tells the element when to switch on and off. Energy Saving Trust says a heating system with a hot water cylinder should have a cylinder thermostat.[6]Energy Saving Trust, Heating controls Gledhill’s manual for one unvented cylinder says the heaters are 3 kW, 240 V, and that the thermostat is supplied set at about 60 to 62°C depending on the model.[3]Gledhill, StainlessLite Plus instruction manual
Why people use one
There are two common reasons. The first is as a back-up: if the boiler breaks down, the element can still heat the cylinder.
The second is a cheap overnight tariff. Energy Saving Trust says some tariffs, such as Economy 7, give cheaper electricity at certain times, usually overnight, and that you can power an immersion heater in the cylinder during that period.[1]Energy Saving Trust, Electric storage heaters Ideal says this lets you wake up to a full tank without using gas, and that it might work out cheaper on that tariff.[2]Ideal Heating, Should You Turn Your Boiler Off at Night?
Cost matters because electricity is dearer than gas. Energy Saving Trust says electricity is around four times more expensive than gas per unit.[7]Energy Saving Trust, Electric heating At standard rates, running an element for long periods is therefore an expensive way to heat water.
Parts and safety features
| Part | What it does |
|---|---|
| Element | Heats the water when electricity flows through it |
| Control thermostat | Switches the element off when the water reaches the set temperature |
| Thermal cut-out | A safety thermostat that cuts the electricity if the control thermostat fails |
| Cylinder insulation | Keeps the heat in so the element switches on less often |
Gledhill says its cylinders are factory fitted with immersion heaters with thermal cut-outs, and that immersion heaters without thermal cut-outs must not be fitted.[3]Gledhill, StainlessLite Plus instruction manual Its thermostats have a manual reset safety cut-out marked with a red button. If it operates, the manual says to investigate the cause before pressing reset.[3]Gledhill, StainlessLite Plus instruction manual
Insulation affects how often the element runs. Ideal says a poorly insulated cylinder leaks heat and the heater comes on more often.[2]Ideal Heating, Should You Turn Your Boiler Off at Night? Energy Saving Trust says an existing jacket should be at least 80mm thick, and a replacement costs about £30.[4]Energy Saving Trust, Insulating tanks, pipes and radiators
When to call a professional
Immersion heater work involves mains electricity and, often, an unvented cylinder. GOV.UK says all electrical work in a dwelling must be carried out in line with Part P of the Building Regulations. It can be certified by an installer who belongs to a competent person self-certification scheme.[5]GOV.UK, Third party certification schemes for domestic electrical work Gledhill’s manual warns of a danger of electrocution and says to isolate the heater at the fuse spur before making any adjustment to the thermostat.[3]Gledhill, StainlessLite Plus instruction manual It also says the temperature and pressure relief valve must not be removed or used for any other purpose.[3]Gledhill, StainlessLite Plus instruction manual See unvented cylinder and our hot water cylinders page.
Example
Ranjit in Bradford has a cylinder heated by his boiler and an immersion heater already fitted, with its own switch. His boiler needs a part in January, so he uses that switch for hot water. Say the element is rated 3 kW, like the Gledhill heaters above.[3]Gledhill, StainlessLite Plus instruction manual Every hour it runs uses 3 kWh of electricity. He leaves the boiler and heating to the engineer until it is repaired.
What people get wrong
An immersion heater is not a cheap way to heat water all day. It earns its place as a back-up or on a cheap overnight tariff, and costs more than gas when it runs at standard rates.
Another mistake is leaving it on with a poorly insulated cylinder. Ideal says that makes the heater come on more often, and Energy Saving Trust says topping a jacket up to 80mm is one of the cheaper fixes.[2]Ideal Heating, Should You Turn Your Boiler Off at Night?[4]Energy Saving Trust, Insulating tanks, pipes and radiators
The bottom line
An immersion heater is an electric element in the cylinder. It heats water without the boiler, so it suits a back-up or a cheap-rate tariff. Electricity costs around four times as much as gas per unit, so insulate the cylinder and use the cheap-rate window if you have one. Replacement is a job for a qualified person, because it involves mains electricity and sometimes an unvented cylinder.
Common questions
Is an immersion heater the same as the boiler heating my water?
No. Ideal explains that the boiler heats cylinder water by circulating hot water round it, while an immersion heater is an element inside the cylinder that uses electricity. The two are separate and the immersion heater does not heat radiators.
Is an immersion heater expensive to run?
It can be on a standard tariff. Energy Saving Trust says electricity is around four times more expensive than gas per unit. Ideal says an overnight cheap-rate tariff can make it work out cheaper.
Can I replace an immersion heater myself?
Electrical work in a home must follow Part P of the Building Regulations, according to GOV.UK, and the heater sits in a cylinder that may be unvented. Use a qualified, registered electrician or plumber.
Why has my immersion heater stopped working?
Gledhill's manual says its thermostats have a manual reset safety cut-out and tells you to investigate the cause before pressing reset. Switch the heater off and ask a qualified person to check it.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Energy Saving Trust. Electric storage heaters. Checked 11 November 2026. ↩
- Ideal Heating. Should You Turn Your Boiler Off at Night?. Checked 11 November 2026. ↩
- Gledhill. StainlessLite Plus instruction manual. Checked 11 November 2026. ↩
- Energy Saving Trust. Insulating tanks, pipes and radiators. Checked 11 November 2026. ↩
- GOV.UK. Third party certification schemes for domestic electrical work. Checked 11 November 2026. ↩
- Energy Saving Trust. Heating controls. Checked 11 November 2026. ↩
- Energy Saving Trust. Electric heating. Checked 11 November 2026. ↩