An immersion heater uses electricity to heat water inside a hot-water cylinder. It may be your main source of hot water or a backup for another heating system.
What Is an Immersion Heater?
An immersion heater is an electrical heating element fitted inside a hot-water cylinder.
It works rather like the element in an electric kettle.
When electricity passes through the element, it heats the surrounding water.
A thermostat controls the temperature. So, once the water reaches the set temperature, the element should stop heating.
In simple terms:
Electricity → heating element → hot water
Where Is the Heater?
Most of the immersion heater sits inside the hot-water cylinder.
You may only see the electrical connection where it enters the cylinder.
Depending on the system, you could have:
- one immersion heater
- two immersion heaters
- an immersion heater plus another heat source
For example, a gas boiler may normally heat your water while an immersion heater provides backup.
So, finding an immersion heater does not necessarily mean it is your main source of hot water.
Why Are There Two Heaters?
Some cylinders have two immersion heaters.
This is particularly common in electrically heated homes designed around off-peak electricity.
A typical arrangement may have:
Lower element → heats more of the cylinder
Upper element → heats a smaller amount as a boost
The lower heater may heat water during a cheaper electricity period.
Then, if you need more hot water later, the upper heater may provide a boost.
However, systems vary.
So, find out what each switch actually controls rather than assuming they all work this way.
What Does Boost Do?
A Boost control gives you extra hot water when you need it.
For example, suppose the stored hot water has been used during the day.
Rather than waiting for the next normal heating period, Boost may heat some additional water.
However, there is an important point.
Boost may use electricity when it is more expensive.
So, think of Boost as:
Extra hot water when needed — not necessarily the cheapest way to heat your water every day.
Check When It Heats
If you have a tariff with cheaper electricity at certain times, your main immersion heater may be set to use those cheaper periods.
For example:
Off-peak period → main cylinder heats
Daytime → stored hot water is used
Boost → extra heating if needed
This can work well because the cylinder stores the hot water until you need it.
However, check your own tariff and controls.
Don’t assume your cheap-rate times are the same as somebody else’s.
Check Your Electricity Rate
An immersion heater can use a significant amount of electricity while it is actively heating.
So, the price you pay per kWh matters.
Suppose a 3 kW immersion heater runs continuously for one hour.
It would use about:
3 kW × 1 hour = 3 kWh
If electricity costs 25p per kWh:
3 kWh × 25p = 75p
So, that hour of active heating would cost about 75p.
These figures are examples only.
Also, the heater may not run continuously for the whole time it is switched on. Once the thermostat reaches its set temperature, the element should stop heating until more heat is needed.
Should You Leave It On?
There is no single answer for every system.
Once the water reaches the set temperature, the thermostat should stop the element heating continuously.
However, hot water gradually loses heat.
So, if the immersion remains enabled, it may switch on again to replace some of that lost heat.
Therefore, keeping a cylinder hot when nobody needs hot water can use energy unnecessarily.
On the other hand, manually switching the immersion on and off may make little sense if a timer or off-peak control is already managing it properly.
A better principle is:
Heat the water you need, at suitable times, using the right controls.
Does Reheating Cold Water Cost More?
Heating cold water needs energy.
However, that does not mean keeping water hot all day is automatically cheaper.
While hot water sits in the cylinder, some heat escapes.
The hotter the water remains compared with its surroundings, the more heat the system has to retain or replace.
So, don’t leave an immersion heater on simply because you have been told that reheating water is always more expensive.
Instead, use the controls to match hot-water heating to when you need it.
How Long Does It Take?
There is no universal heating time.
It depends on:
- cylinder size
- amount of water being heated
- starting water temperature
- immersion-heater power
- thermostat setting
- which element is being used
For example, an upper Boost element heating part of the cylinder may provide some hot water sooner than heating the whole cylinder from cold.
So, learn how your own system behaves rather than relying on a fixed time.
Why Have I Run Out of Hot Water?
A cylinder stores a limited amount of hot water.
If you use that water faster than the system can replace it, you can run out.
For example, several showers or a bath may use much of the stored supply.
You then need to wait for more water to heat.
If you have a Boost control, this may provide additional hot water.
However, check what electricity rate applies when Boost is used.
Check the Cylinder Insulation
Stored hot water gradually loses heat.
Good cylinder insulation helps slow this down.
Modern cylinders are normally insulated. However, some older cylinders may have less effective insulation.
So, if you have an older cylinder, check whether its insulation can sensibly be improved.
Also, insulating suitable hot-water pipes can reduce heat loss.
Use Your Smart Meter
If you have a smart meter, you may be able to see electricity use rise when the immersion heater starts heating.
This can help you understand when it operates.
For example, compare:
Before heating → while heating → after heating
You can also look at overnight use if your immersion heater runs during an off-peak period.
However, your smart meter normally measures electricity use for the whole home.
So, an increase does not prove that the immersion heater alone caused it.
Learn more: Smart Meters Explained →
Don’t Turn the Temperature Too Low
It may seem sensible to turn the cylinder temperature down as far as possible to save energy.
However, stored hot water must also be managed safely.
Very low stored-water temperatures can create hygiene risks. Meanwhile, excessively hot water can increase heat loss and the risk of scalding.
So, don’t simply turn the thermostat to its lowest setting.
Follow suitable guidance for your system. If you are unsure, get professional advice.
If the Water Is Too Hot
Water suddenly becoming much hotter than normal can indicate a problem with the thermostat or controls.
Don’t ignore it.
If the system does not appear to be controlling the water temperature properly, have it checked.
If the Water Is Cold
First, check the controls you normally use.
For example:
- Is the correct switch on?
- Is the timer set correctly?
- Is the Boost working?
- Is electricity available to the system?
- Is another heat source supposed to be heating the water?
If everything appears correct but the water remains cold, the heater, thermostat, electrical supply or another part may need attention.
Do not remove electrical covers or attempt internal repairs yourself.
If You Also Have a Boiler
Some cylinders can be heated by both a boiler and an immersion heater.
In that case:
Boiler → heats the cylinder indirectly
Immersion heater → heats the water electrically
So, switching off the immersion heater does not necessarily mean you will lose your hot water.
Your boiler may still be heating it.
Likewise, the immersion heater may provide backup if the boiler is unavailable.
If You Have Storage Heaters
Storage heaters and immersion heaters are often found together in electrically heated homes.
For example:
Storage heaters → room heating
Immersion heater → hot water
Both may use cheaper off-peak electricity.
So, your heating, hot water, meter and tariff may all need to be understood together.
Learn more: Storage Heaters Explained →
Five Things to Find Out
If you have an immersion-heated cylinder, answer these five questions:
- How many immersion heaters do I have?
- Which switch controls each one?
- When does the main heater operate?
- What does Boost actually do?
- What electricity rate applies at those times?
Once you know those answers, the system becomes much easier to understand and control.
Key Points
An immersion heater uses electricity to heat stored water.
Some homes use it as their main hot-water system. Others use it as backup.
If you have two elements, they may perform different jobs.
Also, when the heater runs can affect the cost if your electricity tariff has different rates at different times.
So, understand your controls, tariff and heating times.
Most importantly:
Heat the water you need, when you need it, at the most suitable available rate.
Next: Understanding Your Hot Water System →