A Simple Guide to Heating Water With Electricity
An immersion heater is one of the simplest ways to heat water in a home.
It uses electricity to heat water stored inside a hot-water cylinder. In principle, it works rather like the heating element inside an electric kettle.
However, immersion-heater systems can sometimes be confusing.
You may have one switch or several. One might be labelled Boost, while another operates automatically overnight. You may also have an Economy 7 or other time-of-use electricity tariff that makes electricity much cheaper at certain times.
As a result, understanding which immersion heater you are using and when it operates can make a significant difference to your energy costs.
What Is an Immersion Heater?
An immersion heater is an electrical heating element fitted directly into a hot-water cylinder.
When electricity passes through the element, it produces heat.
That heat transfers into the surrounding water until the required temperature is reached.
A thermostat then controls the temperature so that the element does not simply continue heating indefinitely.
Therefore, the basic process is:
Electricity → heating element → stored water becomes hot → thermostat stops the heating when the target temperature is reached.
When the water cools sufficiently, the thermostat may allow the element to operate again if the electrical supply to it is still on.
Where Is the Immersion Heater?
The heating element itself is fitted into the hot-water cylinder, so most of it is not visible.
However, you may be able to see the electrical connection or cover where the immersion heater enters the cylinder.
Depending on the system, the element may be installed near the:
- Top of the cylinder
- Bottom of the cylinder
- Side of the cylinder
Some cylinders have more than one immersion heater.
You may also have switches or controls elsewhere in the property rather than directly beside the cylinder.
Why Would You Have an Immersion Heater?
There are several reasons.
In some electrically heated homes, immersion heaters are the main source of hot water.
For example, a home with electric storage heaters may have no gas boiler. Therefore, an immersion heater can heat the water stored in the cylinder.
In other homes, an immersion heater provides a backup.
For example, a gas boiler might normally heat the cylinder. However, if the boiler stops working, the immersion heater may still be able to provide hot water.
Immersion heaters can also form part of systems using heat pumps or other technologies.
Therefore, having an immersion heater does not necessarily mean that electricity is normally your main source of hot water.
How Do I Know If I Have an Immersion Heater?
If you have a hot-water cylinder, look for electrical cables and controls associated with it.
You may find:
- A wall switch
- A fused connection unit
- A timer
- A control marked Boost
- More than one immersion-heater control
However, don’t assume that every switch near a cylinder operates an immersion heater.
If the controls are not clearly labelled, it is worth finding out exactly what they do.
Understanding the system now can prevent considerable confusion later.
Why Do Some Cylinders Have Two Immersion Heaters?
Some cylinders have two electrical heating elements.
This is particularly common in systems designed to use off-peak electricity.
Typically, a lower immersion heater can heat a larger volume of water during a cheaper electricity period.
A second element, often positioned higher in the cylinder, may provide additional hot water when required.
You can think of the arrangement broadly as:
Lower element → heats more of the cylinder
Upper element → heats a smaller amount near the top
However, systems vary.
Therefore, check how your particular cylinder is configured rather than assuming every two-element system works identically.
Why Is One Heater Near the Bottom?
Hot water naturally rises.
Therefore, an immersion element positioned lower in the cylinder can heat a larger proportion of the stored water.
This makes it suitable for providing the main store of hot water.
In an off-peak system, this element may be arranged to operate during cheaper electricity periods so that a large quantity of hot water is ready for the following day.
Why Is Another Heater Near the Top?
An upper immersion heater can heat the water near the top of the cylinder.
Because hot water is normally drawn from the upper part of the cylinder, this can provide a smaller quantity of usable hot water without reheating the entire cylinder.
This is often associated with a boost function.
For example, suppose your household uses most of the stored hot water during the day.
Rather than waiting until the next main heating period, you may be able to use the boost to heat enough additional water for another shower or other immediate need.
What Does the Boost Control Do?
A boost control provides additional hot-water heating outside the normal heating arrangement.
Depending on the system, the boost may:
- Operate an upper immersion heater
- Run for a set period
- Switch off automatically
- Use electricity charged at the rate applying at that time
This last point is particularly important.
If your main immersion heater operates during a cheaper off-peak period, the boost may operate when electricity is more expensive.
Therefore, boost is generally best understood as:
Extra hot water when you need it — not necessarily the cheapest way to heat all your water every day.
What Is Economy 7?
Economy 7 is a type of electricity tariff that provides a cheaper rate for a period of approximately seven hours, usually overnight.
The exact hours depend on factors such as the tariff, meter setup, supplier and location.
During the cheaper period, a home may use electricity to:
- Charge storage heaters
- Heat a hot-water cylinder
- Charge other suitable equipment
Electricity used outside the off-peak period may cost more.
Therefore, households using Economy 7 need to consider when they use electricity, not simply how much they use.
Immersion Heaters and Other Time-of-Use Tariffs
Economy 7 is not the only tariff where electricity prices can vary according to time.
Modern smart tariffs can also offer different prices during different periods.
Therefore, an electrically heated hot-water system may potentially benefit from heating water when electricity is cheaper.
However, the controls, wiring and tariff all need to work appropriately together.
Do not assume that having a smart meter automatically means your immersion heater will start heating water during the cheapest periods.
Automation depends on the particular equipment and tariff arrangement.
Should You Leave an Immersion Heater On All the Time?
There is no universal answer because it depends on the system, controls, tariff and household.
An immersion heater has a thermostat. Therefore, once the water reaches its target temperature, the element should stop actively heating until more heat is required.
So leaving the electrical supply switched on does not necessarily mean the element consumes its full rated power continuously.
However, stored hot water gradually loses heat.
If the immersion heater remains enabled, the thermostat may periodically reheat the cylinder to replace those losses.
Therefore, if nobody needs hot water for a long period, continually maintaining a cylinder at temperature can use energy unnecessarily.
On the other hand, repeatedly switching the system manually may be inconvenient and unnecessary where a suitable timer or off-peak control already manages it properly.
The better principle is:
Heat the water you need, at appropriate times, using the most suitable available tariff.
Does It Cost More to Heat Water From Cold?
Heating colder water requires more energy than heating already-warm water to the same final temperature.
However, this does not mean that continually keeping water hot is automatically cheaper.
While water remains hotter than its surroundings, heat gradually escapes from the cylinder and associated pipework.
Maintaining that temperature means replacing the lost heat.
Therefore, the idea that an immersion heater should always be left on because “reheating cold water costs more” can be misleading.
The best arrangement depends on when the household needs hot water and how well the system stores it.
How Long Does an Immersion Heater Take to Heat Water?
There is no single answer.
Heating time depends on factors such as:
- Cylinder size
- Amount of water being heated
- Starting water temperature
- Immersion-heater power
- Thermostat setting
- Whether one or more elements are operating
- How much hot water is being used while heating
A small upper boost element heating part of a cylinder may provide usable hot water relatively quickly.
Meanwhile, heating a large cylinder from cold can take considerably longer.
Therefore, don’t rely on a universal claim such as “an immersion heater takes one hour.”
Your own system may behave differently.
How Much Electricity Does an Immersion Heater Use?
The amount of electricity used depends on the heater’s power and how long it operates.
For example, suppose an immersion heater is rated at 3 kW.
If it operated continuously at full power for one hour, it would use approximately:
3 kW × 1 hour = 3 kWh
However, that does not mean every hour that the switch is on will necessarily consume 3 kWh.
Once the thermostat reaches its target temperature, the element should stop heating until further heat is required.
Therefore, the actual energy consumption depends on how much water needs heating and how much heat is subsequently lost or used.
Why the Electricity Price Matters
The same amount of electrical energy can cost different amounts depending on your tariff.
For example, heating water using 3 kWh of electricity costs:
3 × your electricity unit rate
Therefore, if your tariff has different rates at different times, heating the same amount of water during an expensive period could cost substantially more than heating it during a cheaper period.
This is why immersion heaters and time-of-use tariffs need to be considered together.
Learn about fixed, variable and smart energy tariffs →
Use Your Smart Meter or App
If you have a smart meter, your in-home display or supplier app may help you understand the effect of your immersion heater.
An immersion heater is a relatively high-powered electrical load while actively heating.
Therefore, you may notice electricity demand increase when it switches on.
You can also use longer-term consumption information to compare different periods.
For example, you might look at:
- Overnight electricity use
- Consumption during the off-peak period
- Electricity use when boost is activated
- Daily consumption after changing the hot-water schedule
However, remember that the smart meter measures the home’s total electricity use.
It does not necessarily know that a particular amount was used specifically by the immersion heater.
Use the information as a clue rather than assuming every increase is caused by one appliance.
Learn more about smart meters →
What Temperature Should an Immersion Heater Be Set To?
The immersion heater normally has a thermostat that controls the temperature of the stored water.
It may be tempting to turn the thermostat down significantly to save energy.
However, stored hot water needs to be managed safely.
If water is stored at unsuitable temperatures, there can be hygiene concerns, including conditions that allow Legionella bacteria to multiply.
Conversely, excessively hot water can increase heat loss and the risk of scalding.
Therefore, do not simply turn the thermostat as low or high as possible.
Follow appropriate guidance for your hot-water system and seek professional advice if you are unsure about the correct setting.
Why Is My Hot Water Too Hot?
If water from the taps suddenly becomes much hotter than normal, there could be a problem with the controls or thermostat.
Do not simply assume that this is normal behaviour.
Some systems also use thermostatic mixing valves to reduce the temperature of water delivered to outlets.
Therefore, unusually hot water can have several possible causes.
If the system is not controlling temperature properly, have it investigated.
Why Is My Water Not Hot Enough?
Several things could be responsible.
For example:
- The immersion heater may not be receiving electricity
- A timer may be incorrectly set
- The thermostat may have operated or developed a fault
- An immersion element may have failed
- The boost may only be heating part of the cylinder
- More hot water may be being used than the system can replace
- Another control may be affecting operation
Start by checking the normal user controls.
However, do not remove electrical covers or attempt internal repairs yourself.
What If the Immersion Heater Stops Working?
If the immersion heater appears not to work, first check the obvious user controls.
For example:
- Is the correct switch on?
- Is the timer programmed correctly?
- Is the boost control operating?
- Is there an obvious electrical supply problem?
- Has another heat source been selected instead?
If the controls appear correct but the water still does not heat, the immersion element, thermostat, electrical supply or another component may need investigation.
Because immersion heaters operate using mains electricity around water, internal electrical work should be left to a suitably competent professional.
Don’t Remove the Immersion Heater Yourself
The immersion heater screws or fits into the hot-water cylinder.
Removing it is not comparable to replacing an ordinary plug-in appliance.
The cylinder contains water and may be part of a pressurised system.
In addition, the immersion heater is connected to the mains electricity supply.
Therefore, do not remove or dismantle it unless you are suitably qualified and competent to carry out the work.
This is particularly important with unvented hot-water cylinders, which have specific pressure and temperature safety requirements.
What If the Immersion Heater Keeps Tripping?
If an electrical protective device repeatedly trips when the immersion heater operates, do not simply keep resetting it.
Repeated tripping may indicate an electrical fault that requires investigation.
Likewise, if a thermostat or safety cut-out repeatedly operates, there may be an underlying problem.
A safety device is there for a reason.
Therefore, identify the cause rather than repeatedly overriding or resetting the symptom.
Immersion Heater or Gas Boiler — Which Should You Use?
Some homes have a cylinder that can be heated by both a gas boiler and an electric immersion heater.
In that situation, the immersion heater may mainly be intended as a backup.
Which heat source costs less depends on:
- Gas price
- Electricity price
- Boiler efficiency
- Tariff structure
- When electricity is used
- System losses
- How much hot water you require
Because electricity is often more expensive per kWh than gas, routinely using a standard-rate immersion heater instead of an efficient gas boiler may cost more.
However, tariffs vary, and very cheap electricity periods can change the calculation.
Therefore, compare the actual cost of the energy available to you, rather than assuming one technology is always cheaper.
Immersion Heaters and Solar Electricity
If you have solar PV panels, surplus electricity can sometimes be used to heat water.
Specialist controls can divert surplus solar generation to an immersion heater rather than exporting that electricity to the grid.
This can effectively store some surplus solar energy as hot water.
However, whether this makes financial sense depends on your system, export arrangements and alternative uses for the electricity.
We cover solar PV properly within Renewables.
Immersion Heaters and Heat Pumps
Some heat-pump hot-water systems also include an immersion heater.
The immersion may provide:
- Backup heating
- Additional heating when required
- Support for particular hot-water cycles
- Emergency hot water if another component is unavailable
However, an immersion heater produces heat differently from a heat pump.
Therefore, routinely relying on the immersion may change the running cost of the system.
Follow the system manufacturer’s operating strategy rather than treating the immersion as an ordinary everyday replacement for the heat pump.
Don’t Confuse the Immersion Switch With the Boiler
If your home has both a boiler and immersion heater, the controls can sometimes be confusing.
Remember:
Boiler
May heat the cylinder indirectly using water circulating through a coil.
Immersion heater
Uses electricity to heat the stored water directly inside the cylinder.
Therefore, switching the immersion off does not necessarily mean you will have no hot water.
The boiler may still be heating the cylinder.
Likewise, switching the boiler off does not necessarily mean the water cannot be heated.
The immersion heater may provide an alternative heat source.
Learn more about your hot-water system →
A Simple Way to Understand Your Controls
If you have an immersion-heated cylinder, find out five things:
1. How many immersion heaters do I have?
2. Which switch controls each one?
3. Is one connected to an off-peak supply?
4. What does the boost control actually operate?
5. What electricity rate applies when each heater operates?
Those five answers can tell you far more than simply knowing that you have an immersion heater.
If the controls are unclear or badly labelled, having them properly identified can make the system much easier to use.
In Short
An immersion heater uses electricity to heat water stored inside a hot-water cylinder.
Some homes use immersion heaters as their main source of hot water.
Others use them as backup for a boiler, heat pump or another system.
If you have two immersion heaters, one may heat a larger volume of water while another provides a smaller boost. In homes with off-peak electricity, when each element operates can have a major effect on cost.
Therefore, don’t think only about whether the immersion heater is on or off.
Think about:
Which element is heating?
How much water is being heated?
When is it heating?
What electricity rate applies at that time?
Most importantly, learn what your switches and controls actually do.
Once you understand them, an immersion heater becomes a very simple piece of equipment to understand — and much easier to use sensibly.
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