How Does an Immersion Heater Work?

By David Tait Reviewed by Cormac Breen

Editorial Manager

An immersion heater is an electrical heating element found inside hot water cylinders and are designed to heat water. They are energy intensive devices and can cost roughly €1.50 per day when used to heat one full cylinder of roughly 120 litres. There are various ways to lower the cost its cost, including insulating the cylinder, utilising timers and using the bath/sink switch.

What Is an Immersion Heater?

An immersion heater is an electric water heater contained within a hot water cylinder and works similarly to a kettle in that electricity is passed through a metal coil which then heats the surrounding water.

The metal element heats the water directly in the tank, with the hot water rising to the top. They are useful for when you need a top-up of hot water in addition to the water heated by your home's heating system.

How Long Does an Immersion Heater Take To Heat Water?

It can take between one and two hours for an immersion heater to heat a tank of water to the desired temperature.

The time varies depending on how powerful the heater is, as well as how much water the tank holds.

Below, you'll find an estimate for the amount of time it takes for an immersion heater to heat enough water for common household task:

Is an Immersion Heater in Ireland Expensive to Run?

Any appliance that heats water will be more expensive to run given the energy required, and immersion heaters are no exception, with the following factors influencing the cost:

1. The amount of water you need to heat.
2. How efficient your immersion is.
3. The price you pay for electricity.

Heating one full cylinder (normally 120 litres) will cost about €1.30 per day. Actual usage costs will vary depending on cylinder size and the demand for hot water.

Heating a smaller quantity (normally 40 litres) using the "sink" switch will cost about €0.45 and take about 50 minutes to fully heat.

Below, you'll find how immersion heaters compare to other expensive household appliances based on a typical use time:

Prices based on Electric Ireland unit rate 34.75c per kWh on an Urban 24-hour meter with VAT included.
Last Updated: January 2025

How Can You Reduce the Costs of Running the Immersion Heater?

There are several things you can do to reduce the cost of your immersion heater, including:

1. Insulating your cylinder
2. Using a timer
3. Using the bath/sink function
4. Using solar panels to power your immersion

Insulation

By insulating the cylinder, you are able to keep water hotter for longer periods of time and thereby use the immersion less often.

Although many modern immersion tanks come with an insulating layer built-in, if you have an older tank, you should definitely invest in a lagging jacket. Lagging jackets can be bought at any local hardware store.

Grants for lagging jackets are available under the SEAI, Fully Funded energy upgrades scheme.

Timers

By using a timer and setting the immersion to turn on and off at certain times, you avoid overusing the immersion as well as forgetting completely to turn it off.

Fitting a timer will only cost around €45 and if you have a nightsaver or smart meter plan, you can take advantage of cheaper night rates for electricity and set your immersion to heat water during the night.

Do not leave your immersion heater running all the time. No matter how well-insulated the immersion tank is, it will still lose heat and the drop in water temperature will cause it to run repeatedly.

Using the Bath/Sink Switch

The immersion heater has two switches: an on-off switch and a sink-bath switch.

In theory, the sink switch provides enough hot water for washing dishes and washing hands etc., while the bath switch is enough for a bath or a shower.

Given that it is now more common to have showers than baths, which require much more water, setting the immersion to the sink side of the switch will use roughly a third of the amount normally heated for a bath.

By switching to the sink mode, your immersion will use much less energy and will also allow you to save on water consumption.

Solar Panels

If you have solar panels installed on your home, you can use the energy generated to directly power your immersion. By doing so, you would eliminate the cost of running your immersion.

Were you to invest in a heat pump, you could power both your home heating and hot water from the energy generated and essentially remove heating as a cost.

How Much Does An Immersion Heater Cost?

In Ireland, the cost of buying a new immersion heater can range from about €50 to €300, depending on various factors

• Brand
• Model
• Special features
• Installation costs

If you're looking at replacing an existing immersion heater, the total cost including labour and parts can vary, typically ranging from €200 to €500.

This variation in price depends on the complexity of the installation and the type of immersion heater you choose.

Some available options include different materials like copper and titanium, which might affect both performance and price.

The Pros and Cons of Immersion Heaters

While immersion heaters often have a bad reputation for being an expensive, albeit an essential home appliance, there are still many benefits to having one:

Are There Alternatives to Immersion Heaters in Ireland?

Immersion heaters are designed to be an addition to your home central heating system and not the primary means of heating water.

If you are looking for an alternative, the best option would be to consider installing a combination-boiler in your home. Combination boilers work by heating your home as would a traditional boiler, but also have a second function whereby they instantaneously heat water from the main supply, giving you hot water on demand.

Combination boilers have several advantages over immersions, including:

• Highly efficient as they only heat the water you are using.
• They save energy, as you don't need to heat water you don't use.
• They are much more compact than immersion heaters and take up less space in your home.
• They are easier to repair.
• More cost-effective as you don't waste money heating excess water.

Combination boilers cost in the range of €2,590-€3,395 and while they would require an initial large investment, the long term savings compared to an immersion heater would make it worthwhile.