Immersion Heaters: Our Complete Guide for 2023

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A boiler with a temperature gauge and a wrench in the background

Immersion heaters heat your water directly rather than relying on the central heating. In our immersion heater guide, find out everything you need to know about immersion heaters from switches, timers, and the age-old sink or bath problem!

What Is an Immersion Heater and How Does It Work?

A large immersion tank

Many people refer to the cylindrical-boiler-like contraption in the hot press as the immersion heater, but actually, the immersion heater is an electric water heater that sits inside the cylinder (also known as the immersion tank).

Strong currents of electricity pass directly into the immersion heater, which then uses the energy to warm the water around it.

Basically, an immersion heater is like a kettle in that they both heat up water using electricity. Some households that are not connected to the gas grid will use immersion heaters as their only source of hot water. Others will use the immersion heater in addition to their central heating/boiler set up as a backup (or “top up”) supply of hot water.

Immersion heaters can be particularly useful for businesses that require a large quantity of hot water (for example, a hairdressing salon) due to the fact that you can get immersion tanks with a large capacity.

For households, an immersion water heater and tank can also be useful in the summer months as the central heating has usually been turned off and people may prefer not to fire up the whole system just for hot water.

How Long Does It Take for an Immersion Heater To Heat a Tank of Water?

It can take between one to 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.

How Much Does It Cost To Run an Immersion Heater?

Rough estimates are that it costs around 50 cents per hour to run an immersion heater. If you have a more efficient immersion heater (say a 6KW element instead of a 3KW one), it will heat the same amount of water and cost the same. However, it will do the job in half an hour instead of a full hour.

Heating water with electricity is much more expensive than heating it with gas. To help you find the best price, we've listed the cheapest electricity offers below per supplier:

Cheapest Electricity Offer per Supplier
SupplierBest OfferEstimated Annual BillTrustpilot Reviews
Yuno EnergyFixed Discount€1,168.123.6 / 5
SSE AirtricityFixed Offer€1,273.321.2 / 5
FlogasNo Discount€1,332.294.2 / 5
Bord Gáis33% on Electricity€1,250.232.4 / 5
Energia31% Selectra Discount 📞€1,340.374.1 / 5
Electric Ireland24% on EO, DF€1,394.351.4 / 5
Community PowerNo Discount€1,448.22N/A
EcopowerNo Discount€1,808.533.9 / 5
PrepayPowerNo Discount€1,835.814.8 / 5
PinergyNo Discount€2,001.734.6 / 5
WaterpowerNo Discountnew customers are currently unable to switchN/A

*Figures are for illustrative purposes only. Calculations based on average consumption figures for an urban home with a 24-hour standard meter unless specified. All discounts and cashback have been applied. This also includes the PSO Levy at €0.00.
Last updated: May 2024

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

a calculator with an electriciy bolt over it

The two most important factors to reduce the cost of running the immersion are immersion heater thermostats and insulation (such as cavity wall insulation). Make sure your immersion heater thermostat functions properly and is set to a reasonable temperature. There is no point in spending money overheating water to the point where you need to mix cold water with it to take a shower.

However, you do need to remember to heat the water in the immersion tank to above 50ºC to kill off any bacteria which may be lurking inside.

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.

Is It Cheaper To Leave the Immersion On?

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 the immersion heater to run repeatedly (like your kettle coming back to the boil over and over again).

Unless you are sure that your immersion heater thermostat is just the ticket, that the tank is insulated as much as possible, and that you need copious amounts of hot water all throughout the day, switch it off until needed. Otherwise, the only thing you will be saving up for is a big (nasty) surprise when you receive your electricity bill.

Fitting a timer will only cost around €45 and will solve the never-ending arguments over who left the immersion on. It will also allow you to heat water for less if you have a Nightsaver meter since you can program the immersion to come on and heat the water during the cheaper night-rate hours. Then it will be ready for everyone's hot shower in the morning. This way you can save even more money.


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How Does an Immersion Switch Work?

To further reduce the costs of running the immersion, you should set it to the sink switch for quick showers. 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.

If you’ve ever kept the plug in the bath while having a shower, you’ll have noticed that you usually only use a third as much hot water as you would for a bath. Setting the immersion to the sink side of the switch will use roughly a third of the amount normally heated for a bath. We recommend setting the switch to sink and taking a shower. Not only will you be using less water, but you’ll also only be using a third of the amount of electricity as when you set it to bath.

Are There Energy-Efficient Immersion Heaters?

No. Due to the nature of the function they perform (which is exchanging electricity energy for heat energy), there are no energy-efficient immersion heaters whereby you can get more heat energy bang for your buck.

When immersion heaters are referred to as being “efficient”, it just means that they are more powerful and can produce more hot water in the same amount of time. However, they still need to use the same amount of energy.

It is possible to use solar panels to power your immersion heaters, and this would result in clean, renewably produced hot water.

Is It Safe To Leave the Immersion Heater On?

It's not necessary to leave the immersion heater on. If you do leave it on constantly, there are two main problems that may occur:

  • The immersion may stop working completely.
  • The water in the cylinder may boil. The cylinder will then push this boiling water into the cold water tank in the attic. The water then coming out of the cold tap will also be boiling.

By leaving the immersion heater on, you risk burning yourself with the boiling water. It will also cost a fortune to leave the immersion on all the time. It's best to run it for a couple hours at night and the water will still be warm in the morning for a shower.

How Much Does It Cost To Replace an Immersion eater?

If your immersion heater is not heating up the water properly, it may be time to get a replacement. Before purchasing a new immersion heater, it's best to arrange for a plumber or heating engineer to assess the issue. They may determine that you can simply replace a part of the immersion heater (such as the heating element) rather than having to replace it entirely.

The cost of replacing an immersion heater varies depending on its size and type. If there are no complications, it will typically cost around €140 excluding VAT.

The Pros and Cons of Immersion Heaters

Initial economical purchase price and later cheap replacement costs of up to €130 only, as the heater can be replaced separately to the tank.Slow to heat up water, requires planning in advance.
If your boiler breaks down you will still have hot water through the immersion, and vice versa.More expensive to run than heating hot water with gas.
Well-insulated immersion tanks can keep water hot for hours after being switched off.The majority of Irish households would need to run the immersion for hours each day to produce enough hot water for their household.
Nightsaver customers can set the immersion to come on during cheaper electricity hours, and wake up to plenty of hot water.Gas boilers are considered to be far cheaper to run.
Easy to operate, simply switch on or off.Electricity dependent - you’ll have no access to hot water if the power goes out.

KRIB: A Viable Alternative To the Traditional Immersion Tank and Heater?

A green lightbulb

As mentioned above, a gas-fired boiler is usually a cheaper alternative to an immersion heater. But did you know there is a new kid on the hot-water block in Ireland, the KRIB device?

KRIB heats water at the top of a water tank rather than the bottom, and it heats it fast, so that only the required amount of hot water is produced, rather than heating up the whole tank. It has a modern digital controller which comes in a variety of colours, and can also be accessed from anywhere via an app on your smartphone.

KRIB can be retrofitted to existing hot water cylinders for €999 (which includes the touchscreen control and app as well as the retrofit equipment) or installed from scratch for €1850. The KRIB is touted to save energy, reduce water consumption, and lower heating costs. It also qualifies for an SEAI Better Energy Home grant.

Initial reports are that while immersion heaters can take up to an hour to heat enough hot water for a bath or shower, the KRIB can do so in just six minutes.

Investing in a KRIB is a much bigger initial outlay than investing in an immersion heating system. This means that it may not be suitable for you if:

  • You’re on a tight budget.
  • You plan to be using the property sporadically.
  • The property is a rental.

However, if you’re planning on living in the property where you’ll be installing a hot water system for a long time, then you could definitely make some considerable savings long-term with the KRIB system.

The services and products mentioned on this website may only represent a small selection of the options available to you. Selectra encourages you to carry out your own research and seek advice if necessary before making any decisions. We may receive commission from selected partner providers on sales of some products and/or services mentioned within this website. Our website is free to use, and the commission we receive does not affect our opinion or the information we provide.

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