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6 Frequently Asked Questions About Immersion Heaters

1. What is an immersion heater?

An immersion heater is an electric heater element made of metal that is fitted inside a hot water cylinder by a plumber to heat up the water in the tank as it is needed. It is shaped like a coil and works similarly to how the metal coil in your electric kettle works by transferring the electrical energy it creates into heat energy into the water it is sitting in. It is easy to control because it is powered by electricity and so it has a switch where it can be turned on or off.

2. How does an immersion heater work?

When the immersion heater element is activated, it heats up the casing that surrounds it, and this heat is then transferred to the water in the cylinder. Cold water flows into your hot water cylinder at the bottom, and as it gets heated by the element, it ascends to the top. The pressure generated at the tap’s outlet is a result of the ‘water head’, which is determined by the height of the cold water tank situated in the attic. The immersion heater operates off the main’s electricity and is controlled by an immersion heater switch or a timer on the wall near it that is connected to an immersion heater thermostat. This allows the heater to be turned on and off to heat the water as needed.

3. How long should an immersion heater be on for?

Unfortunately, this isn’t a simple question to answer as there are numerous factors that can affect how quickly the water in your hot water cylinder, with the immersion heater, takes to come up to temperature. The duration for which an immersion heater should be on mainly depends on the size of your water tank and the temperature you would like. Here are the factors that can affect its heating time:

Water tank temperature

Depending on when last your immersion heater was used, the temperature of the water in the water tank can vary significantly. If your immersion heater has only been off for a few hours, it’s likely the water will only be a few degrees below your thermostat settings. But if you had a long, hot bath before turning off your water heater, the tank would have refilled with cold water, meaning it would take much longer to heat the water back up to temperature.

If your immersion heater is heating up from cold, it can take up anywhere from 1.5 hours to 4 hours to reach your desired temperature. Groundwater will also play a role here as the groundwater in winter entering the tank is much colder than in summer and will drastically increase the heating time.

Thermostat setting & the size of the tank

Two factors come into play here that can affect heating time. Firstly, the higher your thermostat setting is on your immersion heater, the longer the water will take to come up to temperature. Secondly, the size of your tank will affect heating time; the more water in the reservoir, the longer it will take to heat the water. So a large tank will with a high thermostat setting will take considerably longer to heat up than a small tank with a lower thermostat setting.

If the tank is surrounded by insulation

Another factor that can affect heating time is insulation. For example, if your immersion heater is sitting in a cold cupboard in your home, it may take longer to heat than a water cylinder surrounded by isotherm in your attic. At the same time, a hot cylinder that has good insulation will maintain the water temperature in the reservoir tank more easily as the insulation will prevent heat from escaping.

Condition of your water heater

The condition of your immersion heater can play a role in how efficiently it heats the water in the tank. If you have an older water heater, there may be issues with the heating coil, pipes, thermostat or even the structural integrity of the tank, which may all affect its ability to heat water and maintain the water temperature.

Immersion heater timers

You could also consider an immersion heater timer to automate the process, turning the heater on for a few hours before you usually use hot water. The immersion heater timer will bring the water up to temperature, and then it will turn the water heater off again. Plumbing companies in your area will be able to help with the installation of a timer.
It’s crucial not to leave the immersion heater on unnecessarily, as it can lead to increased energy consumption and higher utility bills, as the electric water heater will be constantly heating up to maintain the temperature set on your thermostat.

As a final note, we recommended trialling the heater by turning it on when the water is completely cold and then testing the water every hour only to see how long it takes to come up to temperature. It is critical that you do not check more frequently, as you will force cold water into the tank and only delay the process. This experiment will help you get a clear estimate of the time it takes to heat up the water in your water heater.

4. Is it cheaper to use an immersion heater or a gas heater?

Using a hot water cylinder with an immersion heater could work out to be more expensive than using gas, simply because the cost of electricity is higher than gas. There are a few ways in which you could save money if you do have or prefer to have an immersion heater tank. 

Using immersion heater timers is one way to save a lot of money as the element will only be using electricity when it is heating, and this will only be when it is needed. It will not need to constantly keep the water at the required temperature. By also ensuring that the immersion heater timers are set to only heat up during off-peak, cheaper tariff hours, you are able to keep the costs down even more. Off-peak tariff hours for Economy 10 tariff are usually 3 hours in the afternoon, 2 hours in the evening and 5 hours in the early hours of the morning. On an Economy 7 tariff, they usually fall between 10 pm and 8 am, but it is best to check with your energy supplier to find out which specific hours are applicable in your area.

Another way to save on your energy bill is to ensure you turn down the temperature set on your thermostat in summer, so you can shower using the hot water tap only. Just remember to turn it back up as the colder months approach.

Making sure your hot water cylinder with the immersion heater is very well insulated is another way to keep costs down. If the insulation proves to be insufficient and your water heater cools too quickly, then it would be a good idea to upgrade the insulation by using plumbing services, such as Absolute Plumbing Ltd, to do so.

Despite the higher cost of electricity, gas heaters do require regular maintenance, and they can be expensive to repair or replace, so those costs should be factored in as well when deciding between immersion tank heaters or gas boilers.

5. Why do I have an immersion heater and a boiler?

It is common for homes to have both an immersion heater and a boiler (sometimes referred to as an immersion boiler). This setup provides flexibility and a backup option for heating water. The boiler is typically part of the central heating system, providing hot water throughout the house as well as heating the home during colder months. 

On the other hand, an immersion heater is usually housed within a hot water cylinder and heats only the water inside that cylinder so that if the boiler fails, then you would still have access to hot water. This can be especially useful in situations when the boiler breaks down or is being serviced, where the immersion heater can serve as a standalone unit for water heating until the emergency plumber, such as Absolute Plumbing Ltd, has fixed your boiler. 

Using an immersion heater may also be more energy-efficient during summer months when you might not want to turn on the entire central heating system just to get hot water. It’s always good to have options, and having both an immersion heater and a boiler ensures you’re never without hot water.

6. How much does an immersion heater cost to run?

From 1 July 2023, the price per kilowatt will be fixed at 30p per kilowatt for electricity and at 8 kilowatt for gas. These prices will, of course, fluctuate a little depending on where you are in the UK, so it is a good idea to check the specific energy tariff that you are being charged. 

An electric immersion heater uses, on average, 3 kilowatts per hour (approximately 90p per hour at the current tariff). But this depends on the model you have, the size of the tank, the heat set on your thermostat and the cost per kilowatt that you pay.

Now that you know how much an immersion heater costs to run, you can use these figures to decide if it would be a viable option for your household.

In need of a plumber?

Gone are the days of searching for “plumbers near me” when you are looking for reliable plumbers in your area! Absolute Plumbing Ltd services areas across London, and our team of specialised and dedicated professionals take pride in delivering fast and reliable plumbing services to all our clients.

If you need plumbers to install an immersion heater or are in need of emergency plumbers because yours isn’t working, call the friendly experts at Absolute Plumbing Ltd to assist. We have over a decade of experience installing, repairing and upgrading immersion heaters. Contact Absolute Plumbing Ltd by calling 073 4154 7739 or sending a query via our website.