What Is an Immersion Heater, and Why Have One and a Boiler?

How an immersion heater works

The element is a long metal loop, sealed in a sheath, screwed into a threaded boss on the cylinder so that it sits in the water. When the switch is on, current flows through the element, it heats up, and the heat passes into the surrounding water. Hot water rises, so the cylinder heats from the element upwards.

Fitted alongside the element is a rod thermostat with a dial, set to the temperature you want the water held at. When the water reaches it, the thermostat cuts the power. As the water cools or is used, it switches back on. Modern immersion thermostats also have a separate safety cut out that trips permanently if the water gets far too hot, and has to be reset by hand once the cause is found.

Many cylinders have one element entering from the top and reaching down inside. Others have two: one near the top for a quick top up of the hottest water, and one lower down to heat the whole cylinder, typically overnight on a cheaper rate. On some unvented cylinders the elements are specific to the make, rather than the standard size a merchant keeps on the shelf. Whether you need one element or two depends on how the cylinder is heated, which our guide to the dual immersion heater works through.

Why do I have an immersion heater and a boiler?

Because a cylinder heated by a boiler has no other way of heating water if the boiler breaks down. Most installations with a gas boiler and a cylinder include an immersion as standby. On a normal day the boiler heats the cylinder through a coil inside it, which is cheaper, and the immersion stays off.

Some households also use it in summer when they do not want the heating side running, or to boost the water quickly when the boiler timer is not on. Unvented cylinders are commonly supplied with an immersion as standard for the same backup reason.

If you have a combi boiler, you do not have an immersion heater, because there is no stored hot water for it to heat. The water is heated on demand as it passes through the boiler.

Should the immersion be on all the time?

Not if the boiler is working. Leaving it on constantly heats the water electrically around the clock, which on most tariffs costs more than letting the boiler do it. The usual approach is to leave it switched off and use it when the boiler is out of action, or on a timer if you are on an off peak tariff and the cylinder is designed to be heated that way.

Running cost depends on the element rating and how long it is actually on, not on how long the switch is on. A standard element is often rated at 3kW, which means it uses 3kWh for every hour it is actively heating. Multiply that by the unit rate on your electricity bill to see what an hour costs you. Once the water is up to temperature, the thermostat keeps the element off for most of the time. Our guide to immersion heater running cost works through the cost per hour and per tank for common cylinder sizes.

Stored hot water should be kept hot enough to control bacteria. Guidance for domestic cylinders is generally to store water at 60°C. Turning the immersion or cylinder thermostat right down to save money is not a good trade.

How long does an immersion heater take to heat water?

It depends on the size of the cylinder, the element rating, and how cold the water is to start with. A full cylinder heated from cold on a single element takes a good while, often a couple of hours for a typical family cylinder. A top element heating just the upper part is quicker. Our guide on how long hot water takes to heat up goes through it in more detail.

Signs your immersion heater has stopped working

These are the usual signs, and each points to a slightly different fault.

• Switched on, but the water never gets hot: a failed element, a failed thermostat, or the safety cut out has tripped.
• The water gets hot but runs out much sooner than it used to: on a dual element cylinder, one element may have failed, or scale has built up around it.
• The fuse or breaker trips when the immersion is switched on: an element that has failed to earth, which should be left off until tested.
• Water is too hot, or the cylinder is making loud noises: the thermostat may have stuck on, which needs attention.
• Water weeping around the boss where the element enters the cylinder: the seal has failed.

Do I need a plumber or an electrician?

It depends on the fault. Replacing an element or a thermostat on an existing circuit is part of hot water work, and it involves draining down part of the cylinder and resealing a joint, which is plumbing. Our engineers do that routinely, including on unvented cylinders where the work also has to respect the safety controls. A fault in the circuit feeding the immersion, a new supply, or work at the consumer unit is an electrician job.

In practice, if the immersion is not heating, a hot water engineer is the right first call, because the test that proves whether it is the element, the thermostat or the supply takes a few minutes with a meter, and the fix is usually a part in the cylinder. Our immersion heater repair page explains the visit, and the immersion heater not working guide covers the checks you can make first.

Switching the immersion on and off at its own switch is a normal household task. Removing the cover, resetting the cut out or testing the element involves live electrical parts and a vessel of very hot water, and is not a homeowner job.

If yours needs attention

If the immersion is not heating, keeps tripping, or is leaking at the cylinder, call us or send an enquiry with a photo of the cylinder label and the immersion switch. We have 20 engineers across London, and the photo lets us check whether your cylinder takes a standard element or a make specific one before we arrive. Our rate is on the pricing page and agreed before anyone attends.