What Is an Immersion Heater and How Does It Work?

Updated July 2026 · Emergency Plumber Chelsea

Where to find it. Look in the airing cupboard or wherever your hot water cylinder lives. The immersion is the round cap, usually on the top of the cylinder or on its side near the bottom, with a heavy flex running to a switch on the wall. The switch is normally a large double-pole switch marked water heater, sometimes with a neon light that glows when it is on. Some homes have a timer or a two-way switch labelled bath and sink instead. If you have a combi boiler and no cylinder, you do not have an immersion heater at all, because a combi heats water on demand as it flows to the tap.

How it works. The element is a metal rod or loop that sticks down into the water. When the switch is on, electricity heats the element and the element heats the water around it. Hot water rises, so the cylinder warms from the level of the element upwards. A thermostat built into the element head measures the water temperature and cuts the power once it reaches the setting, then switches back on when the water cools. A separate high-limit cut-out sits alongside it and trips for safety if the water ever gets far too hot, for example if the thermostat sticks on. For how the immersion fits alongside the boiler coil, thermostat and valves, see our guide to how a hot water cylinder works.

Single and dual elements. A single immersion usually enters through the top of the cylinder and runs down most of its length. A dual immersion either has two elements in one head, one long and one short, or two separate elements, one near the top and one near the bottom. The short or top element heats a small amount of water quickly for a sink or a quick shower. The long or bottom element heats the whole tank, and on an economy or off-peak tariff it is often set to come on overnight when electricity is cheaper. The bath and sink switch you see in many older Chelsea flats is simply choosing between those two elements.

Immersion heater or boiler? In a home with a regular or system boiler and a cylinder, the boiler heats the water through a copper coil inside the cylinder, and the immersion sits there as an electric alternative. When the boiler is working you usually leave the immersion off and let the boiler do the job, because heating water with gas is normally cheaper than heating it with electricity. When the boiler breaks down, the immersion keeps you in hot water until it is repaired. In flats without gas, which covers a fair number of Chelsea mansion blocks and converted period buildings, the immersion is the only source of hot water. Our guide to vented and unvented cylinders explains which type of cylinder you are likely to have.

How long it takes to heat up. It depends on the size of the cylinder, the power of the element, how cold the incoming water is and how much hot water is already in the tank. A full tank heated from cold can take a couple of hours or more; topping up a partly used tank is much quicker. If it seems to take far longer than it used to, or the water never gets properly hot, the element may be caked in limescale or the thermostat may be failing. Our guide to hot water not hot enough covers the usual causes.

Should you leave it on all the time? Usually not. The thermostat stops it overheating the water, but a cylinder slowly loses heat through its walls, so an immersion left on all day keeps reheating water you may not use. A timer set to heat the tank shortly before you need it, or a boost button for occasional use, is normally the more economical way to run it. A well-insulated modern cylinder loses heat much more slowly than an old copper tank with a thin jacket. Our guide on how much an immersion heater costs to run works through the sums.

What temperature to set it to. Stored hot water should reach at least 60°C, because a cylinder kept cooler than that can allow legionella bacteria to grow. Most immersion thermostats have a small dial under the cap, and 60°C is a sensible setting. Going much higher wastes energy and speeds up limescale build-up, which is already a problem in London's hard water. If young children or older people use the taps, a thermostatic mixing valve at the outlet keeps the water at the tap at a safer temperature while the cylinder stays hot enough. When an element or thermostat does fail, our guide to immersion heater replacement cost covers what the job involves.

Limescale and London water. Every time the element heats, some of the minerals in hard water bake onto it as scale. Over the years that crust insulates the element, so it works harder, heats more slowly and fails sooner. Crackling or fizzing noises from the cylinder while the immersion is on are a common sign. Our guide to hard water and limescale in London explains the wider effect on taps, showers and boilers.

Safety. An immersion heater combines mains electricity with a tank of hot water, so treat it with respect. It is fine to switch it on and off, set a timer, and press the reset button on the thermostat once if the cut-out has tripped. It is not a DIY job to open the element head, change the element or thermostat, or rewire the switch. If the switch or flex feels hot, you smell burning, you see scorching around the cap, the fuse or breaker keeps tripping, or water is weeping from the element boss, switch it off at the consumer unit and get it looked at. On an unvented cylinder, hot water discharging from the tundish, the small funnel in the pipework, means the water has overheated: switch off the immersion and the boiler and call an engineer.

When it stops working. The common faults are a tripped cut-out, a failed thermostat, a burnt-out element, a faulty switch or timer, and a tripped circuit. Our guide to an immersion heater not working walks through the checks in the order to do them. Emergency Plumber Chelsea tests and replaces immersion elements and thermostats, drains cylinders to the right level and sorts out the cylinder plumbing in one visit across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We explain the fault and the options before any work starts, and there is no call-out fee on jobs over 3 hours. See our plumbing repairs page or call 020 3002 6667.

Quick tips

• The immersion switch is usually a large wall switch marked water heater near the airing cupboard.
• If you have a working gas boiler, use the boiler for hot water and keep the immersion as a backup.
• Use a timer or boost rather than leaving the immersion on all day.
• Keep stored hot water at 60°C or above to control legionella.
• Never open the element head or rewire the switch yourself; switch it off at the consumer unit if anything smells hot.

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