Immersion Heaters: How They Work, Faults and Running Costs
An immersion heater is an electric element inside a hot water cylinder that heats the water directly, independently of the boiler.
An immersion heater is an electric element inside a hot water cylinder that heats the water directly, independently of the boiler. Most cylinders have one as a backup, some rely on it entirely, and it is the thing that keeps hot water available when a boiler fails. It is also one of the more expensive ways to heat water, so understanding when to use it and when it has failed is worth knowing before the boiler breaks down.
What it is and where it sits
The element screws into a boss on the cylinder and is immersed in the stored water, with a thermostat that switches it off at the set temperature and a thermal cut-out as a safety backstop. Some cylinders have two, a lower one for heating the full cylinder and an upper one for heating just the top portion quickly. Each is wired on its own circuit and controlled by a switch, sometimes with a timer on an off-peak tariff.
How you know it has failed
The most common symptom is no hot water from the immersion when the switch is on, with the boiler side unaffected. Water that is lukewarm rather than hot usually means the thermostat has drifted or the element is partially scaled. Repeated tripping of the circuit breaker points at an element that has broken down electrically and is leaking to earth, which needs disconnecting rather than resetting. A thermal cut-out that has operated indicates the thermostat failed to switch off, and both parts want replacing together.
Limescale is the usual killer
In hard water areas, scale builds on the element surface and insulates it, so it runs hotter, heats less efficiently and eventually fails. A scaled element sometimes announces itself with crackling or popping noises from the cylinder as water flashes to steam against the hot surface. Descaling an old element is rarely worth it compared with fitting a new one, and where water is very hard an incoloy element lasts significantly longer than a standard one.
Running cost, honestly
Electricity is generally a more expensive fuel per unit of heat than mains gas, so heating a full cylinder by immersion typically costs more than heating the same water with a gas boiler. That does not make it wrong — it makes it a backup or an off-peak tool. If your tariff has a cheaper overnight rate, timing the immersion to heat the cylinder overnight and relying on the cylinder's insulation through the day is far more sensible than heating on demand at peak rate.
Replacing one properly
The cylinder has to be isolated and drained down to below the element boss, which takes time on a full cylinder. Old elements often seize into the boss and need a proper immersion spanner and care to avoid distorting the cylinder. The gasket is renewed, the new element and thermostat fitted, and the temperature set sensibly — high enough for safe stored water, not so high that it scalds or accelerates scaling. It is electrical work as well as plumbing, so it must be done and tested competently.
Getting the best from it
Check the cylinder jacket or foam insulation is intact, because a well-insulated cylinder holds heat for many hours. Do not leave the immersion switched on permanently unless it is your only heat source. Use the upper element where fitted for a quick partial heat rather than heating the entire cylinder. And have it tested during an annual visit, because finding a dead immersion in January when the boiler fails is the worst possible time.
Immersion heater not heating?
Tell us the cylinder type and what happens when you switch it on, and we will get the element and thermostat checked.
Questions we get asked
Is it expensive to run an immersion heater?
Generally more expensive per unit of heat than mains gas, which is why it is best used as a backup or on a cheaper overnight tariff.
Should I leave the immersion on all the time?
Not usually. Timed heating with a well-insulated cylinder is more economical unless the immersion is your only source of hot water.
Why does my immersion keep tripping the electrics?
Typically an element that has broken down and is leaking to earth. It should be isolated and replaced rather than repeatedly reset.
Can a scaled element be cleaned?
Occasionally, but it is rarely worth it. In hard water areas an incoloy replacement element is a better long-term answer.
Can I replace an immersion heater myself?
It combines draining a cylinder with electrical work, and seized elements can damage the cylinder if forced. It is a job worth having done properly.