An immersion heater that has stopped heating water almost always comes down to one of three things: the switch feeding it, the thermostat, or the element itself. Working through them in that order finds the fault fastest, and tells you early on whether this is something you can safely check yourself or something to leave alone.
How an immersion heater actually works
An immersion heater is an electric element that sits inside the hot water cylinder, heating the water around it directly rather than through a boiler. A thermostat clipped to the top of the element controls the temperature, switching the element off once the water is hot enough and back on as it cools. Most cylinders that use one have their own isolator switch nearby — often a switched fused connection unit on the wall, separate from any ordinary light switch in the airing cupboard.
Start with the switch and the circuit
Check the isolator switch is actually on — it is easy to knock, and easy for someone else in the house to have turned it off without mentioning it. If that checks out, look at the consumer unit. A tripped breaker or RCD on that circuit will stop the immersion heating with nothing else in the house obviously affected, since immersion heaters are usually wired on their own circuit.
Where the immersion runs on an off-peak tariff such as Economy 7, check whether its circuit sits on a second consumer unit rather than the main one. It is easy to overlook a second board, and easy for a fault there to go unnoticed for a long time.
The thermostat is the most common failure
The thermostat clipped to the element fails more often than the element itself. A stuck thermostat can leave the heater running when it should not — worth checking if the water is coming out noticeably hotter than usual — or stop switching the element on at all. Most immersion heater thermostats have a small manual reset button under a cover on the head of the unit. One that trips repeatedly rather than resetting once and running normally is telling you the thermostat has failed, not just tripped.
The element itself
If the switch, the circuit and the thermostat all check out and the water still is not heating, the element has likely failed — most often burnt out, or so scaled up with limescale in a hard water area that it can no longer pass heat into the water properly. An electrician can test the element for continuity to confirm that before replacing anything, rather than fitting a new part that was never the actual fault.
When to call an electrician
Book someone in if the switch and circuit are fine, the thermostat's reset button has not fixed it, or you are not confident telling the two faults apart — replacing a thermostat costs a good deal less than replacing an element, so it is worth confirming rather than guessing. We repair and replace immersion heater thermostats and elements as part of our heater repair work, and where a circuit keeps tripping rather than the heater simply failing to heat, a proper fault-finding visit finds the actual cause rather than replacing parts on a guess.
If you are unsure, it is always cheaper to ask than to guess. Give us a call and we will tell you straight.