Immersion heater not working

What it usually means

An immersion heater is the simplest water heating there is: an electric element screwed through the side of a hot water cylinder, sitting in the water, with a thermostat clamped to it and a safety cut-out above that. It is normally three kilowatts, it has its own circuit and its own switch on the wall rather than a plug, and it exists either as the only way a cylinder is heated or as the backup for a boiler or heat pump. Because it is so simple, the diagnosis is a short list and most of it can be done without tools.

Start by finding out whether it is being asked to work at all. Many immersion heaters are on a timeswitch or an off-peak circuit, which means they heat overnight on a cheaper rate and do nothing during the day by design — the classic complaint of no hot water at six in the evening is very often a tank that heated at two in the morning and has since been used. Others are on a switch marked boost, and plenty of cylinders have two elements with two switches, a short one near the top for a quick bath and a long one lower down for the whole tank. If the switch you are using heats only a third of a cylinder, you may simply be using the wrong one.

Then check the supply. The switch usually has a neon in it, and a lit neon means the circuit is live. No neon means the breaker or fuse for that circuit, and immersion circuits are frequently switched off at the board deliberately by someone who did not want the expense. If the neon lights and nothing warms up in an hour, feel the pipe leaving the top of the cylinder: a working three kilowatt element brings a normal domestic tank up over roughly one to two hours, so an hour with no warmth anywhere is a genuine failure.

At that point there are three candidates and they behave differently. A failed element gives no heat at all. A thermostat set too low gives water that is warm but never properly hot, and it is worth knowing that stored water should be kept at sixty degrees or so rather than turned down for economy, because stored water below fifty is where legionella becomes a real consideration. And a tripped thermal cut-out gives no heat at all with everything apparently intact — that device is there because a thermostat that fails closed would otherwise take a cylinder towards boiling, so it is the safety net rather than a nuisance, and a cut-out that has operated has done so for a reason worth finding rather than merely resetting.

Two things kill elements. In hard water areas, scale builds on the element, insulates it and makes it overheat until it fails, which is why a heater in Kent or Cambridgeshire lasts a fraction as long as one in Glasgow. And running one in a cylinder that has been drained destroys it in seconds, so after any work on the plumbing the tank has to be full before that switch goes on.