Immersion Heater Not Working? Checks to Make First

An immersion heater is an electric element that sits inside a hot water cylinder, much like the element in a kettle. When it stops heating, the fault is almost always in one of five places, and the first three can be checked without opening anything up.

Short answer: Check the switch is on and the timer is calling for heat, then check the consumer unit for a tripped circuit and the fuse in the switch. If power is reaching it and the cylinder stays cold, the thermostat or the element has failed. Both live behind a cover on top of or on the side of the cylinder, and that is the point to switch off and get it checked properly.

1. The switch and the timer

Most immersion heaters have a double-pole switch near the cylinder, often with a neon light, and many also have a timer or a separate switch for cheaper night-time electricity. A timer knocked out of step after a power cut is one of the most common causes of cold water in the morning. Check the clock is right and the on period is set.

2. The circuit and the fuse

Look at the consumer unit. If the immersion circuit has tripped, reset it once. If it trips again straight away, leave it off: that is a fault, not a fluke. Some switches also have a cartridge fuse in a small drawer that can be replaced with one of the same rating.

3. The thermostat

The thermostat sits under the cover with the element and decides when to switch it off. Many have a small reset button for an overheat cut-out, which trips if the stat has stuck on and the water got too hot. A cut-out that trips once may have been a one-off; one that keeps tripping means the thermostat needs replacing. The cover hides live terminals, so only take it off with the power isolated at the switch, and if you are not confident with electrics, do not open it at all.

The HSE’s guidance is to store hot water at 60°C, which is also the setting Thames Water suggests for keeping limescale down. A thermostat turned much lower than that heats the water, but not enough to feel properly hot at the tap.

4. The element

If power reaches the heater and the cylinder stays stone cold, the element has usually burned out. If the electrics trip every time it switches on, it has probably failed to earth. Either way it needs replacing, which means draining the cylinder and unscrewing the element from its boss. Old elements can be seized solid, and forcing one can damage the cylinder, so this is a job where experience pays.

5. Two elements, one working

Some cylinders have two elements: one near the bottom for a full tank, one near the top for a quick top-up. If only the top one works, you get a small amount of hot water that runs out quickly. That symptom often gets reported as “not heating enough” rather than “not working”.

Why London is hard on immersion heaters

Thames Water’s supply is hard, and scale forms on hot surfaces fastest. A coat of scale on an element insulates it, so it runs hotter, heats more slowly and fails sooner. Keeping the stored temperature around 60°C rather than hotter reduces scale build-up.

When to call someone

If the switch, timer and circuit are all fine and the water is still cold, the remaining causes are behind the cover. Our water heater repair team in London replaces thermostats and elements, and if you have no hot water at all and need it back today, see our no hot water service.