How does an immersion heater work, and what do I do if it stops heating?

What the element, the thermostat and the thermal cut-out each do, the safe checks when a cylinder stops heating electrically, and where the electrician's job starts.

Difficulty: beginner
Time: 20 min
Read: 5 min
Safety: warning

Short answer

An immersion is an electric element screwed into the cylinder with a thermostat down its centre and a thermal cut-out behind the cap. If it stops heating, the usual causes are a tripped cut-out, a failed thermostat, or a scaled and failed element. Check the switch and the circuit breaker yourself, then call an electrician — the cap is live behind it and the element needs the cylinder drained.

An immersion heater is the simplest heat source in the house: a big kettle element in the side of your hot water cylinder. It matters far beyond its running cost because it is the backup when the boiler fails, the only heat source in many flats, and the single most useful diagnostic tool you have when a cylinder is not getting hot — it heats the water completely independently of the boiler and its controls.

Safety

An immersion heater is a high-current fixed electrical appliance on its own circuit. Switching it off at its double pole switch is householder work; removing the cover, testing the element or replacing it is not — the terminals sit directly behind the cap and the element cannot be changed without draining the cylinder. Never switch an immersion on if the cylinder may be empty: a dry element burns out in seconds and can be a fire risk.

Step by step

Understand the three parts.The element itself, a sheathed loop reaching down into the water. A rod thermostat running down the middle of it, which switches the element off when the water around it reaches the set temperature. And a thermal cut-out, a small safety switch behind the cap that trips permanently if the thermostat fails and the water overheats.
Find the switch and the circuit.Immersions have a dedicated double pole switch, usually a large switch with a neon on the airing cupboard wall, fed by its own circuit at the consumer unit. Some houses have two elements — a short top one for a quick small volume, and a long bottom one for the whole cylinder — with two switches.
When it stops heating, check the switch first.Neon lit means the switch is passing power. No neon means either the switch is off, the neon has failed, or the circuit has tripped. Check the consumer unit for a tripped breaker or RCD on that circuit.
Check the timer or the tariff.Many immersions are on an economy tariff timer or a boost switch, so nothing happens outside the cheap-rate window unless you press boost. This is a surprisingly common 'fault' after a house move or a power cut resets a timer.
Give it long enough.A full cylinder from cold takes a substantial time on an immersion — commonly an hour and a half to three hours depending on cylinder size and element rating. Judging it after twenty minutes tells you nothing.
Feel the cylinder to see how far it heated.Hot at the very top only means the top element is working and the bottom one is not, or the water is only being heated locally. Cold all over after a long run means no heat is going in at all.
Stop at the cap.That is where householder diagnosis ends. Behind the plastic cap are live mains terminals, the thermostat and the cut-out. Resetting a cut-out, testing an element or replacing one is electrician's work, and replacement also requires the cylinder to be drained down and the element unscrewed with a proper immersion spanner.
Tell the electrician what you found.Whether the neon lights, whether the breaker holds, whether the cylinder gets warm at all and for how long you ran it. That turns a long diagnostic visit into a short one.

Tips

If your boiler dies in midwinter, the immersion is the reason you can still have a bath. Test it once a year even if you never otherwise use it, so you know it works.
In a hard water area, elements scale up and their life shortens. A scaled element runs hot, is less efficient and eventually fails.
Never switch the immersion on after draining down without checking the cylinder has refilled completely.
An immersion running continuously is an expensive way to heat water. If you are relying on it while the boiler is broken, use the boost and a short window rather than leaving it on.

If it doesn't work

Neon on, cylinder stays cold for hours

Cause: Failed element, or a tripped thermal cut-out that has not reset — Fix: Both are behind the cap. Switch it off and call an electrician. A cut-out that has tripped also means the thermostat failed, so both usually get replaced together.

Breaker trips as soon as the immersion is switched on

Cause: The element has failed to earth — the sheath has perforated and water is in contact with the element — Fix: Leave it off and leave the breaker off. This is a genuine electrical fault and needs an electrician; do not keep resetting it.

Water gets scalding hot no matter what the dial says

Cause: The rod thermostat has failed closed and the element never switches off — Fix: Switch the immersion off at the double pole switch now. Running it in this state will eventually trip the thermal cut-out, and on a vented cylinder can produce steam. Electrician.

Only heats a small amount of water

Cause: The short top element is being used rather than the long bottom one, or the bottom element has failed — Fix: If you have two switches, try the other. If you only have one and it used to heat the whole cylinder, the element has partly failed.

Worked before, nothing since a power cut

Cause: An economy tariff timer or the boost circuit has reset — Fix: Reset the timer to the correct time and check the cheap-rate window. Costs nothing and is by far the most common cause after an outage.

Questions people ask

Is it cheaper to heat water with the immersion or the boiler?

Almost always the boiler on gas, because electricity costs several times more per unit of heat. The immersion earns its keep as a backup, on an economy tariff overnight, or where a small amount of water is needed and firing the boiler for it makes little sense.

Can I replace the element myself?

No. It requires the cylinder drained, a dedicated immersion spanner, correct sealing and reconnection of a fixed high-current circuit. It is electrical work on a fixed appliance and the cylinder side has to be refilled and vented correctly afterwards.

Why does mine have two switches?

Two elements. A short one heating just the top of the cylinder for a quick modest amount, and a full-length one heating the whole thing. Useful for economy tariffs, where you heat the whole cylinder overnight cheaply and top up briefly during the day.