Immersion Heater Stopped Working: Fuse, Stat or Element?

Start with the power: the checks you can do safely

Most immersion heaters that suddenly stop are not broken at all. Power is not getting to them. Work through these before assuming the worst. None of them means taking a cover off.

• The switch: most immersions are wired through a 20 amp double-pole switch near the cylinder, often marked water heater, with a neon light. Check it is on and the light is lit.
• Dual switches: older cylinders have a bath and sink switch that selects the long lower element or the short upper one. Try both positions. If one works and the other does not, one element has failed.
• The timer: many immersions run through a timer or a smart controller. Check it is on, set for now, and has not lost its time after a power cut. Override it to constant on as a test.
• Off-peak supply: on Economy 7 or a similar tariff, the lower element may only get power overnight from a separate circuit. If the upper element works in the day and the water is cold in the morning, the night circuit or its timer is the suspect.
• The consumer unit: look for a tripped breaker labelled immersion or water heater, or a tripped RCD that has taken out a group of circuits.
• The fuse: if the immersion runs from a fused spur rather than a double-pole switch, the fuse in the spur may have blown. Switch off before removing it.

An immersion heater runs at 230 volts and draws around 13 amps. Do not remove the cap on top of the element or touch the wiring unless the circuit is switched off at the consumer unit and you are competent to work on it. If anything smells burnt or the switch feels hot, turn it off at the consumer unit and leave it.

Breaker trips as soon as you switch on

This is the most definite symptom of the lot. If the breaker or RCD trips instantly when the immersion is switched on, the element has almost certainly failed. The metal sheath has cracked or corroded, water has reached the heating wire inside, and current is leaking to earth.

Leave it switched off at the consumer unit. Do not keep resetting the breaker, because each reset puts current into a failed element sitting in water. A replacement element is the fix, and the thermostat is usually changed at the same time.

If the breaker trips only after the immersion has been on for a while, the element can still be the cause, but a loose or overheated connection at the switch or on the element terminals is also possible. Scorch marks or a brown, brittle switch plate point to the connection.

Power is on but the water stays cold

If the switch light is on, nothing trips and the water stays cold, the thermostat or the element is at fault. The thermostat is the rod that sits inside the element, with a dial under the cap, usually set to about 60°C. That setting matters: stored hot water should be kept at 60°C to control Legionella bacteria.

Modern immersion thermostats have a built-in safety cut-out with a manual reset button, often a small red or white button beside the dial. It trips if the water gets far too hot, which only happens when the thermostat has stuck on. So a tripped cut-out is a symptom, not the fault. If it has tripped, the thermostat has usually failed and needs replacing, even if resetting it brings the hot water back for a while. Our guide on how to reset an immersion heater explains the safe way to do it with the power off.

If the thermostat is fine and power reaches the element, the element itself has gone open circuit. The wire inside has burnt through and it simply does not heat. A standard 3kW element should measure somewhere around 17 to 20 ohms across its terminals when tested cold, and an open circuit reads as infinite. The full test is in our guide to testing an immersion heater element.

Water too hot, or boiling in the pipes

Scalding water, a cylinder that bangs or hisses, or hot water spurting from the vent pipe over the loft tank mean the thermostat has stuck on. Switch the immersion off at the switch straight away and leave it off.

If you have an unvented cylinder, water discharging into the tundish, the small funnel on the pipe from the safety valves, means the cylinder has overheated or over-pressured. Switch off the immersion and any boiler supply to the cylinder, keep clear of the hot discharge, never cap or block the tundish, and call an engineer the same day. Our page on water running into the tundish covers what that means.

On an unvented cylinder, a replacement element or thermostat must be the one the cylinder maker specifies, with its own thermal cut-out. A generic part can defeat one of the cylinder’s safety layers.

Why immersion elements fail in London

Scale is the main cause. London water is hard, and every time an element heats, it drops a layer of limescale onto its own surface. The scale holds the heat in the element, the sheath runs hotter than it should, and in time it splits or burns through. You often hear it coming: a crackling or fizzing from the cylinder while the immersion is on.

Elements in hard water areas are often made with Incoloy or titanium sheaths, which cope with scale far better than the older copper ones. If your old element lasted only a few years, ask for one of those when it is replaced.

An element that has been switched on while the cylinder was empty or part-drained will also fail within seconds. That is called dry firing, and it is why the cylinder is always refilled and vented before the power goes back on after any work.

What the engineer does and what it usually costs

The engineer isolates the circuit and proves it dead, then tests the thermostat and the element with a meter, including an insulation test on the element to find a leak to earth. If the element has failed, the cylinder is drained down below the element boss using the drain valve, the old element is unscrewed with an immersion spanner, and the new one goes in on a fresh washer. Most elements screw into a standard 2¼ inch boss. The cylinder is then refilled and vented, and the element checked for leaks before the power is switched back on.

The visit is the £60 call-out, and the repair price is agreed with you on site before any work starts. Our immersion heater repair and replacement service covers elements, thermostats, switches and timers. For comparison, these are typical UK market ranges, parts and labour together.

• Thermostat replaced: typically about £80 to £150
• Element and thermostat replaced on a vented cylinder: typically about £150 to £300
• Element on an unvented cylinder, maker-specified part: typically about £200 to £400
• Double-pole switch or timer replaced: typically about £80 to £180

If the cylinder itself is corroded around the element boss or weeping from its seams, a new element can disturb it. The engineer will say so before starting. Our guide to immersion heater replacement cost covers the bigger jobs.