Immersion Heater Not Working? How to Find the Fault

Nine times out of ten, an immersion heater that has stopped working has not failed at all. The thermostat has tripped out on its safety cut-out, and it needs resetting rather than replacing. That is a small job. The element itself failing is the expensive outcome, and it is the less likely one.

Knowing which you are dealing with saves you buying a part you did not need. Here is how the two look different, plus the switch and supply faults that account for most of the rest.

What an immersion heater actually is

An electric element, usually 3kW, screwed into the side or top of your hot water cylinder, sitting in the water. A thermostat pushed down inside the element's pocket senses water temperature and switches the element on and off to hold your set temperature.

Built into that same thermostat is a thermal cut-out. If the water gets dangerously hot, typically because the thermostat has failed to switch off, the cut-out latches open and kills the element. It stays open until someone presses the reset button. That is a safety device working correctly, and it is the single most common reason an immersion "stops working".

Larger cylinders often have two elements, a short one near the top for a quick partial heat and a long one near the bottom to heat the full tank. If you have two switches, that is why, and one can fail while the other still works.

Start with the switch and the supply

Before touching the cylinder, rule out the boring causes.

• Is the switch on? The immersion switch is usually a 20A double-pole switch with a neon, in the airing cupboard or just outside it. If the neon is dark, either there is no supply or the neon has failed, which happens.
• Has the breaker tripped? An immersion is on its own circuit at the consumer unit. Look for a switch that has dropped, or an RCD that has tripped. Reset it once. If it goes again, stop, because a failing element leaking to earth is exactly what makes an RCD trip.
• Is there a timer involved? Plenty of immersions run through a time switch or an off-peak tariff. A timer that has lost its programming after a power cut looks exactly like a dead immersion.
• Any heat at the switch? A warm or discoloured immersion switch means the contacts are struggling. A 3kW load through a tired switch is a fire risk, and the smell is unmistakeable. Same warning as we give about a burning smell from any socket or accessory.

If the breaker keeps going, our guide to why your fuse box keeps tripping covers what the pattern tells you, and what an RCD is explains why an earth leakage trip is different from an overload.

The thermostat cut-out, and how to spot it

Classic signs the cut-out has operated:

• The water went unusually hot shortly before it stopped working.
• Everything else on the circuit is fine, nothing has tripped, the switch neon is lit.
• It has done this before and someone "pressed a button" to fix it.
• You have recently had a power cut or a plumbing issue that let the cylinder run dry.

The reset is a small button behind the cover on the end of the element. Access means removing a cover plate, which means live terminals, which means isolating the circuit at the consumer unit properly first. If you are not confident isolating and proving dead, this is a fifteen minute job for an electrician and not one to improvise on.

What matters more than the reset is the reason it tripped. A cut-out that operates once after a power cut is one thing. A cut-out that operates every few weeks means the thermostat is failing to control temperature, and the correct fix is a new thermostat, not repeatedly pressing reset. Each trip is the safety device catching water that got hot enough to scald.

When it really is the element

Elements do fail, usually in one of two ways.

Open circuit. The element burns through internally. No current flows, no heat, nothing trips. From outside it looks the same as a cut-out that has operated, which is why it gets tested rather than guessed.

Leaking to earth. The insulation inside the element breaks down and current finds its way to the cylinder. This trips the RCD, often the moment you switch on. Scale accelerates it, and most of London is a hard water area, so elements here work harder and die sooner than the manufacturers' figures suggest.

An electrician confirms which with an insulation resistance test and a continuity test across the element. Two minutes with the right tester and there is no more guessing. That is the point of getting it tested rather than swapping parts hopefully.

Replacing an element is a plumbing job as much as an electrical one. The cylinder has to be drained below the element, the old one unscrewed with an immersion spanner, and in older cylinders they seize solid. Sometimes the element comes out cleanly. Sometimes the cylinder is damaged getting it out, and you find out the cylinder was the real problem.

Heats, but not enough

A separate symptom worth naming. If you get some hot water but it runs out quickly, or the tank never gets properly hot:

• Only the top element is working, on a twin-element cylinder. You get the top third of the tank and no more.
• The thermostat is set low. Most sit around 60 to 65 degrees. Lower and you notice.
• The element is heavily scaled, so it transfers heat poorly and takes far longer to bring the tank up.
• Cylinder insulation has failed, so the tank heats and then loses it. Older cylinders with a loose jacket rather than sprayed foam are the usual culprits.

What you can safely do yourself

• Check the switch is on and the neon lit.
• Check the breaker at the consumer unit, and reset once only.
• Check any timer or programmer has not lost its settings.
• Note whether anything else on the same RCD went off at the same time.

Leave the cover on the element. Behind it are exposed terminals carrying 13 amps, in a cupboard, next to a tank of water.

Repair or replace

Rough rule from doing plenty of these. A thermostat reset or replacement is a small job and always worth doing. An element replacement on a cylinder under about ten years old is usually worth it. On an older cylinder, particularly a vented copper one with a tired jacket, the cost of the element plus the risk of damaging the cylinder makes replacing the cylinder the better conversation to have, and that brings a plumber in alongside us.

If your immersion is the backup to a boiler and you only use it occasionally, run it for an hour every month or two. Elements that sit unused in hard water seize and scale far faster than ones in regular use.

Frequently asked questions

Why has my immersion heater stopped working but not tripped anything?

That points at the thermostat cut-out having operated, or the element having gone open circuit. Neither trips a breaker. They look identical from outside the cover, so it comes down to a continuity test to tell them apart.

Where is the reset button on an immersion heater?

Behind the cover on the exposed end of the element, on the thermostat itself. It is usually a small red or black button you press in until it clicks. Reaching it means removing a cover over live terminals, so isolate the circuit at the consumer unit first and prove it dead.

How long does an immersion heater element last?

Commonly around five to ten years, though hard water shortens that noticeably and most of London is hard water. An element that is heavily scaled works harder for less output and fails sooner.

Why does my immersion heater keep tripping the RCD?

Almost always an element leaking current to earth through broken-down insulation. It is a genuine fault and the RCD is doing exactly what it should. Repeatedly resetting it puts the fault back into a tank of water. Get it tested.

Can I replace an immersion heater myself?

The electrical connection needs to be done and tested properly, and the job also involves draining the cylinder and dealing with an element that may be seized into it. It is not notifiable work in the way a new circuit is, but it is a poor DIY choice, and a badly reseated element floods airing cupboards.

Should I leave the immersion heater on all the time?

Generally no. A well-insulated cylinder holds heat for hours, so heating on a timer to suit when you actually use hot water costs less than holding it at temperature around the clock. If you are on an off-peak tariff, the timer matters even more.

No hot water and not sure whether it is the thermostat, the element or the switch? We test rather than guess, so you pay for the part that actually failed. See our fault finding and repairs service or call 020 3653 2600. If there is any smell of burning at the immersion switch, turn the circuit off and ring us straight away.