Immersion Heater Keeps Tripping? Why, and What to Do

First: what exactly is tripping?

Three different things get called tripping, and they mean different faults. Open the consumer unit, usually under the stairs or in a hall cupboard, and look at which switch has dropped. Then look at the immersion switch on the wall near the cylinder.

• The RCD has dropped, taking several circuits with it: current is leaking to earth somewhere on the immersion circuit. The element is the usual suspect.
• Only the immersion's own breaker has dropped: either an earth fault on a circuit with its own protection, or too much current, which points to a short in the element or wiring.
• The fuse in the immersion switch keeps blowing, or the switch is scorched: the switch, the fuse or the connections are overheating. This is a fire risk. Leave it off.
• Nothing in the consumer unit has moved, but the immersion has stopped heating: that is probably not a trip at all but the cut out on the immersion thermostat, which is covered in our guide on how to reset an immersion heater.

When it trips tells you even more

Watch the timing. It is one of the most useful clues an engineer gets, and you can give it over the phone.

• The moment you switch it on: a hard earth fault. Almost always the element has broken down, or water is sitting in the terminal head.
• After ten minutes to an hour of heating: the element's insulation is breaking down as it warms up. It will get worse, not better.
• Only the first time after it has been off for weeks, then fine: the element is starting to fail and is taking on moisture while it sits cold. Get it tested before it fails completely.
• Only when the oven, kettle or washing machine is on as well: the immersion may be sharing a circuit it should not, or an RCD is close to its limit because of small leakages across several appliances.

The usual cause: a failed element

An immersion element is a heating wire packed inside a metal tube with an insulating powder. When the tube corrodes through or splits, water gets into that powder and current leaks through it to the metal of the cylinder and the earth wire. That leak is exactly what an RCD is designed to spot.

Elements fail faster in hard water. Scale builds up on the element, it runs hotter than it should to push heat through the scale, and eventually the sheath fails. In most of London that is the main reason elements do not last, and our guide to hard water and hot water cylinders in London explains what scale does to the rest of the cylinder too. A failed element cannot be dried out or repaired. It has to be replaced, which means isolating the cylinder and partly or fully draining it first.

Water in the terminal head

The element screws into a boss on the cylinder, and the wiring connects under a plastic cap on top. If the seal around the element weeps, or water drips onto the cap from a fitting above, it can track inside and trip the RCD just as convincingly as a failed element. Look for damp, rust or white crusty deposits around the immersion head, and water marks on the cylinder jacket beneath it.

If that is the cause, the repair may be a new seal and a dry out of the connections rather than a new element, which is one reason the element should be tested before anyone replaces it. Our guide to an immersion heater leaking shows where the water usually comes from.

The switch, the fuse and the cable

A 3 kilowatt immersion element draws close to 13 amps for hours at a time. That is a heavy, continuous load. Immersions are normally on their own circuit with a 20 amp double pole switch. Where one has been wired through an ordinary 13 amp fused spur, the fuse runs at its limit, gets hot, and eventually blows or scorches the switch. The flex from the switch to the cylinder can also be damaged where it touches the hot pipework or the immersion boss.

A scorched switch plate, a burning smell near the cylinder or a fuse that keeps blowing all need the circuit switched off at the consumer unit and left off until it has been looked at.

Do not take the cap off the immersion head or open the switch unless the circuit is isolated at the consumer unit and you are competent to prove it dead. A tripping immersion is an electrical fault on a cylinder full of water.

What you can safely do now

None of these involve opening anything electrical.

• Switch the immersion off at its wall switch and leave it off.
• Reset the RCD or breaker so the rest of the house has power. If it will not reset with the immersion switched off, the fault is elsewhere or in the wiring, and you need an electrician.
• Use the boiler for hot water if your cylinder has a coil, and our guide on what an immersion heater is explains how the two work together.
• Look for water around the top of the cylinder and on the immersion cap, without touching the wiring.
• Note what tripped and when, and photograph the cylinder label and the immersion switch.

What the repair involves

The engineer tests the element with an insulation resistance tester, which shows whether current is leaking to earth, and checks it for continuity. The switch, flex and connections are checked at the same time. If the element has failed, it is replaced with the correct length and type for your cylinder, and in hard water it is worth fitting an element designed for scale. The cylinder is drained enough to get the element out, refilled and vented, and the new element is tested before it goes back into use.

On a tank fed cylinder this is a routine job, and our immersion heater repair page covers it. On an unvented cylinder the element sits in a sealed, pressurised vessel, so the work must be done by an engineer with the G3 unvented qualification, under our unvented cylinder repair service. If the immersion has simply stopped heating without tripping anything, our page on an immersion heater not working takes you through that instead.

When to call us

Call us on 020 4652 5111 or send an enquiry once you have switched the immersion off. Tell us what tripped, when it tripped, and whether you have loft tanks or a tundish near the cylinder, and send a photo of the cylinder label if you can. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.