Boiler tripping the electrics: why it happens and what to do

This guide is for when a switch at your fuse box flips off because of the boiler. If the boiler stops by itself and shows a fault code, but the fuse box is fine, that's a lockout. See our guide to a boiler that keeps turning off instead. If the checks below don't help, you'll need a boiler repair from a Gas Safe engineer, or an electrician. This guide is part of our common boiler problems series.

Why does my boiler trip the electrics?

A boiler trips the electrics when one of its electrical parts leaks power. A safety switch at the fuse box spots the leak and cuts the power in a split second. A worn pump, fan or control board can cause it. So can water dripping onto the electrics.

A few words you'll see in this guide:

• Consumer unit. The modern name for your fuse box. It holds the switches for each circuit in your home.
• RCD. A switch in the consumer unit that cuts the power if electricity leaks. It protects you from shocks.
• MCB, or circuit breaker. A smaller switch for one circuit. It trips if the circuit draws too much power, or if wires touch.
• Fault to earth. When power leaks out of a part and into the metal case or earth wire. This is what an RCD looks for.
• Fused spur. The switch on the wall near the boiler that powers it. It has a small fuse inside.

Most boiler trips flip the RCD, because the usual fault is a leak to earth. On many fuse boxes, one RCD covers several circuits. So when it trips, other things in the house go off too.

What should I check first?

First, note which switch tripped and when. Then look for water near the boiler, and try turning the boiler off at its fused spur (see the safe test below). These checks are all done from the outside. Never open the boiler case or the fused spur.

• Which switch tripped? Is it an RCD, or one of the smaller MCBs? Most fuse boxes label them. A photo of the fuse box helps whoever comes out.
• When does it trip? As the heating comes on, when you run a hot tap, at night, or after rain? Think back to any past trips too, and note them down.
• Is there water? Look under and around the boiler for drips, damp or rust stains. If you see any, see our guide to a leaking boiler.
• Has anything else tripped it? Has the same switch tripped before when you used the kettle, washing machine or an outside light? If so, the boiler may not be the problem.

Is it the boiler or the wiring?

A simple test can show if the boiler is to blame. Before you reset, turn the boiler off at its fused spur. Use your one reset for this test. If the switch stays on with the boiler off, the fault is likely the boiler. If it trips anyway, it's likely the wiring or something else on that circuit. Either way, leave it as it is and call.

1. If your boiler's spur has no switch, skip this test. Leave the tripped switch off and call.
2. Make sure your hands are dry, and that the spur and the wall round it are dry too. If the spur or the wall round it is wet, don't touch it. Leave the power off at the fuse box and call.
3. Turn the fused spur next to the boiler to off. Don't take out its fuse or unscrew it.
4. At the fuse box, push the tripped switch fully down, then back up. This is your one reset.
5. If it trips again, even with the boiler off, leave it off. The fault is likely in the wiring or something else on that circuit.
6. If it stays on, leave the boiler off at the spur. Don't turn the boiler back on to test it. The boiler side is the likely cause.
7. Either way, call us and say what happened.

The test isn't perfect. Some faults only show up now and then, such as in wet weather. And the boiler's spur often powers the thermostat, timer and valves too, so those are part of the "boiler side". Still, it gives a useful clue to share when you call.

When does it trip, and what does that tell you?

The moment it trips is the best clue to the cause. This table shows the usual links. It's a guide, not a diagnosis.

Why does my boiler trip the electrics when the heating comes on?

When the heating comes on, several electrical parts start at once. If one of them has a fault to earth, the RCD trips as soon as it gets power. That's why the trip often happens at the same time each day, when the timer clicks on.

• The pump. It pushes hot water round your radiators. Its motor can wear, or a leaking seal can wet its electrics.
• The fan. It pushes the fumes out through the flue. A worn fan motor can leak power.
• The ignition. It makes the spark that lights the gas. A worn lead can spark to the case instead.
• The control board, or PCB. The boiler's main circuit board. Damp or a burnt part on it can trip the power.
• Valves outside the boiler. On systems with a hot water cylinder, motorised valves open when the heating starts. These are zone valves. Their small motors can fail too.

All of these are parts of the boiler or the heating system. Anything inside the boiler case needs a Gas Safe engineer. If it only trips when you run a hot tap on a combi, say so when you call. It helps narrow down which parts were running.

Why does my boiler trip the electrics in wet or cold weather?

In wet weather, the usual cause is water getting where it shouldn't. Rain can get into the boiler through the flue or a gap in the wall, and reach the electrics. In cold weather, damp in outside wiring can be the cause.

• Rain through the flue. The flue is the pipe that takes fumes outside. If its seal or cover is damaged, water can run back into the boiler.
• A leak inside the case. A dripping joint or pump seal can drip onto the PCB or wiring.
• A blocked condensate trap. Condensing boilers make waste water as they run. It drains through a trap inside the case. If the trap blocks or cracks, water can spill inside.
• Damp outside wiring. An outside light, socket or garden supply on the same RCD can trip it when it rains.
• Frost protection. Some systems have a frost thermostat. It turns the boiler on when it gets very cold. If a part has a fault, the trip happens on frosty nights only. A boiler in a garage or loft is more open to damp and cold.

If the cold weather has frozen the condensate pipe outside, the boiler locks out instead of tripping. See our guide to a frozen condensate pipe.

Is it safe to keep resetting the trip switch?

No. Reset it once, and if it trips again, leave it off. Use that one reset for the spur test above. The switch is doing its job. It has found a leak of power that could give someone a shock or start to heat up a part.

• Never tape, wedge or hold a switch in the on position.
• Never open the fused spur or change its fuse for a bigger one.
• Never take the cover off the boiler to look inside.
• If the boiler is off for a while in winter, use a safe plug-in heater. Never use a gas cooker or oven to heat a room.

If one RCD powers your fridge or freezer too, and it won't stay on, call sooner rather than later. Our article on why RCDs matter explains how they keep you safe.

Who should I call, an electrician or a gas engineer?

Call a Gas Safe engineer if the fault is in the boiler, and an electrician if it's in the wiring. The trouble is, it's often hard to tell which, even after the spur test. A fault can sit in the boiler, its controls, the cable to it, or something else on the same RCD.

That's where someone who does both trades helps. We are NICEIC approved electricians and Gas Safe registered engineers. So one visit can cover the boiler, its controls and the wiring that feeds them.

For boiler faults, see our boiler repair page. If other circuits trip too, see our electrician page. An EICR (a report on the safety of all your wiring) can show if older wiring needs work.

Do you need a new boiler if it keeps tripping the electrics?

Usually not. A boiler that trips the electrics most often needs one electrical part fixed or swapped, such as a pump, fan or PCB. And sometimes the boiler isn't at fault at all. A wiring fault or another appliance can be to blame.

A new boiler is worth talking about when:

• water has got inside an older boiler and harmed more than one electrical part
• the faulty part is no longer made for your model, so a repair isn't possible
• the boiler trips again after earlier repairs, as other parts wear out in turn

If water caused it, find where it came from first. A new boiler won't stay dry if the flue or a pipe still lets water in. We'll talk through the options with you and tell you which one makes more sense. If it's a new boiler, our boiler installation page explains the free survey and fixed price.

When to call a Gas Safe engineer or electrician

Call for help if the switch trips again after one reset. A trip that keeps coming back means a fault is still there, even if the power stays on for a while. By law, paid gas work must be done by someone on the Gas Safe Register.

• the switch stays on only while the boiler is off at its spur
• it trips each time the heating comes on, or each time it rains
• you see water, rust or stains on or under the boiler
• the boiler is dead, but nothing has tripped
• you smell burning, or see scorch marks on the boiler, spur or fuse box

Tell us which switch tripped, when it happens and your boiler make and model. That's only a starting point. The engineer first runs a thorough check of the boiler and wiring. That finds the fault and the parts needed. See our boiler repair page for the faults we fix. We repair all makes.

A yearly boiler service helps catch problems early. The engineer checks for leaks inside the case and looks at the parts that can wear.

Other boiler guides

• Boiler keeps turning off: lockouts, short cycling and when to reset.
• Leaking boiler: what to do first, and where the water is coming from.
• Boiler not working after a power cut: how to reset it, and set the timer clock.
• Boiler won't turn on: the checks to try first, and what a pilot light is.
• Frozen condensate pipe: how to thaw it safely.
• Boiler making noise: what banging, gurgling and kettling mean.
• Hot water but no heating: thermostat, timer and valve checks.
• Heating works but no hot water: checks to try, and what a diverter valve does.
• Boiler losing pressure: what the gauge should read and how to top it up.
• Boiler pressure too high: what is too high, and how to lower it safely.
• Cold radiators: what cold at the top or bottom means.
• All common boiler problems.

Boiler still tripping the electrics? Book a boiler repair and we'll find the cause.