Boiler not working: what to check before you call

First, what exactly has stopped?

"Not working" covers several different faults, and the pattern points to where the problem is. A combi boiler that gives neither heating nor hot water has usually stopped as a whole. A system that still gives hot water but no heating, or the other way round, is often a valve, pump or control problem rather than a dead boiler. Before you start, note which of these describes your home. If you have hot water but no heating, or no hot water but heating works, our separate guides to those two faults go into more detail.

Check 1: is the boiler getting power?

A blank display usually means no electricity. Check the fused switch next to the boiler is on and, if the boiler is plugged in, that the plug is in and switched on. Then look at your consumer unit for a tripped switch. If a circuit trips again as soon as you reset it, leave it off and call, because something on that circuit has an electrical fault.

If there has been a power cut, the boiler may be fine but the programmer may have lost its settings. Our guide to heating controls after a power cut explains how to get the times and temperatures back without upsetting anything else.

Check 2: are the controls asking for heat?

A surprising number of call-outs end at the thermostat. Turn the room thermostat up a few degrees above the room temperature and listen for a click. Check the programmer or timer is set to on or constant, not off, and that the clock shows the right time. If you have a smart thermostat, check its batteries and whether the app shows it as connected. A wireless thermostat that has lost contact with its receiver will not call for heat even though its screen looks normal.

On a boiler with a separate hot water cylinder, the heating and hot water are usually controlled independently, so check both channels on the programmer.

Check 3: what does the pressure gauge say?

Most modern boilers are on a sealed system and will not run if the pressure is too low. Look at the gauge on the front or underneath the boiler. For most boilers, the cold reading should be somewhere around 1 to 1.5 bar, but check the handbook for yours. If the needle is close to zero, the boiler has probably locked out to protect itself.

You can top up the pressure using the filling loop if you are comfortable doing so and the handbook explains how. But pressure does not disappear on its own. If you have had to top it up more than once in a few weeks, there is a leak somewhere, often a radiator valve, the pressure relief valve or the boiler itself. Our guide to a boiler losing pressure covers where that water goes.

Check 4: is the gas on?

Check another gas appliance, such as the hob, if you have one. If that will not light either, the problem is the gas supply rather than the boiler. On a prepayment meter, check there is credit, including emergency credit. The emergency control valve next to the meter should have its handle in line with the pipe when it is on.

Do not try to work out a gas fault yourself beyond that. If you smell gas at any point, open the windows, do not use light switches or anything that could spark, turn the gas off at the meter if you can do it safely, leave the property and call the National Gas Emergency Service on 0800 111 999.

If you smell gas, or anyone in the house has headaches, dizziness or nausea that ease when they go outside, stop and call 0800 111 999. Do not wait for a boiler repair appointment. Our guide to carbon monoxide at home lists the signs to look for.

Check 5: has the condensate pipe frozen?

Condensing boilers produce water that drains away through a plastic pipe, usually running outside to a drain. In a hard frost that pipe can freeze, the water backs up and the boiler shuts down, often with a fault code and a gurgling sound. It is one of the most common reasons for a boiler not working in the first cold snap of winter.

If the outside section of pipe is frozen, pour warm water, not boiling, over it slowly, starting at the end nearest the drain. Then reset the boiler once, following the handbook. If the pipe keeps freezing, it is probably too narrow, too long outside or badly insulated, and an engineer can reroute or upgrade it.

Should you reset the boiler?

Once you have fixed an obvious cause, such as low pressure or a frozen pipe, one reset following the manufacturer's instructions is reasonable. Our guide on how to reset a boiler explains where the button usually is and what it does.

Resetting over and over is a different matter. A boiler that locks out, restarts and then locks out again is telling you a fault has not gone away. Repeated resets can make the fault harder to diagnose and, depending on the fault, are not safe. If it locks out a second time, leave it off and note the code, the time it happened and what the boiler was doing. We explain what that pattern usually means in boiler lockout keeps returning.

When it is time to call an engineer

Call if the checks above have not brought the boiler back, if the same code keeps returning, if there is water leaking from the boiler, if it is making loud banging or grinding noises, or if the pressure keeps dropping. Any work inside the boiler's casing, or on its gas supply, must be done by a Gas Safe registered engineer. That is a legal requirement, not a formality.

It also helps to know when a repair is worth doing. On an older boiler, a big part such as a heat exchanger can cost enough that replacement is worth comparing. Our guide on whether to repair or replace a boiler sets out how to weigh that up, but get a diagnosis first. Many faults on older boilers are still cheap and quick to fix.

Getting your boiler fixed

Loyal Plumbers works across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Gas work is carried out by Gas Safe registered engineers. When you call, tell us the boiler make and model if you can see it, any code on the display, what the pressure gauge reads, whether you have heating, hot water or neither, and which of the checks above you have already tried. That lets the engineer arrive with a better idea of what to look for.

Our call-out charge is £60. The engineer will explain what they have found and what the repair involves before any work starts. For a vulnerable household with no heating or hot water in cold weather, tell us when you call so we can take that into account. See our boiler repair and servicing page for more on how we handle breakdowns.