Not every boiler that has stopped is broken. A good share of the calls that come in on the first genuinely cold night of the year turn out to be a frozen pipe, a reset clock or a pressure gauge that has quietly drifted down over the summer. None of those needs an engineer, and all of them are checkable in about five minutes.
What follows is the order worth working through. Before any of it, though, there are two situations where you should stop reading and act, and they come first for a reason.
Stop immediately if either of these applies
If you can smell gas: do not look for the source, do not turn anything electrical on or off, and do not use a phone inside the property. Turn the gas off at the meter if you can reach it safely, open doors and windows, get everybody outside, and ring the National Gas Emergency Service on 0800 111 999. That line is free and staffed around the clock, and it is the right first call ahead of any plumber, ourselves included.
If a carbon monoxide alarm sounds, or if people in the house have headaches, nausea or drowsiness that get better when they leave: turn the appliance off, open the windows, get everyone out and ring the same number. Carbon monoxide has no smell and no colour, and the symptoms are easy to mistake for a winter bug. Nothing in the rest of this guide is worth weighing against either of those.
The seven checks, in order
Start with the pressure gauge, because it is the most common single cause. Below about half a bar on a cold system and most boilers will not fire. Our separate guide covers what the reading should be and how to bring it back safely.
Second, the controls. A power cut resets the clock on many programmers, and a boiler obediently following a schedule that now thinks it is Tuesday at four in the morning looks exactly like a broken boiler. Check the programmer, the room thermostat and its batteries, and any smart thermostat's connection — a battery-flat wireless thermostat simply stops asking for heat.
Third, the electrics. Boilers have their own fused spur, often a switch beside the appliance or in an airing cupboard, and it gets turned off by accident more often than you would think. Check the consumer unit for a tripped breaker or RCD while you are there.
Fourth, other gas appliances. If the hob and the fire are dead too, the problem is upstream of the boiler — the supply, the meter, or a prepayment meter that has run out. That saves you a wasted call-out.
Fifth, in cold weather, the condensate pipe. This is the plastic pipe running out of the bottom of the boiler, usually down an outside wall. When it freezes the boiler locks out, and it is the single biggest cause of breakdowns on the first hard frost. A warm cloth wrapped around the outside of the pipe, or warm — never boiling — water poured slowly along it, will usually clear the blockage. Once the boiler restarts, that pipe is worth insulating or rerouting so it does not happen every winter.
Sixth, the fault code. Most modern boilers display one, and it is worth writing down before you reset anything. It tells an engineer on the phone what to bring, and it is the difference between one visit and two.
Seventh, reset once. Almost all boilers have a reset button and almost all faults that are going to clear will clear on the first attempt. If it locks out again, stop. A lockout is a protective response on a gas appliance, and repeatedly forcing past it is precisely the wrong instinct.
- Pressure gauge — below half a bar cold and it will not fire
- Programmer, room thermostat, thermostat batteries
- The boiler's own fused spur, and the consumer unit
- Do other gas appliances work? If not, it is the supply
- Condensate pipe frozen — warm cloth or warm water, never boiling
- Write down the fault code before resetting
- Reset once. If it locks out again, ring somebody
What the common symptoms usually mean
Hot water but no heating, or heating but no hot water, on a combi is very often the diverter valve — the component that decides which of the two the boiler is serving. It is a repair, not a replacement.
Radiators warm at the top and cold at the bottom is sludge that has settled in the radiator, and it does not respond to bleeding. Cold at the top and warm at the bottom is air, and that one does respond to bleeding. The two look similar from the doorway and mean completely different things.
Banging or kettling noises from the boiler is usually scale or debris in the heat exchanger, common in London's hard water. A boiler that fires briefly then shuts down repeatedly is short-cycling, which can be an oversized appliance, a blockage or a controls fault. None of those is an emergency, but none of them fixes itself either.
When it is worth ringing straight away
If the property has no heating and no hot water and there is a young child, an elderly resident or anyone unwell in it, do not spend the evening working through a list. Ring somebody. The same applies to any water escaping that you cannot stop, and to any fault that has already been reset once and returned.
We work evenings and weekends, which covers most of the hours people actually discover these things, and for a genuine emergency we come out at night as well. What we will not do is take the booking and let you find out: we say on the phone whether we can reach you tonight or in the morning, and if it is the morning, that is worth knowing in the first minute of the call rather than the eighth hour of waiting.