Boiler breakdown: the first hour

When the boiler stops, the house notices quickly: the radiators cool, then the hot taps run cold, and routines built around hot water and heating fall apart. Most breakdowns are genuine faults that need a qualified engineer. Even when the fault is real, though, the checks below are the same information a heating engineer will ask for when you call, so working through them first tends to shorten the job whichever way it goes.

First, the safety checks

These come before everything else in this sheet. If you can smell gas, or hear hissing near the appliance or the meter, follow the official gas emergency instructions and leave the property. Do not go hunting for the fault and do not operate switches or appliances. Gas work afterwards, including relighting, reconnecting and repairing, belongs to a Gas Safe registered engineer. GOV.UK and the Health and Safety Executive pages on domestic gas safety set out the legal requirements and the emergency procedure; follow those as the primary source rather than any number quoted second-hand.

Six checks, in the order a heating engineer will ask for them

Work down this list and note what you find. The notes matter later, both for your own record and for whoever you call.

1. The pressure dial. Read it with the system cold and compare it with the figure in your boiler's own manual, since the correct cold pressure and the level at which a particular model locks out both vary by make and model. Many sealed-system boilers that have dropped well below their stated cold pressure lock out instead of running, and a low reading is a common reason a boiler appears to have broken down. If the manual calls for a top-up, the filling loop procedure in that manual is the one to follow. We keep a separate sheet on what the dial is telling you.
2. The power. Is the switch at the wall on, and has a tripped fuse or breaker cut the spur the boiler sits on? A boiler with no lights at all is an electrical question before it is a plumbing one.
3. The controls. Thermostat turned down, timer slipped onto the wrong programme, a room thermostat sitting in direct sun or next to a draught, batteries gone in a wireless one. Set everything to "constant" and see if the boiler responds before you assume the worst.
4. The condensate pipe. In freezing weather, the white plastic pipe running from the boiler to a drain can freeze solid, and many boilers shut down rather than spill. Warm the outside run gently, a hot water bottle or low hair-dryer heat, never a naked flame, and the boiler may restart itself.
5. The error code. If the display shows a letter-and-number code, write it down exactly, then look it up in your boiler's own manual. The code tells an engineer the fault family before they arrive, which changes what goes in the van.
6. The water. Are the cold taps running? A mains stoppage or a burst elsewhere can look exactly like boiler failure.

When it is worth resetting

Your boiler's manual will say whether a reset is permitted and how many attempts are reasonable on your model, so follow that rather than a general rule. If a condensate thaw or a pressure top-up has cleared the cause, a reset may bring the boiler back and you can watch it. What is best avoided is the reset-every-morning routine: a boiler that locks out nightly is telling you something, and repeatedly forcing it on without fixing the cause is the kind of thing manufacturers warn against, because an underlying fault is left to run.

What a breakdown visit involves

A firm that quotes a repair over the phone, before anyone has seen the boiler, is quoting on guesswork. What you can reasonably expect instead: a diagnosis fee agreed before the visit, the fault named in writing afterwards, and then a quote for the fix with the parts and the labour separated, which you approve before any repair work starts. If the honest answer is that the boiler has reached the end of its life, that should be said plainly too, with replacement versus repair compared openly rather than pushed one way.

The two faults that never wait

Water pouring from the boiler, or a flue that has come loose from the wall, are same-hour problems: isolate the water at the stop tap, switch the appliance off at the wall if it is safe to reach, and call a qualified engineer. WaterSafe Register approved contractors are a useful starting point for the water-side side of this. Any smell of gas goes back to the safety checks at the top of this sheet, then the official emergency procedure.

What this sheet does not cover

This sheet does not include specific fault-code diagnosis, because codes differ between manufacturers and interpreting them safely needs the maker's own documentation for your model. It also does not cover oil boilers, which are an OFTEC matter, or the annual service that prevents many of these calls. If your boiler is still under warranty, read the manufacturer's terms before booking anyone, because some warranties require their own engineer and can otherwise be affected. For any work on a gas appliance, use an engineer on the Gas Safe Register. For water-side work on leaks or the stop tap, WaterSafe Register approved contractors are a useful starting point.