If you smell gas, or a CO alarm sounds
Do not use light switches, electrical appliances or naked flames. Open doors and windows, turn the gas supply off at the meter if you can reach it safely, leave the property and call the National Gas Emergency Service on 0800 111 999 — a free 24-hour national emergency line. If anyone feels unwell, seek medical advice.
Four things worth checking first
None of these involve opening the boiler, and between them they account for a large share of callouts that turn out not to need one.
- Power and controls. Is the boiler powered? Has a fuse or breaker tripped? Is the programmer actually calling for heat, and has the time reset after a power cut? Clocks going back or a brief outage account for a surprising number of "broken" boilers.
- System pressure. The gauge should sit in the normal band, typically somewhere around one bar when cold. Too low and the boiler will not fire; too high and the pressure relief valve may discharge.
- Gas supply. Are your other gas appliances working? If nothing gas-fired works, the problem may be upstream of the boiler — including a prepayment meter that has run out.
- The condensate pipe, in cold weather. A frozen external condensate run will shut a boiler down safely and is one of the most common winter faults. Thawing it gently, from the outside, often restores normal operation.
What common symptoms usually mean
These are the usual suspects rather than diagnoses. The point is to help you describe the problem accurately, which makes any visit shorter.
- No heating and no hot water
- Check the obvious first: power, the programmer and time settings, system pressure, and whether the gas supply is on. If everything looks normal and the boiler will not fire, it needs diagnosing.
- Heating works but no hot water (or the reverse)
- On a combi this often points to a diverter valve. On a stored system it is more often the cylinder, its thermostat or a motorised valve.
- Pressure keeps dropping
- Topping up more than very occasionally means water is leaving the system — a leak on the pipework or a radiator, or a failed expansion vessel or pressure relief valve.
- Boiler locks out and shows a fault code
- Note the exact code before resetting. Codes narrow the diagnosis considerably, and repeated lockouts on the same code are a pattern rather than a coincidence.
- Kettling — a sound like a boiling kettle
- Usually scale or sludge restricting flow through the heat exchanger. It is a system condition problem as much as a boiler one, so a flush is often part of the fix.
- Radiators cold at the top, or cold downstairs only
- Air in the system, or circulation and balancing problems. Frequently not a boiler fault at all, which is worth establishing before anyone quotes to replace it.
- Nothing works after a cold night
- A frozen condensate pipe is a very common winter cause. The external run freezes and the boiler shuts down safely.
- Water visibly leaking from the boiler
- Turn off the water supply to the boiler and switch it off. Do not run it while it is leaking — water and electrics are a poor combination.
Fault codes are worth writing down
Modern boilers display a code when they lock out, and that code is genuinely informative — it narrows the fault to a subsystem before anyone has looked at the appliance. Write down the exact code, and note whether it appears immediately on firing or after the boiler has been running.
Resetting once is reasonable. Resetting repeatedly is not: the lockout is a protective function, and repeatedly overriding it both hides the underlying fault and can make the eventual repair larger.
When it is the system, not the boiler
A significant share of "boiler problems" are heating system problems. Cold spots, slow warm-up, noisy pipework and radiators that never quite get there are frequently caused by sludge, air, poor balancing or a tired pump rather than by the appliance.
This matters commercially. A boiler quoted for replacement because "it cannot heat the house" may be perfectly serviceable behind a system that needs cleaning. It is a fair question to ask anyone who recommends replacement.
Judging whether a repair is worth it
The honest test is the cost of the repair set against the remaining life of the appliance. A single component failure on a boiler with years left in it is usually worth fixing. Repeated faults, an ageing appliance, or parts that are becoming difficult to source all point the other way — at that point the money is better spent on something that comes with a warranty.
Our boiler replacement page sets out that decision in more detail, including a rough guide to where the line usually falls.
Most repairs are preventable
Annual servicing catches a good proportion of these faults while they are still small, inexpensive and not happening in February. See boiler servicing for what a service actually covers.