Why Is My Boiler Not Firing Up?
A boiler that won't fire up is usually a gas-supply, ignition or condensate issue. Here's the order to check things in — and when to stop and call an engineer.
First: check your gas supply
Before assuming the boiler is broken, confirm gas is reaching it. Light a hob or gas fire — if it works, gas is fine and the fault is on the boiler. If no gas appliance works, check your prepayment meter has credit and the emergency control valve is open, then call your gas supplier or National Gas Emergency on 0800 111 999. This single check avoids hundreds of unnecessary call-outs.
The 7 reasons a boiler won't fire up
Once gas is confirmed, the cause is almost always one of these:
- No or low gas supply to the boiler only (a partly closed isolation valve)
- A frozen condensate pipe in cold weather — the most common winter cause
- A dirty or worn ignition electrode failing to spark
- A failing flame-sensing electrode reporting 'no flame' even when lit
- Low system pressure locking the boiler out (below 0.5–0.6 bar)
- A blocked or poorly terminated flue
- A failing gas valve or control board
The frozen condensate pipe — the winter classic
Condensing boilers produce a trickle of slightly acidic water that drains outside through a plastic pipe. In freezing weather that pipe can ice up, the boiler can't drain, and it locks out with an ignition fault code (Worcester EA, Vaillant F28, Baxi 133, Ideal F1/L2). If you can safely reach the external pipe, pour warm — not boiling — water over it to thaw it. Long term, lagging the pipe or rerouting it to a frost-protected drain stops it recurring.
What to do before calling an engineer
A few quick checks can save a call-out, or at least speed one up:
- Confirm other gas appliances work and the meter has credit.
- Check the boiler pressure gauge — top up to 1.0–1.5 bar if it's low.
- Press the reset button once, and only once. If the fault returns, stop.
- Note the exact fault code shown on the display — this tells an engineer a lot before they arrive.
When not to keep resetting
Repeatedly resetting a boiler that won't ignite can damage the gas valve and, in some faults, is unsafe. If a reset doesn't clear the code the first time, the boiler is telling you it needs an engineer. Stop retrying and call — repeated ignition attempts on a faulty gas valve are a known cause of avoidable damage.
Frequently asked questions
Why won't my boiler fire up but the heating is on?
If the radiators heat but the boiler won't fire for hot water, the fault is often the diverter valve or the hot-water temperature sensor (e.g. Worcester Bosch CD). If neither works and the boiler won't ignite at all, suspect gas supply, ignition or a frozen condensate pipe.
Can a frozen condensate pipe stop a boiler working?
Yes — it's the single most common winter call-out. The condensate drain ices up, the boiler can't empty it, and it locks out with an ignition fault. Thaw the external pipe gently with warm water; lagging it prevents it happening again.
How do I reset my boiler?
Press and hold the reset button (usually on the front panel) for 3–5 seconds. The boiler will attempt to relight. Press it only once — if the fault code returns, don't keep resetting; call a Gas Safe engineer.
What does it mean if my boiler tries to light then clicks off?
That click-and-out pattern is a classic ignition or flame-detection fault — the spark fires but the boiler can't confirm the flame, so it shuts the gas valve for safety. It needs a Gas Safe engineer to inspect the ignition and flame-sensing electrodes.