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Pilot Light Keeps Going Out
A standing pilot on a fire, back boiler or water heater lights, then blows out minutes or hours later.
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What's happening?
A standing pilot is a small permanent flame that lights the main burner on demand and, more importantly, keeps a thermocouple hot. That thermocouple generates a tiny voltage that holds the gas valve open; when the pilot goes out and the thermocouple cools, the valve shuts and the appliance is safe. A pilot that will not stay lit is therefore two different problems wearing the same face: either the pilot itself is being disturbed — draught, blockage, dirt — or the thermocouple is no longer producing enough to hold the valve. HSE specifically lists a pilot light that frequently blows out as one of the signs that combustion is not right, which is why it is worth a service call rather than repeated relighting.
What usually causes it?
- A worn or failing thermocouple no longer generating enough millivolts to hold the valve
- A partly blocked pilot injector, so the flame is small and does not envelop the thermocouple tip
- Lint, dust or spider webbing in the pilot assembly — very common in airing cupboards and garages
- A draught down an oversized or unlined chimney knocking the flame out
- A downdraught in windy weather at a poorly sited or damaged flue terminal
- A loose thermocouple connection at the gas valve, or a kinked lead
- Low gas pressure at the appliance, often alongside other appliances misbehaving
Is it dangerous?
The safety device is doing its job, so a pilot going out is not itself a leak — the valve shuts. The risks are indirect: repeated relighting attempts push unburnt gas into the room, and a pilot being extinguished by downdraught means the flue is not working, which is the same condition that spills carbon monoxide when the main burner fires. Do not attempt to hold the valve open, wedge anything, or bypass the thermocouple. If the appliance is an open-flued fire or back boiler and you also see soot or condensation, treat it as the higher-risk fault.
Can I fix it myself?
Relighting to the manufacturer's printed sequence is a homeowner job on appliances that have a pilot lighting procedure — hold the knob in for the full period stated, usually twenty to thirty seconds, before releasing. You can also close windows and doors on the windward side to see whether wind is the trigger, and check that a nearby extractor is not running. Do not dismantle the pilot assembly, clean the injector or replace the thermocouple: those are Gas Safe tasks.
When should you call a gas engineer?
Call a Gas Safe registered engineer if it goes out more than occasionally, if you have to relight it more than once a day, or if it fails in windy weather — that last one is a flue issue, not a part. Call urgently instead of relighting if you smell gas, if a CO alarm has sounded, or if there is any staining around the appliance. On an appliance older than about fifteen years, ask whether the pilot assembly is still available before paying for a diagnosis.
What will the gas engineer actually do?
- Measure the thermocouple output in millivolts and the valve's drop-out point, then replace the thermocouple if it is weak
- Clean or replace the pilot injector and the pilot assembly, and reset the flame so it wraps the thermocouple tip
- Check gas pressure at the appliance under working and standing conditions
- Run a flue flow and spillage test to prove the flue is not the cause
- Check ventilation to the room and any extract that could be pulling the flame about
- Relight, run the appliance for a full cycle and record combustion readings before leaving
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