What does the pilot light do?
On older gas boilers, the pilot light is a small flame that burns all the time. When the boiler needs to heat, the main burner opens and the pilot lights it. Next to the pilot sits a thermocouple, a metal probe that produces a tiny electrical current when heated by the flame. That current holds the gas valve open. If the pilot goes out, the thermocouple cools, and the valve closes to stop unburnt gas escaping. It is a simple and important safety system.
Many older boilers in Birmingham’s Victorian, Edwardian and inter-war homes, including back boilers behind gas fires, still work this way.
Why does the pilot keep going out?
- Faulty thermocouple. The most common cause. A worn thermocouple produces too little current to hold the valve, so the pilot drops out after a few seconds or minutes.
- Dirty or partly blocked pilot jet. A weak, yellow or flickering pilot flame may not heat the thermocouple properly.
- Draughts. A strong wind down the flue or a draught through the case can blow the pilot out, especially on open-flued appliances.
- Gas supply problems. A prepayment meter running out, or low gas pressure, will put the pilot out.
- Faulty gas valve. Less common, but the valve itself can fail to stay open.
What is safe to do yourself?
- Check other gas appliances and any prepayment meter.
- Look at the flame through the window if your boiler has one. Crisp blue is good; yellow or orange is a warning sign.
- Relight it once only if your user instructions describe how, and follow them exactly.
- If it goes out again, leave it off and send an enquiry.
Any sign of soot, staining around the boiler, a yellow flame or a carbon monoxide alarm means turning the appliance off and getting it checked straight away.
Is it time to replace an old boiler?
Not necessarily. A thermocouple is often an inexpensive repair. But a boiler old enough to have a pilot light is likely to be much less efficient than a modern one, and parts for some old models are getting harder to find. The engineer will give you an honest view of repair versus replacement. The boiler repair cost guide has typical prices for both.