Boiler Pilot Light Keeps Going Out

What the pilot light and thermocouple actually do

Only older boilers have a permanent pilot light: typically floor-standing boilers, back boilers behind a gas fire such as the Baxi Bermuda, and wall-hung models fitted before electronic ignition took over. Modern condensing boilers light with a spark each time and have no pilot, so if yours has a digital display and no small blue flame visible through a window, this page is not your fault. Our guide to a boiler that will not fire up covers those.

The pilot is a small flame that burns all the time and lights the main burner when the heating calls. Sitting in that flame is the tip of the thermocouple, a thin metal probe joined to the gas valve by a narrow copper lead. Heat makes it produce a tiny electrical current, measured in millivolts, which holds a magnet in the gas valve open. If the flame goes out, the current stops, the magnet lets go, and the gas shuts off. It is a flame failure safety device, and it is doing its job every time the pilot goes out.

Is it safe to relight the pilot light yourself?

Relighting a pilot is a normal user operation on these boilers, and the instructions are usually printed inside the lower cover or on the front panel. Follow those for your model. The general sequence is to turn the gas control knob to the pilot position, press and hold it, light the pilot with the built-in igniter button, and keep holding the knob for the time the instructions give, often somewhere between 15 and 60 seconds, so the thermocouple heats up before you release it.

• Smell of gas at any point: do not relight. Open windows, turn the gas off at the meter if safe, use no switches or flames, then leave the property and call the National Gas Emergency Service on 0800 111 999
• Pilot flame yellow, orange, lazy or leaving black soot: turn the boiler off and do not use it. It is burning badly and can produce carbon monoxide
• Sooting or staining on the boiler case or around the flue: same, switch off and call an engineer
• The pilot has gone out several times this week: one relight to get heat tonight is reasonable, but book a repair

If an attempt fails, turn the knob to off and wait at least as long as the instructions say, usually a few minutes, before trying again so unburnt gas can clear. If you smell gas, stop, ventilate, turn off at the meter if it is safe to, and call 0800 111 999 from outside. Never light the main burner directly, and never wedge the gas control knob down.

Why the pilot light keeps going out

The way it goes out tells you most of what you need to know. Work through the patterns in this order.

It lights, but goes out the moment you let go of the knob. This is the classic thermocouple fault. The flame is there but the thermocouple is not producing enough current to hold the valve open. Thermocouples wear out with years of heat, and a tip that has burnt thin, cracked or become crusted gives a weaker signal. A loose nut where the thermocouple screws into the gas valve can do the same.

It stays lit for hours or days, then goes out on its own. That points to a flame that is only just keeping the thermocouple warm. The usual reason is a partly blocked pilot jet, from dust, spider webs or corrosion, which gives a small or wavering flame that does not fully wrap the thermocouple tip. Pilot jets on boilers in garages and outbuildings suffer most.

It goes out when it is windy. Open-flued and balanced-flue boilers can be blown out by strong gusts, particularly where the flue terminal is in a corner or near a gap between buildings. Occasional wind blow-outs are a nuisance. Frequent ones, or any sign of fumes coming back into the room, mean the flue needs checking, because a flue that lets air back down may also let combustion gases back in.

It goes out when the main burner lights. A drop in gas pressure when the burner opens can starve the pilot. If the gas has stopped altogether on a prepayment meter, top up and press the meter’s button to reopen the supply. Otherwise it can be an undersized gas pipe, a failing regulator at the meter, or a fault in the gas valve itself.

What the engineer does on the visit

An engineer first looks at the pilot flame. A healthy one is crisp blue and wraps the top 10 to 13 millimetres of the thermocouple. If it is small or lifting away, they clean or replace the pilot jet before blaming anything else.

Next they test the thermocouple output with a multimeter while the pilot is lit. A low reading means a new thermocouple, which is a universal or model-specific part and usually quick to fit. If the output is good but the valve still drops out, the fault is the magnet unit in the gas valve. On some valves only that unit is replaced, on others the whole valve.

Any visit to a flame fault on an older boiler should include checking gas pressure at the valve and a combustion and flue check, because a pilot that keeps going out can be the first sign of a flue problem. Replacing a thermocouple or pilot assembly means working on the gas valve, which is gas work. Gas work is carried out by Gas Safe registered engineers. Our boiler repair visit covers pilot and thermocouple faults on older boilers.

What it usually costs, and whether it is worth fixing

The £60 call-out covers getting an engineer to you, and the repair price is agreed with you on site before any work starts. The full rate card is on our pricing page. These are typical UK market ranges, parts and labour together.

• Pilot jet cleaned and flame set up: usually a short job
• Thermocouple replaced: typically about £80 to £160
• Pilot assembly or igniter replaced: typically about £100 to £250
• Gas valve or magnet unit replaced: typically about £200 to £450, where parts are still made

Pilot-light boilers are now old, and some parts are getting hard to find. A thermocouple is almost always worth replacing. A gas valve on a boiler that is also sooting or leaking is often the point to weigh up a replacement. Our guide to repairing or replacing a broken boiler sets out how to decide.

Keeping warm if it will not stay lit tonight

If the pilot will not stay lit and you are waiting for an engineer, leave the gas control knob in the off position so no gas is released. Use portable electric heaters rather than anything that burns fuel indoors, and if you have a hot water cylinder with an immersion heater, switch that on for washing.

Older people, babies and anyone unwell should not spend a cold night without heat. Our guide to a boiler breakdown in winter with someone vulnerable at home covers what to prioritise, and what to do when the boiler breaks down at night covers keeping the house safe until the morning.