Why a Boiler Pilot Light Keeps Going Out: Causes and What to Do
While most modern boilers no longer use a standing pilot light, a significant number of older boilers still installed in London homes rely on a continuously burning pilot flame to ignite the main burner. When this pilot light repeatedly extinguishes, the boiler loses the ability to fire and produces no heat or hot water. Understanding the common causes helps London homeowners decide whether a reset, a cleaning attempt, or an engineer is needed.
Which Boilers Still Have a Standing Pilot Light
Modern condensing boilers, which represent the vast majority of boilers installed in London homes since 2005, do not have a standing pilot light. Instead they use an electronic spark ignition system that generates a spark to light the burner each time heating or hot water is demanded. However, many London homes, particularly those in Victorian, Edwardian, and post-war period properties, still have older boilers installed. Boilers manufactured before approximately 2000, including older Potterton, Glowworm, Ideal, Baxi, and Vokera models, frequently used a continuously burning pilot flame. These boilers are now well beyond their design service life and replacement is the recommended course of action, but while they remain in service, understanding the pilot light system is useful.
A standing pilot light burns continuously and is held in place by the thermocouple, a safety device that generates a small electrical current when heated by the flame. This current holds open a safety valve that allows gas to flow to the pilot. If the pilot flame extinguishes for any reason, the thermocouple cools, the current ceases, the safety valve closes, and gas supply to the pilot is cut off. This is the fundamental safety mechanism that prevents unburned gas from accumulating if the pilot goes out. To relight the pilot, the homeowner must press and hold a button or valve that manually bypasses the safety valve, apply a flame, and hold the button for long enough for the thermocouple to heat up and generate sufficient current to hold the valve open independently.
Common Reasons the Pilot Light Extinguishes Repeatedly
The most common cause of a pilot light that repeatedly goes out on an older boiler is a degraded or failed thermocouple. The thermocouple is a consumable component with a typical service life of two to five years. As it ages, the junction between the two dissimilar metals degrades and the millivoltage it generates when heated decreases. When the output falls below the minimum required to hold the safety valve open, the pilot flame extinguishes shortly after the manual hold button is released. Thermocouple replacement is a straightforward repair for a Gas Safe engineer and the component itself is inexpensive, making it a cost-effective fix on an otherwise functional older boiler.
A second common cause is a partially blocked pilot jet. The pilot jet is a small brass fitting with a very fine orifice through which gas flows to the pilot flame. Over years of operation, the jet can accumulate a partial blockage from corrosion products or small debris in the gas supply, reducing the gas flow to the pilot and producing a small, weak flame that is more easily extinguished by draughts or condensate. Cleaning or replacing the pilot jet resolves this issue. A strong draught entering the boiler casing through a gap in the flue or through the boiler casing itself is a third cause, particularly in older boilers where the casing seals have deteriorated. Any pilot light fault on an older boiler should be assessed by a Gas Safe engineer who can also confirm that the boiler flue is drawing correctly and that combustion products are not spilling into the room. Prestige Engineers service and repair older boilers in London while also advising homeowners on the significant efficiency and reliability benefits of replacement with a modern condensing boiler.