A pilot light is a small, permanently burning flame inside an older gas boiler. Its sole purpose is to ignite the main burner when heating or hot water is demanded. If it goes out, the boiler will not fire - leaving you without heating or hot water until it is relit.
The relighting procedure is straightforward on older manual boilers and is within the scope of any responsible homeowner. However, the process is entirely different for modern boilers installed in the last 15 or so years. Confusing the two wastes time at best - and can cause damage at worst. This guide explains both, step by step.
What is a Boiler Pilot Light?
A pilot light is a continuously burning gas flame, typically around 2-3cm in size, housed inside the boiler's combustion chamber. It acts as a permanent ignition source. When the boiler calls for heat - triggered by a thermostat or hot water demand - gas flows to the main burner, which the pilot light ignites instantly.
Most boilers with a pilot light were manufactured before approximately 2000-2005. They are less energy efficient than modern systems because the pilot burns gas continuously, even when no heating is required. This is one reason why manufacturers moved to electronic ignition systems on modern boilers.
Older Boilers vs Modern Boilers: Know the Difference
Before doing anything, confirm what type of boiler you have. The difference dictates the entire approach.
Manual Pilot Light
Typically 15 or more years old. Has a small inspection window through which you can see a permanent small blue flame. Examples include older Potterton Kingfisher, Glow-worm, and back boiler units behind a gas fire.
Fix: Follow the step-by-step relight procedure below.
Electronic Ignition (No Pilot)
Most boilers installed in the last 15 years - Worcester Bosch, Vaillant, Baxi, Ideal, and others - use an electronic spark igniter. There is no permanent pilot flame. When they fault, they display an error code and require a reset, not a relight.
Fix: See Section 4 - Modern Boiler Reset.
| Feature | Older Boiler (Pilot Light) | Modern Boiler (Electronic) |
|---|---|---|
| Ignition | MANUAL Permanent pilot flame | AUTOMATIC Electronic spark on demand |
| When it fails | Pilot flame visible through window is absent | Error or fault code displayed on control panel |
| User remedy | Relight using control knob and igniter | Press reset button - do not manually light |
| Key safety component | Thermocouple | Flame sensor / ionisation probe |
| When to call an engineer | Pilot won't stay lit after correct procedure | Fault code won't clear after 2-3 resets |
How to Relight an Older Manual Pilot Light
If you have confirmed you have an older boiler with a manual pilot, follow these steps carefully. Always refer to the specific manufacturer's instructions where available, as control layouts vary between models.
Locate the Gas Control Knob
Find the main control dial on the front or underside of the boiler casing. It will be marked with positions for "Off", "Pilot" (sometimes shown as a spark or star symbol), and "On". Take a moment to identify each position before proceeding.
Turn to 'Off' and Wait
Turn the dial fully to the "Off" position. Wait a minimum of five minutes - ten is preferable. This is a non-negotiable safety step that allows any residual unburnt gas inside the combustion chamber to fully disperse before a flame is introduced. Do not skip or shorten this step.
Turn to 'Pilot' and Press In
Turn the dial to the "Pilot" position. Press the control knob or button firmly inward and hold it there. Pressing in manually opens the gas valve to allow a small flow of gas to reach the pilot jet. You must maintain this inward pressure continuously throughout steps 4 and 5.
Ignite the Pilot Flame
Whilst continuing to hold the knob depressed, ignite the pilot using one of two methods depending on your boiler model:
- Piezo igniter button: Press the separate ignition button (usually red or black, marked with a spark symbol) repeatedly until the pilot lights.
- Long-reach lighter or match: If no separate igniter button is present, carefully introduce a lit long-reach lighter to the pilot jet opening.
Hold for 30-60 Seconds, Then Release
Once the pilot flame ignites, do not release the control knob. Continue holding it pressed in for 30 to 60 seconds. This is the time required for the thermocouple to heat up and generate enough electrical current to hold the gas valve open independently.
Slowly release the knob. If the flame remains alight, turn the dial from "Pilot" to "On". The boiler should now operate normally.
Resetting a Modern Boiler (No Pilot Light)
If your boiler is a modern combi or system boiler - such as a Worcester Bosch Greenstar, Vaillant ecoTEC, Baxi 600, or Ideal Logic - it does not have a pilot light. These boilers use an electronic spark to ignite the main burner on demand. When they develop a fault, they lock out and display a fault code on the control panel.
Common Fault Codes by Brand
- Worcester Bosch: E133, EA, 227
- Vaillant: F.22, F.28, F.29, F.75
- Baxi: E1, E119, E168
- Ideal: F1, F2, F4
- Potterton: E1, E2 (modern range)
How to Reset
- Note the fault code displayed
- Locate the reset button - usually clearly labelled
- Press or hold the reset button as directed (typically 3 seconds)
- Listen for the fan and then an ignition click
- If it fails after 2-3 attempts, stop and call an engineer
Troubleshooting: Why Won't the Pilot Stay Lit?
If you followed the procedure correctly but the flame drops out the moment you release the control knob, one of these faults is almost certainly responsible.
Faulty or Worn Thermocouple
The number one cause of a pilot that will not stay lit. The thermocouple is a wear component that degrades over time and with repeated heating and cooling cycles. When it fails, it cannot generate sufficient electrical current to hold the safety gas valve open, so the valve closes and the flame extinguishes. Thermocouple replacement is a routine, cost-effective repair carried out by a Gas Safe registered engineer.
Blocked or Dirty Pilot Jet
Carbon deposits and airborne debris can partially block the small orifice that feeds gas to the pilot. The resulting weak or intermittent flame may not make reliable contact with the thermocouple tip, meaning it never heats adequately. An engineer can clean or replace the pilot assembly during a service visit.
Draughts and Air Infiltration
Older floor-standing boilers and some wall-hung models can develop gaps in the casing over time. Draughts entering the combustion chamber can extinguish the pilot, particularly during windy weather or in exposed locations. Casing inspection and draught sealing are required.
Yellow or Orange Pilot Flame
Do's & Don'ts
Do
- Check for the smell of gas before any action
- Turn off and wait at least 5 minutes before attempting to light
- Hold the control knob in for the full 30-60 seconds
- Check the pilot flame colour is blue before using the boiler
- Refer to your boiler's instruction manual for model-specific steps
- Call a Gas Safe engineer if the pilot will not stay lit after 2-3 attempts
- Ensure carbon monoxide detectors are fitted and working
Don't
- Attempt to relight if you can smell gas
- Rush or skip the waiting period after turning off
- Release the control knob too early before the thermocouple heats up
- Try to manually light the pilot on a modern electronic ignition boiler
- Repeatedly reset a modern boiler that won't restart
- Use the boiler if the pilot flame is yellow or orange
- Attempt gas valve or thermocouple replacement yourself
Quick Reference Checklist
Before You Start
- Confirmed there is no smell of gas in the property
- Identified boiler type - older manual pilot or modern electronic ignition
- Located the control dial and identified Off / Pilot / On positions
- Turned dial to Off and waited at least 5 minutes
- Have a long-reach lighter or know where the piezo igniter button is
After Relighting
- Pilot flame is steady and burns blue - not yellow or orange
- Control dial turned from Pilot to On
- Boiler fires normally when thermostat demands heat
- Carbon monoxide detector is installed and functioning
- If pilot keeps going out - contacted a Gas Safe registered engineer
