Glow-worm F.1 Fault Code
The F.1 fault is distinct from F.28 — the Glow-worm successfully established a flame, but the ionisation probe stopped detecting it during operation. This points to a different fault pathway: the ignition system is functioning, but something is disrupting the flame after it establishes.
What is the Glow-worm F.1 fault code?
The Glow-worm F.1 fault is triggered when the boiler establishes a flame on startup but the ionisation probe loses the flame signal during the firing cycle. Because ignition was initially successful, the ignition electrode and spark generation are typically not the issue. The investigation focuses on why the flame was lost: a dirty ionisation probe that loses sensitivity mid-cycle, an intermittent gas valve that cannot sustain gas flow, a condensate backpressure issue disrupting combustion, or a flue restriction that starves the flame of air during operation. A Gas Safe engineer is required to distinguish between these causes.
What causes the F.1 fault?
The probe detects the flame by measuring electrical conductivity through the flame itself. If it is coated with combustion deposits, it may detect the flame initially but lose the signal as the boiler warms up. Common in South London hard water areas where combustion deposits accumulate faster.
An ageing gas valve may open fully for ignition but intermittently restrict gas flow during operation. The flame establishes but weakens and goes out under load, triggering F.1.
A partially blocked or frozen condensate pipe creates backpressure in the combustion chamber. The boiler lights correctly but the backpressure disrupts combustion air flow and extinguishes the flame shortly after ignition.
A partially blocked flue terminal restricts the escape of combustion gases. The boiler lights but the flue restriction builds backpressure during the firing cycle, extinguishing the flame. The F.9 proving sequence may have passed during fan startup before the restriction became apparent under load.
A loose or corroded connector at the ionisation probe can cause intermittent loss of signal during operation, triggering F.1 even when the physical probe and flame are functioning correctly.
What you can safely check yourself
Test another gas appliance. An intermittent gas supply from the network can cause the flame to establish and then go out — but this is rare and usually affects all appliances simultaneously.
In cold weather, a frozen or partially blocked condensate pipe is the most common cause of F.1. Pour warm water over the external condensate pipe to thaw it, then attempt one reset.
One reset is acceptable. If F.1 returns on the first or second heating cycle, call a Gas Safe engineer. The flame loss pattern will help the engineer identify the cause.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Clean or replace the ionisation probe
- Inspect ionisation probe connector and wiring for corrosion
- Test gas valve for consistent flow during a firing cycle
- Check gas inlet and manifold pressure under load
- Inspect flue terminal and condensate trap for partial blockages
- Test PCB ionisation sensing circuit if probe and wiring test normal
- Commission and test boiler through multiple heating cycles after repair
F.1 flame loss after ignition is more frequent in South London than in low-hardness areas. Thames Water's calcium-rich supply accelerates combustion deposit build-up on the ionisation probe across Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley. An annual service that includes probe cleaning is the most effective way to prevent F.1 from recurring. Winter F.1 faults from condensate backpressure are also common across all of Reion's South London coverage area.
How much does a F.1 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Glow-worm boiler showing F.1
Do not reset if you smell gas — leave the property and call 0800 111 999.
In winter, check the condensate pipe — thaw if frozen.
Hold the reset button for 3–5 seconds.
Allow a full heating cycle — note whether the boiler lights before F.1 appears.
If F.1 appears on the first or second cycle, call a Gas Safe engineer.
Ionisation probe replacement is a low-to-mid cost repair — typically £80–£160 all in. On any Glow-worm boiler it is worth doing. Gas valve replacement at £260–£440 is worth considering on boilers under 11 years old; on older boilers Reion will give an honest repair-vs-replace comparison at the diagnostic visit.
Common questions about the Glow-worm F.1 fault
What is the difference between Glow-worm F.28 and F.1?
Can a Glow-worm F.1 fault fix itself?
Why does my Glow-worm boiler show F.1 in cold weather?
How much does a Glow-worm F.1 repair cost in London?
Why does my Glow-worm F.1 fault keep coming back?
Other Glow-worm fault codes
Need a Glow-worm engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
