Glow-worm F29 Fault: Flame Loss During Operation — Causes & What To Do
The F29 fault on a Glow-worm boiler means the burner flame was lost while the boiler was running. It often points to a gas pressure issue, a flue problem, a faulty gas valve, or a fan fault. You may get heating that works then cuts out.
What does the Glow-worm F29 fault code mean?
F29 indicates the burner flame was lost during operation. The boiler detected the flame disappear and locked out for safety. You may get intermittent heating that runs for a while then stops.
Why does my Glow-worm boiler keep losing the flame (F29)?
The most common causes, in order of how often we see them:
- • Incorrect gas pressure — a dip or instability in the gas supply can blow the flame out faster than the boiler can recover.
- • Flue issue — a blockage, restriction or draft in the flue can extinguish the flame.
- • Faulty gas valve — the gas valve may close or adjust intermittently, starving the burner.
- • Fan fault — if the fan isn't clearing flue gases correctly, the boiler shuts the flame down for safety.
- • Flame detection electrode — a failing electrode can make the boiler think the flame has gone when it hasn't.
Can I reset the F29 fault?
You can reset the boiler and it may run again for a while. But if F29 keeps recurring, there's an underlying cause — a gas pressure, flue, gas valve or fan issue. Repeatedly resetting without fixing the cause risks the boiler locking out more often; book an engineer if it returns.
Can I fix this myself?
Safe homeowner checks
- Check whether other gas appliances still work
- Check the boiler pressure gauge
- Read the fault code and note when the boiler cuts out
- Try one reset — but if F29 returns, don't keep resetting
Engineer required
- Diagnosing or replacing the gas valve
- Checking the flue for blockage or restriction
- Testing the fan and flue-gas clearance
- Replacing the flame detection electrode
- PCB diagnosis and any work inside the boiler casing
Gas work legally requires a Gas Safe Registered engineer. Tampering with gas appliances is dangerous and unlawful.
Safe checks you can make first
- • Check other gas appliances — if they're also behaving oddly, the gas supply may be unstable. Contact your gas supplier.
- • Check the pressure gauge and repressurise to 1.2 bar if it's low, then reset.
- • Note whether the boiler runs for a set time before cutting out — that pattern helps the engineer diagnose a flame sensor or overheating issue.
- • If F29 returns after a reset, stop resetting and book a Gas Safe engineer.
Need this fault diagnosed? Book a Gas Safe engineer.
What should you NOT attempt yourself?
The gas valve, flue connections and fan sit inside or on the boiler casing and involve gas. Any work inside the casing or on the flue is gas work that must be done by a Gas Safe Registered engineer.
What will a Gas Safe engineer check?
- • Check gas pressure and supply at the boiler under operation.
- • Inspect the flue for blockages, restrictions or drafts.
- • Test the gas valve stays open and adjusts correctly under load.
- • Test the fan and flue-gas clearance.
- • Test the flame detection electrode and PCB flame signal.
- • Replace the faulty part, then re-commission with a combustion check.
How Urban Suburban can help
Our Gas Safe Registered (594547) engineers repair every Glow-worm model — Betacom, Ultracom, Flexicom and Energy — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors, fans and ignition parts on the van. Most Glow-worm fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.
Related Glow-worm fault codes
Frequently asked questions
What does the Glow-worm F29 fault code mean?
F29 means the burner flame was lost while the boiler was running. The boiler detected the flame disappear and locked out. You may get intermittent heating that runs then cuts out.
What's the difference between F29 and F28?
F28 means ignition was unsuccessful — the boiler couldn't light at all, so no flame was established. F29 means the boiler lit successfully but the flame went out during operation. They can share causes (gas supply, flue) but F29 often points to a gas pressure, gas valve or fan issue.
Is an F29 fault dangerous?
The boiler locks out for safety when the flame is lost, so it's not immediately dangerous — but it shouldn't be ignored. If you can smell gas when it cuts out, turn off the gas at the meter and call the National Gas Emergency Service on 0800 111 999.
How much does it cost to fix a Glow-worm F29 fault?
Labour typically runs £90–£220 plus the £150 call-out (which includes the first hour). If a gas valve, fan or flue part needs replacing, that's quoted and agreed before fitting.
Technical content & review
Technical content by URBAN SUBURBAN GAS
Gas Safe Registration: 594547 · NVQ Qualified engineers
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