Glow-worm F29 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F29 fault code mean?
The F29 code on a Glow-Worm boiler means the burner managed to light but the flame then cut out unexpectedly while the boiler was running. The boiler's flame-sensing circuit detected the loss and triggered an automatic safety lockout to prevent unburned gas from building up. It is worth distinguishing F29 from the closely related F28 code: F28 means the boiler failed to establish a flame in the first place, whereas F29 means ignition succeeded but the flame could not be maintained. Both codes point broadly to gas supply or ignition issues, but F29 specifically indicates the problem occurs after the initial spark. The boiler will stop producing heat and hot water until the underlying cause is resolved and the unit is reset.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Frozen or blocked condensate pipe Common
During cold spells, the plastic condensate pipe that runs from the boiler to an outside drain can freeze solid. This blocks the boiler's ability to expel acidic condensate water, causing it to shut down as a precaution. It is the single most frequent reason for F29 faults in winter and is something a homeowner can often resolve without calling an engineer.
- Interrupted or low gas supply Common
If the gas pressure reaching the burner drops too low — whether because a prepayment meter has run out of credit, a supply issue affecting the street, or a partially closed gas isolation valve — the flame will be starved and extinguish. Checking whether other gas appliances in the home are working normally will quickly confirm or rule this out.
- Faulty or worn ignition electrodes Sometimes
The spark electrode ignites the burner, and a separate flame-sensing electrode (or the same probe in some designs) confirms the flame is present. If either electrode is cracked, coated with deposits, or positioned incorrectly, the boiler may light briefly but then decide no flame is present and shut down. This is a relatively straightforward repair for a Gas Safe engineer.
- Faulty gas valve Sometimes
The gas valve controls the flow of gas to the burner. A valve that sticks, fails to open fully, or closes erratically will cause the flame to be unstable or extinguish mid-cycle, triggering F29. Gas valve faults require a Gas Safe engineer to diagnose and replace.
- Blocked or partially obstructed flue Sometimes
If the flue terminal is blocked by debris, a bird's nest, ice, or accumulated combustion residue, the boiler cannot expel exhaust gases safely. Some boilers respond by cutting out, which can register as a flame loss. A visual check of the external flue terminal is a safe first step for a homeowner.
- Faulty PCB (printed circuit board) Rare
The PCB interprets signals from the flame-sensing electrode. If the board develops a fault — through age, moisture ingress, or a component failure — it may incorrectly report that the flame has been lost even when combustion is normal. PCB issues typically surface only after other causes have been eliminated and require professional diagnosis.
How to fix it
- Check whether other gas appliances are working DIY safe
Try your gas hob or gas fire. If nothing gas-powered works, the issue is with your supply rather than the boiler itself. Contact your gas supplier or check whether a prepayment meter needs topping up. If only the boiler is affected, move to the next step.
- Inspect the condensate pipe for freezing (cold weather only) DIY safe
The condensate pipe is a white or grey plastic pipe, typically 21–32 mm in diameter, that exits the boiler and runs to an outside drain or gulley. In freezing temperatures, pour warm (not boiling) water along its length from top to bottom to melt any ice blockage. You should hear a gurgling sound as the pipe clears. Never use boiling water, as this can crack the pipe. Once thawed, proceed to reset the boiler.
- Check system pressure is between 1 and 1.5 bar DIY safe
Look at the pressure gauge on the front of the boiler. If it reads below 1 bar, the system may need topping up via the filling loop — a braided flexible hose beneath the boiler with one or two small valves. Open the valve(s) slowly, watch the gauge rise to around 1.2–1.5 bar, then close them. An F29 fault is not typically caused by low pressure alone, but it is good practice to confirm the system is correctly pressurised before resetting.
- Reset the boiler DIY safe
Press and hold the reset button (usually marked with a flame-and-line symbol) for approximately three seconds until the boiler attempts to restart. Allow it a full ignition cycle before judging whether it has cleared. If it locks out again immediately, attempt one further reset. If it continues to lock out after two attempts, stop resetting — repeated forced ignition attempts can stress the PCB and burner components. At this point, a Gas Safe engineer is needed.
- Check the flue terminal externally DIY safe
From outside the property, visually inspect the flue outlet (usually a white plastic terminal on an external wall or roof). Look for obvious blockages such as leaves, debris, ice, or a bird's nest. Do not attempt to clear a roof-mounted terminal yourself. If the terminal looks clear, the issue lies elsewhere.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the above checks have not resolved the fault, a Gas Safe engineer will need to attend. They will test gas pressure and flow rate at the burner, inspect and test the ignition and flame-sensing electrodes, check the gas valve operation, examine the PCB, and clear any internal flue or heat exchanger blockage. Do not attempt to open the boiler casing or work on any gas components yourself — this is a legal requirement in the UK.
Parts you may need
- Flame sensing / ignition electrode · from £35
- Ignition lead set · from £25
- Gas valve · from £120
- Condensate trap · from £20
- PCB (printed circuit board) · from £200
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£350, depending on the underlying cause.
Frequently asked questions
What is the difference between Glow-Worm F28 and F29?
F28 means the boiler attempted to ignite and never established a flame at all — it could not get started. F29 means ignition was successful but the flame then went out while the boiler was running. In practice both codes often share the same root causes (gas supply, ignition components), but F29 more commonly points to a flame-sensing electrode fault or an intermittent gas supply issue, since the burner clearly did light initially.
Can I fix the Glow-Worm F29 fault myself?
There are a handful of checks any homeowner can safely carry out: confirming the gas supply is on and other appliances work, thawing a frozen condensate pipe with warm water, checking boiler pressure, and performing a reset (no more than twice). Beyond that, any work involving gas components — testing the gas valve, replacing electrodes, inspecting the PCB — must be done by a Gas Safe registered engineer. Attempting gas-related repairs yourself is illegal in the UK and extremely dangerous.
Why does my Glow-Worm boiler keep showing F29 every winter?
If F29 appears reliably each cold snap, the condensate pipe is almost certainly freezing. The long-term fix is to have a Gas Safe engineer insulate the external section of the pipe, or re-route it so it discharges internally. This is a relatively inexpensive job (typically £50–£100 for insulation) and will prevent the same winter lockout recurring year after year.
How much does it cost to fix a Glow-Worm F29 fault?
For most people, the repair falls between £100 and £350 including parts and labour. A condensate pipe thaw or electrode replacement sits towards the lower end; a gas valve replacement sits in the middle to upper end of that range. PCB replacement is a less common outcome for F29 and tends to cost more — typically £300–£500 — but this is usually only recommended after all other causes have been ruled out. Glow-Worm also offer their own fixed-price repair service if you prefer to go direct to the manufacturer.