Protherm F.28 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.28 fault code mean?
The F.28 code appears on Protherm boilers — including the Gepard and Panther ranges — when the burner fails to light on its first attempt. The boiler tries to ignite, detects no flame, and shuts itself down as a safety precaution. After several unsuccessful attempts the unit enters a full lockout, cutting off heating and hot water entirely. You may hear repeated clicking as the boiler tries and fails to spark. Because Protherm is part of the Vaillant Group and shares much of its engineering platform, F.28 carries the same ignition-failure meaning across Protherm, Vaillant ecoTEC, and Glow-Worm boilers.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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
- Interrupted gas supply Common
If no gas is reaching the boiler it simply cannot ignite. Check whether other gas appliances in the property — hob, gas fire — are working normally. Prepayment meter users should confirm there is credit on the meter, as a topped-up meter that has run out is one of the most common and cheapest-to-fix causes of an F.28 callout. Also verify that the gas isolation valve on the supply pipe to the boiler is in the open (inline) position.
- Frozen or blocked condensate pipe Common
In cold weather the condensate pipe — which carries acidic wastewater from the boiler to the drain — can freeze solid, particularly where it runs externally. A blocked condensate pipe causes the boiler to shut down and can trigger ignition-related fault codes. A telltale gurgling sound from the boiler often accompanies this fault. This is one of the few causes a homeowner can safely address themselves.
- Fouled or damaged ignition electrode / ionisation sensor Common
Carbon deposits can build up on the ignition electrode or the ionisation sensor that confirms a flame is present. Even a thin layer of soot reduces electrical conductivity enough to prevent a reliable spark or cause the boiler to incorrectly believe no flame exists. A cracked ceramic insulator or an electrode that has moved out of the correct position (the gap should typically be no more than 4–5 mm) will produce the same result.
- Faulty or sticking gas valve Sometimes
The gas valve controls the flow of gas to the burner. Over time — especially on older boilers — the valve can stick, fail to open fully, or develop an electrical fault in its solenoid. Even when the external gas supply is confirmed, a faulty valve means insufficient gas reaches the burner at ignition. The valve position and wiring connections are checked by a Gas Safe engineer using specialist test equipment.
- Blocked burner or clogged injectors Sometimes
Debris or residue inside the burner or its injector ports can restrict gas flow enough to prevent ignition. The injector apertures are extremely small, so even minor contamination is enough to disrupt the gas-to-air mixture that a clean ignition requires. This is more common on boilers that have not been serviced regularly.
- Poor earth connection or wiring fault Sometimes
The boiler's electrical earth and the wiring loom connecting the PCB to the gas valve and ignition components must all be sound. A loose connector, corroded terminal, or failed earth can disrupt the ignition sequence at the point of control rather than at the component itself. The PCB may falsely report an ignition failure when the underlying problem is a wiring issue rather than a mechanical one.
- Faulty PCB (printed circuit board) Rare
The PCB coordinates the entire ignition sequence. If it is damaged — through power surges, moisture ingress, or component failure — it may fail to send the correct signals to the gas valve or ignition electrode, resulting in a no-flame condition even when all other components are serviceable. PCB faults are usually identified only after all other causes have been ruled out.
How to fix it
- Check that other gas appliances in your home are working DIY safe
Turn on a gas hob ring or gas fire. If they also fail to light, the fault is with your gas supply rather than the boiler. Check that the main gas isolation valve is open (handle parallel to the pipe) and, if you have a prepayment meter, confirm there is credit. Contact your gas supplier if there is a wider supply issue.
- Thaw a frozen condensate pipe if cold weather is present DIY safe
Locate the condensate pipe — usually a white or grey plastic pipe (often 21.5 mm or 32 mm diameter) that exits the boiler casing and runs to an external drain. If the pipe feels solid with ice, wrap it in a warm (not boiling) wet cloth or apply a hot-water bottle along the frozen section, working from the drain end back towards the boiler. Once thawed, reset the boiler. Insulating the pipe afterwards helps prevent a repeat.
- Reset the boiler — but limit attempts to two or three DIY safe
Press and hold the reset button (usually marked with a flame or reset symbol) for approximately three seconds, then release and allow the boiler to run through its full startup sequence. If the boiler fires and holds, monitor it over the next few hours. If F.28 returns immediately or within a short time, do not keep resetting — repeated failed ignitions without a diagnosis put unnecessary wear on ignition components and will not resolve an underlying fault.
- Check that the boiler's gas isolation valve is fully open DIY safe
The isolation valve is typically located on the gas supply pipe directly beneath or beside the boiler. The handle should be in line with the pipe to indicate it is fully open. If it has been partially or fully closed — perhaps after recent work on the property — open it carefully and attempt a reset.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the steps above have not resolved the F.28 code, the fault requires professional diagnosis. A Gas Safe engineer will pressure-test the gas supply at the boiler, inspect the ignition electrode and ionisation sensor (cleaning or replacing them as needed), test the gas valve for correct operation and wiring continuity, check all earth connections, and assess the PCB. Do not attempt to open the boiler casing, touch the gas valve, or tamper with any internal components — this is illegal for anyone who is not Gas Safe registered and is extremely dangerous.
Parts you may need
- Ignition electrode · from £25
- Ionisation sensor / flame sensing electrode · from £30
- Gas valve (Protherm compatible) · from £120
- PCB / control board · from £180
- Condensate pipe insulation sleeve · from £8
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
Can I fix the Protherm F.28 fault myself?
There are a few safe checks any homeowner can carry out: confirm the gas supply is on and credited, thaw a frozen condensate pipe, and reset the boiler up to two or three times. Beyond that, work on internal boiler components — including the ignition electrode, gas valve, wiring, and PCB — must only be carried out by a Gas Safe registered engineer. It is a legal requirement in the UK.
Why does my Protherm boiler show F.28 only in cold weather?
When outdoor temperatures drop below freezing, the condensate pipe is the most likely culprit. This pipe carries acidic wastewater from the boiler to a drain, and external sections are vulnerable to freezing solid. Once blocked, the boiler cannot expel its waste and shuts down. Thawing the pipe with warm water or a hot-water bottle usually resolves it quickly. Lagging the exposed section of pipe helps prevent it happening again.
How much does it cost to fix a Protherm F.28 fault?
Most F.28 repairs in the UK fall between £100 and £350, covering the engineer's call-out, labour, and common replacement parts such as electrodes or wiring. A gas valve replacement typically adds £200–£350 in parts and labour on top of the call-out. PCB replacement — if eventually diagnosed as the cause — can push the total to £500 or more, though this is less common. If your boiler is over 15 years old and facing a major component repair, it may be worth comparing the repair cost against a new boiler installation.
Is the F.28 code the same on Vaillant and Glow-Worm boilers?
Yes. Because Protherm is part of the Vaillant Group and shares its engineering platform, the F.28 ignition-failure code means the same thing across Protherm (Gepard, Panther), Vaillant (ecoTEC range), and Glow-Worm boilers. The diagnostic steps and likely causes are identical, and parts are often shared between the ranges.