Protherm F.36 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.36 fault code mean?
The F.36 code appears on chimney-type (open-flue, natural draught) Protherm boilers when the boiler's internal safety system detects an abnormal draught condition in the flue or chimney. This could mean draught has reversed, is weaker than expected, or the pressure switch that monitors it has signalled a problem. The boiler shuts down as a precaution to prevent unsafe combustion. Note that F.36 is specific to chimney-version models — it is separate from F.35 (flue system error) and F.37 (fan RPM out of tolerance), though all three relate to the same area of the boiler.
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
- Chimney or flue blockage restricting draught Common
Soot accumulation, a bird or insect nest, ice build-up, or debris at the flue terminal can all choke off the natural draught the boiler relies on. A poorly sized or excessively long flue duct can have the same effect. This is the most frequent trigger of F.36 and the first thing a Gas Safe engineer will check.
- Faulty or moisture-affected pressure switch Common
The pressure switch is the sensor that monitors draught. If it develops a fault, becomes waterlogged, or its connecting tubes fill with condensate or become disconnected, it can trigger a false F.36 even when the chimney itself is clear. An engineer will test the switch electrically and inspect the tubing.
- Fan or ventilator problem Sometimes
Even on natural-draught chimney models, a small fan or ventilator may assist flue gas movement. Clogged fan blades, a seized shaft, or damaged wiring between the fan and control board can prevent normal draught conditions and cause this fault — sometimes alongside F.37.
- Inadequate room ventilation Sometimes
Open-flue boilers draw combustion air directly from the room they are installed in. If the room has been sealed up, extractor fans are running nearby, or ventilation grilles have been blocked, there may not be enough air reaching the boiler for safe combustion, disrupting the draught pattern.
- Adverse weather conditions Sometimes
Strong winds, heavy frost, driving rain, or snow at the flue terminal can temporarily disrupt natural draught, causing a nuisance F.36 that clears once conditions improve. If the fault reappears only in bad weather, the flue terminal position or design may need reviewing.
- PCB or wiring fault Rare
If the control board is misreading the pressure switch signal, or if wiring connections to the draught-sensing components are corroded or loose, a spurious F.36 can appear even with no real draught problem. This is usually only identified once all other causes have been ruled out.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button as described in your Protherm user guide. If the boiler fires up and runs normally, monitor it over the next hour or so. If F.36 returns promptly or the boiler will not stay lit, move on — repeated resets without finding the cause are not recommended.
- Check that the gas supply is on DIY safe
Confirm that other gas appliances in the property (hob, fire) are working normally and that the gas meter isolation valve is open. If you suspect a wider gas supply issue, contact your gas supplier rather than attempting boiler repairs.
- Check for obvious external flue obstruction DIY safe
From outside, take a visual look at where the flue exits the building. If you can see a bird nest, heavy frost, or debris at the terminal without going onto a roof or ladder, note this and report it to the engineer. Do not attempt to clear internal flue blockages yourself.
- Check room ventilation around the boiler DIY safe
Ensure the room housing the boiler is not sealed. Any permanent air vents or grilles should be unobstructed. Switch off nearby extractor fans temporarily and see if the boiler resets successfully. The boiler needs at least 1.2 m³ of fresh air per hour per kW of output — a sealed utility room can starve it.
- Arrange a Gas Safe engineer to inspect and sweep the flue and chimney Gas Safe engineer
A registered engineer should carry out a full flue inspection and, where needed, a chimney sweep. They will check for blockages, verify the flue dimensions are correct for the boiler output, and confirm the terminal is properly positioned and undamaged.
- Have the engineer test and inspect the pressure switch and its tubing Gas Safe engineer
The engineer will check for moisture or condensate in the pressure switch tubes, ensure all tubes are correctly and securely connected, and electrically test the switch itself. A faulty switch will be replaced. This is a relatively straightforward repair.
- Have the engineer inspect the fan and associated wiring Gas Safe engineer
If the fan is implicated — especially if F.37 is also showing — the engineer will check the fan blades for clogging, assess the motor and shaft, and test the wiring harness back to the PCB. Cleaning or replacing the fan may resolve the issue.
- Have the engineer inspect wiring, connections, and the PCB if the fault persists Gas Safe engineer
If no mechanical fault is found, the engineer will check for corroded or loose electrical connections throughout the draught-sensing circuit. As a last resort, a faulty PCB may need to be replaced — this is typically the most expensive repair for this fault code.
Parts you may need
- Pressure switch (draught sensor) · from £45
- Pressure switch connecting tubing/hose set · from £15
- Flue fan / ventilator assembly · from £95
- PCB (control board) · from £220
- Flue terminal and adapter kit · from £55
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 £80–£320, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.36 fault dangerous?
It can indicate an unsafe combustion condition, which is why the boiler shuts itself down. An open-flue boiler that runs with poor draught risks incomplete combustion and carbon monoxide production. Do not repeatedly reset the boiler and run it if F.36 keeps returning — get a Gas Safe engineer to investigate promptly.
Can I fix the Protherm F.36 fault myself?
The only safe DIY steps are: resetting the boiler once or twice, confirming the gas supply is on, checking that ventilation grilles are unblocked, and taking a visual look at the external flue terminal. Any work on the flue, chimney, pressure switch, fan, or internal wiring must be carried out by a Gas Safe registered engineer. Flue work is legally classified as gas work in the UK regardless of how minor it looks.
How much does it cost to fix a Protherm F.36 fault?
The most common repairs — clearing a blocked flue or chimney (£60–£150 including a sweep) or replacing the pressure switch (£80–£200 parts and labour) — fall within a typical total spend of £80–£320. If the fan needs replacing, expect £150–£300. In the less common scenario where the PCB is at fault, the part alone can cost £180–£400 plus labour, so it is worth getting a quote before committing to a PCB replacement on an older boiler.
Why does F.36 only appear in cold or windy weather?
Natural-draught chimneys are sensitive to external conditions. Strong winds can cause downdraught at the terminal, and ice or frost can partially block the flue outlet. If F.36 appears mainly in bad weather and clears once conditions improve, this suggests the flue terminal may be in an exposed position or the chimney design is marginal. A Gas Safe engineer can advise on terminal guards or flue modifications to improve resilience.