Ferroli F20 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F20 fault code mean?
The F20 fault code appears on certain Ferroli boiler models — most notably the Domiproject D/F range and the DOMIcondens HE 26C — and indicates that the boiler has detected a problem with combustion quality or combustion control. In plain terms, the boiler has sensed that the burner is not burning gas in the way it should, so it has shut down as a precaution. Because F20 begins with 'F', Ferroli classifies it as a blocking fault rather than a full lockout: it may clear on its own if the underlying condition resolves, but if it keeps returning you need a Gas Safe engineer to investigate. Do not confuse this with A-series codes (such as A01 or A02), which are hard lockouts always requiring a manual reset. Important note: Ferroli fault code meanings can differ between model ranges (Modena, Bluehelix, Optimax, i-Series, Domiproject). Always cross-reference your boiler's own user manual to confirm the exact meaning for your unit.
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
- Gas supply or pressure issue Common
If the gas pressure reaching the boiler is too low, or if the supply has been interrupted at the meter or emergency control valve, the burner cannot achieve correct combustion and the boiler raises F20. A quick check is to try lighting a gas hob — if it fails, the issue lies upstream of the boiler entirely.
- Flue or air duct obstruction Common
A blocked or partially blocked flue terminal — caused by debris, bird nesting, or ice in cold weather — restricts airflow through the combustion chamber. The boiler detects poor draught and shuts down. The condensate pipe freezing in winter can also cause back-pressure that disrupts combustion.
- Ionisation electrode fault or unstable flame Common
The ionisation electrode monitors the quality of the flame during each firing cycle. If the electrode is worn, coated in carbon deposits, or positioned incorrectly after a service, it may report poor or absent combustion even when the burner fires. This is a very common cause of F20 on older Domiproject units.
- Fan failure or reduced fan speed Sometimes
The combustion fan draws air into the burner and expels flue gases. A failing fan motor, worn bearings, or a faulty fan speed sensor can result in insufficient airflow for correct combustion, triggering F20.
- Gas valve fault or PCB communication error Sometimes
If the PCB cannot send or receive signals to and from the gas valve controller, the boiler cannot regulate gas flow correctly. A damaged valve or a control board fault can both cause F20, though these are typically discovered after simpler causes have been ruled out.
- Poor water circulation Rare
On some Ferroli models, very low water flow through the heat exchanger triggers a thermal protection response that can contribute to combustion-related shutdowns. This is more likely where a pump is failing or system filters are heavily blocked.
How to fix it
- Check whether other gas appliances in your home are working DIY safe
Turn on a gas hob ring or check another gas appliance. If nothing is working, your gas supply has been interrupted — contact your gas supplier (e.g. Cadent, SGN) rather than a boiler engineer. If the hob lights normally, the gas supply is fine and the issue is with the boiler itself.
- Inspect the external flue terminal for visible blockages DIY safe
From ground level, look at the flue outlet on the outside wall. Check for obvious obstructions such as leaves, a bird's nest, ice, or anything pressed against the terminal. Do not climb ladders or attempt to remove the flue yourself — if you spot a blockage you cannot safely clear from the ground, report it to your engineer.
- Check for a frozen condensate pipe in cold weather DIY safe
In freezing temperatures, the plastic condensate pipe that runs outside (or through an unheated space) can ice up and cause the boiler to fault. You can safely thaw it by pouring warm — not boiling — water over the pipe or wrapping it in a warm cloth. Once thawed, attempt a reset. Consider lagging the pipe to prevent a repeat.
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with an 'R' or a flame symbol) for around 5–10 seconds, then release. Allow the boiler a full minute to attempt a restart. If it fires up and runs normally, monitor it closely over the next few hours. If F20 returns, do not reset again — repeated resets on a combustion fault risk damaging the heat exchanger and fan.
- Check and restore boiler system pressure if low DIY safe
Look at the pressure gauge on the boiler. It should read between 1.0 and 1.5 bar when cold (check your manual for the exact recommended range). If pressure is below 0.8 bar, top it up using the filling loop — a pair of small valves under the boiler connected by a braided hose. Open both valves slowly, watch the gauge, and close them once pressure reaches around 1.2 bar. Dry and replace the loop afterwards.
- Call a Gas Safe registered engineer if F20 persists or returns Gas Safe engineer
If the fault code comes back after a single reset, or if none of the above steps resolve it, you need a qualified Gas Safe engineer. They will inspect and test the ionisation and ignition electrodes, verify gas inlet pressure and valve operation, check fan speed and airflow, examine the PCB for communication faults, and clear any internal condensate trap blockage. Do not continue resetting the boiler in the hope it clears — on a combustion fault this can cause costly secondary damage.
Parts you may need
- Ionisation / ignition electrode · from £35
- Combustion fan (flue fan motor) · from £95
- Gas valve · from £130
- PCB (main control board) · from £180
- Condensate trap assembly · from £25
- NTC temperature sensor · from £20
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Is the Ferroli F20 a lockout or a blocking fault?
F20 is a blocking fault, not a lockout. Ferroli uses codes starting with 'A' for hard lockouts that always need a manual reset. F-series codes like F20 may clear automatically if the root cause resolves itself — for example, a frozen condensate pipe that thaws out. However, if F20 keeps reappearing, it will not fix itself and you should book a Gas Safe engineer.
Can I keep resetting my Ferroli boiler to clear F20?
You should attempt no more than one or two resets. F20 is triggered by a combustion quality problem, and forcing the boiler to attempt firing repeatedly without fixing the cause can overheat the heat exchanger and burn out the fan — both expensive components to replace. If the fault returns after a single reset, leave the boiler off and call a Gas Safe engineer.
Does F20 mean the same thing on all Ferroli boilers?
Not exactly. F20 is specifically documented as a combustion quality fault on the Domiproject D and F models, and as a combustion control fault on the DOMIcondens HE 26C. Other Ferroli ranges such as the Modena, Bluehelix, and Optimax may use different codes to describe similar faults, or may not use F20 at all. Always check your model's user manual or ask a Gas Safe engineer to confirm the exact meaning for your boiler.
How much does it cost to fix a Ferroli F20 fault in the UK?
Most people pay somewhere between £120 and £350 all-in, depending on what the engineer finds. A diagnostic visit to clean the electrodes and clear a flue or condensate blockage typically costs £120–£180. Fan replacement usually runs £180–£250 including parts and labour, and a gas valve swap tends to be £250–£350. A PCB replacement can cost more — the board alone can be £180 or above on some Domiproject models, with fitting on top of that — so if the control board has failed on an older Ferroli, it is worth getting a like-for-like replacement quote alongside a new boiler quote before committing.