Ferroli F35 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F35 fault code mean?
The F35 code on a Ferroli boiler indicates that the boiler's control board has detected an irregular or unstable mains electrical frequency. The UK grid should supply alternating current at a steady 50Hz; when the frequency deviates outside acceptable limits, the boiler raises this fault to protect itself. F35 is an F-type blocking fault on Ferroli boilers, which means it may clear on its own once the underlying condition resolves — unlike A-type lockout codes, which always need a manual reset. Because the fault points squarely at the incoming electricity supply rather than at the gas or heating circuit, the first call should be to a qualified electrician rather than a Gas Safe engineer.
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
- Unstable or irregular mains electricity supply Common
The most frequent trigger for F35 is genuine instability in the incoming mains supply — caused by grid fluctuations, a loose or corroded consumer-unit connection, a failing spur socket, or local wiring faults. A qualified electrician can measure the supply frequency with a meter to confirm or rule this out quickly.
- Faulty PCB (printed circuit board) Sometimes
If a qualified electrician confirms the mains supply is stable and at the correct 50Hz, the next most likely cause is a faulty control board inside the boiler. The PCB monitors incoming frequency; if the monitoring circuit on the board has failed it will generate a false F35 even when the power supply is healthy. PCB replacement must be carried out by a Gas Safe registered engineer.
- Transient power surge or one-off grid disturbance Sometimes
A brief spike or dip on the mains — from a large appliance switching on, nearby roadworks, or a momentary grid event — can trip the F35 code as a one-off. Because F35 is a blocking fault, it may clear automatically once supply stabilises; a single successful reset with no recurrence suggests this was the cause.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Ferroli boiler — it is usually marked with an 'R' or a flame symbol. Press and hold it for around 5–10 seconds. If the boiler fires up normally and the fault does not return, the issue was likely a brief, one-off mains disturbance. Do not reset more than two or three times in total; repeated resets without a fix will not solve the underlying problem and can stress the boiler further.
- Check for any obvious electrical issues in your home DIY safe
Look at your consumer unit (fuse box) to see whether any circuit breakers have tripped. Check that other mains-powered appliances in the home are working normally. If lights are flickering or other devices are behaving oddly, this supports a mains supply problem rather than a boiler fault.
- Call a qualified electrician to test the incoming mains frequency DIY safe
Ask a qualified electrician to measure the frequency of the electricity supply reaching the boiler with a calibrated meter. They should confirm it is stable at 50Hz and check the boiler's dedicated fused spur for loose connections, corrosion, or wiring faults. This step should happen before any boiler components are replaced, as it may identify a cheap fix.
- If mains supply is confirmed healthy, call a Gas Safe registered engineer to inspect and replace the PCB Gas Safe engineer
Once an electrician has confirmed the incoming supply is normal and stable, the fault almost certainly lies within the boiler's control board. A Gas Safe registered engineer will test the PCB, confirm the diagnosis, and replace it with a compatible unit. This is not a DIY job — do not attempt to open the boiler casing or handle internal electrical components yourself.
Parts you may need
- Ferroli PCB / control board · from £180
- Boiler fused spur / isolation switch · from £15
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 £130–£320, depending on the underlying cause.
Frequently asked questions
Is the Ferroli F35 a lockout fault or will it clear by itself?
F35 is an F-type blocking fault on Ferroli boilers, which means it can clear automatically once the underlying condition — in this case an unstable mains frequency — goes away. This is different from an A-type lockout code, which always needs a manual reset button press before the boiler will fire again. If your F35 disappears on its own and does not return, a brief grid disturbance was most likely responsible. If it keeps coming back, you need an electrician or Gas Safe engineer to investigate.
Can I fix the Ferroli F35 myself?
The only homeowner action that is safe and appropriate is a single boiler reset (no more than two or three times). Beyond that, checking the mains frequency requires specialist electrical test equipment, and replacing the PCB requires opening the boiler — both need qualified professionals. Call a qualified electrician first to test the supply frequency; if they confirm the supply is fine, book a Gas Safe registered engineer to assess the control board.
How much does it cost to fix a Ferroli F35 fault?
Most people pay somewhere between £130 and £320 all-in. An electrician visit to test the mains supply typically costs £50–£100, and if the PCB turns out to be the culprit, parts and labour for a replacement board usually run to £200–£320. In rare cases — particularly on older or discontinued Ferroli models where the PCB has to be sourced specially — the board alone can exceed £300, pushing total costs higher. If your boiler is over ten years old and the repair is approaching 40% of the cost of a new boiler, replacement is often the more sensible long-term investment.
Why does my Ferroli boiler show F35 even though the rest of my house electrics seem fine?
The boiler's PCB is sensitive to frequency variations that would not be noticeable to other household appliances. Everyday devices such as televisions and kettles tolerate small fluctuations without any visible effect, whereas the boiler's control board is programmed to flag even relatively minor deviations. It is also possible the PCB's own frequency-monitoring circuit has developed a fault, causing it to report a problem that does not actually exist in the supply. An electrician can rule out a real supply issue, after which a Gas Safe engineer can determine whether the board itself needs replacing.