BoilerFaultCodesFerroli F35 Fault Code
The Ferroli F35 fault code means: your boiler has detected an issue with the quality of the electricity coming into your home, meaning the power supply is not stable enough for it to operate safely.. It appears on 11 Ferroli models in our database.
Technical description: Irregular mains frequency – electric supply problem
What causes the Ferroli F35 fault?
This fault occurs when the boiler's internal computer detects that the incoming electrical supply is fluctuating outside of the standard 50Hz frequency required in the UK. The most common reason is a temporary instability in the National Grid or a local power surge, though it can also be triggered by a degrading main control board failing to read the power correctly.
This fault may be more frequent during periods of extreme weather, such as heavy storms or lightning, which can cause temporary fluctuations and interference in the local electrical grid.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Ferroli models that show the F35 fault code
Click a model for the full fault code list and model-specific repair notes.
- Arean Repair: £100-250
- Arena 30A Repair: £120-250
- Domi Compact Repair: £150-250
- Domiproject Repair: £100-350
- Econcept 100 Repair: £120-250
- iBoiler 24 Repair: £90-250
- iBoiler 28 Repair: £120-350
- Maxima 35 S Repair: £120-250
- Modena HE Repair: £100-350
- Optima 201 Repair: £100-250
- Optimax Repair: £100-£350
Source
Ferroli F35 fault code information on this page is sourced and cross-checked against the official Ferroli service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026