Ferroli F35 Fault Code (i25 / i29)

Safety disclaimer

Fault code summary

Manufacturer

Ferroli

Fault code

F35

Fault type

Electrical

Severity

temporary

Reset

Ferroli states that F faults reset automatically when the value returns to the normal range.

Applicable models

Ferroli i25, Ferroli i29

Quick answer

Faulty mains frequency.

What does this fault code mean?

Faulty mains frequency.

Common causes

• Electrical mains trouble.

Symptoms

• The display flashes F35 and the boiler temporarily shuts down.

What can the homeowner safely check?

• Check only whether the property has a wider power interruption.

Boiler reset information

Ferroli states that F faults reset automatically when the value returns to the normal range.

When should I call an engineer?

Contact a suitably qualified heating engineer when the fault persists, the boiler locks out, or the manufacturer instructions do not resolve it. For gas appliance work in the UK, use a Gas Safe registered engineer.

Engineer diagnosis

• Verify mains frequency and boiler power electronics.

Applicable boiler models

• Ferroli i25
• Ferroli i29

Related fault codes

• A01 — i25 / i29

  No burner ignition.

  Read fault code
• A02 — i25 / i29

  Flame-present signal with the burner off.

  Read fault code
• A03 — i25 / i29

  Overtemperature protection intervention.

  Read fault code
• A06 — i25 / i29

  No flame after the ignition phase.

  Read fault code
• A09 — i25 / i29

  Gas-valve fault.

  Read fault code
• A15 — i25 / i29

  Air-pressure-switch fault F05 generated five times in the last 24 hours.

  Read fault code
• A16 — i25 / i29

  Gas-valve fault.

  Read fault code
• A23 — i25 / i29

  Card parameter fault.

  Read fault code

Manufacturer documentation

Table 9: F35 — Faulty mains frequency; electric mains trouble.

Last verified

2026-09-12

Frequently asked questions