What the FH Fault Code Means on Your Ferroli Boiler
The Ferroli boiler code FH is not a fault code at all — it stands for 'flush heating' (also described as the air-purge or self-check cycle). When FH appears, the boiler is circulating water through the primary circuit to purge trapped air before it will allow the burner to fire. On Ferroli Optimax, Modena, Domina and Divacondens models this cycle typically lasts between two and eight minutes.
FH only becomes a problem when it never clears. A boiler stuck on FH indicates the appliance cannot complete the purge because there is genuinely no flow — a seized pump, an airlock it cannot shift, low pressure, a blocked filter or a faulty flow sensor.
Why A Ferroli Boiler Gets Stuck On FH
Common causes
- System pressure below 1 bar, so the pump cannot establish flow.
- Seized or airlocked pump — the pump hums but does not circulate.
- Large airlock in the primary circuit or a radiator loop.
- Blocked magnetic filter, strainer or plate heat exchanger.
- Closed isolation valve, service valve or a stuck diverter valve.
- Faulty flow switch, pressure sensor or NTC thermistor.
- PCB fault preventing the purge routine from completing.
How To Clear The Ferroli FH Code
Step-by-step
- Wait up to ten minutes — a normal FH purge clears on its own.
- Check the pressure gauge cold and top up to 1.0–1.5 bar via the filling loop.
- Bleed every radiator, starting downstairs, and bleed the pump if it has a bleed screw.
- Verify all isolation and service valves under the boiler are fully open.
- Power the boiler off for a minute and back on. If FH still will not clear, book a Gas Safe registered engineer to test the pump, flow sensor and diverter valve.
Ferroli D3 Fault Code
D3 on a Ferroli boiler indicates a flame or ignition-related lockout on the control board — the appliance has attempted ignition and failed to prove a stable flame signal, or has detected a flame when it should not be present. Typical causes are gas supply issues, a dirty or worn ignition/flame-sense electrode, a faulty gas valve, condensate blockage or a damp PCB.
Reset the boiler once. If D3 returns, do not keep resetting — the electrode, gas valve and gas working pressure must be tested with a manometer by a Gas Safe registered engineer.
Ferroli Fault Codes At A Glance
| Code | Meaning | Typical fix |
|---|---|---|
| FH | Air purge / flush cycle in progress | Normal; if stuck, check pressure, air, pump and flow sensor. |
| D3 | Ignition / flame signal lockout | Gas supply, electrode, gas valve and condensate checks. |
| A01 | No flame detected on ignition | Check gas supply, electrode and gas valve. |
| A03 | Overheat / safety thermostat trip | Check pump, flow and heat exchanger for blockage. |
| A06 | Flame signal lost during operation | Electrode, condensate blockage or gas pressure fault. |
| F37 | Low system water pressure | Re-pressurise to 1.0–1.5 bar and check for leaks. |
Ferroli FH & D3 FAQs
What does FH mean on a Ferroli boiler?
FH means the boiler is running its air-purge or flush cycle, circulating water to remove trapped air before firing the burner. It normally clears within two to eight minutes and is not a fault.
Why is my Ferroli boiler stuck on FH?
A boiler stuck on FH cannot establish circulation. The usual causes are low system pressure, a seized or airlocked pump, a blocked filter or heat exchanger, a closed valve or a faulty flow sensor.
What does D3 mean on a Ferroli boiler?
D3 is an ignition or flame-signal lockout. The boiler either failed to prove a flame on ignition or detected an unexpected flame signal. Reset once; if it repeats, the electrode, gas valve and gas pressure need testing.
Can I fix a Ferroli FH code myself?
You can safely check pressure, bleed radiators and confirm the valves are open. Anything involving the pump, gas valve, electrode or PCB must be handled by a Gas Safe registered engineer.
Need this fault fixed?
Our Gas Safe registered engineers diagnose and repair this fault across London and the M25. Book a boiler repair, arrange a boiler service, or call us out for commercial boiler repair. Call 0208 935 5572.
