F.23 — Safety switch-off: Temperature difference too great

What does F.23 mean?

The boiler has detected an excessively large temperature difference between the flow and return pipes, indicating a circulation problem. This triggers a safety shutdown.

• Circulation pump is blocked or malfunctioning.
• Insufficient pump output, preventing proper water flow.
• Air trapped within the boiler or heating system.
• Flow and return NTC sensors have been incorrectly interchanged during installation or repair.

Click steps to track your progress.

1. 1

   Bleed air from your radiators and heating system, then check the boiler pressure and top up if necessary.

2. 2

   Press the fault clearance key on the boiler to reset it.

3. 3

   If the fault persists, contact Vaillant Service Solutions as this may indicate a pump issue or sensor fault.