Boiler displays F23, may cycle on and off rapidly, poor heating performance

Safety: Isolate power before removing pump covers or filters. Water may be hot; allow to cool.

• Check system pressure: Ensure pressure is within normal range (1.0–1.5 bar cold).
• Open valves: Confirm all isolation valves near the boiler and pump are fully open.
• Bleed radiators: Remove trapped air from radiators and any high points in the system.
• Check magnetic filter: If a system filter (e.g., on return pipe) is fitted, isolate and clean it following manufacturer instructions.
• Listen to pump: A noisy or silent pump may be seized or airlocked.
• Engineer tasks:
  • Check pump performance and replace if necessary.
  • Powerflush or chemically clean a heavily sludged system.
  • Verify correct pipe sizing and bypass arrangements.

Ignoring F23 can lead to heat exchanger stress and premature failure.