Boiler locks out with F.24 shortly after starting

Safety: Turn off power and allow boiler to cool before touching pipework.

• Verify valves and TRVs: Ensure all isolation valves under the boiler are open and that not all TRVs are closed. At least one radiator should be fully open.
• Bleed air: Bleed radiators and any automatic air vents near the boiler. Air in the heat exchanger can cause rapid temperature spikes.
• Check pump speed: On models with adjustable pump speed, ensure it is set according to Vaillant’s installation instructions (often medium or auto mode).
• Inspect pump: Confirm the pump is running and not excessively hot. If it is very hot and noisy with poor circulation, replacement may be needed.
• Descale if in hard water area: A scaled primary heat exchanger can cause F.24. A professional can chemically flush or replace the exchanger.