4C (224) - Heat exchanger or flue gas temperature limiter tripped, or differential pressure switch open after ignition

What does 4C mean?

This is a safety lockout, indicating that a temperature limiter (either for the heat exchanger or flue gas) has tripped, or the differential pressure switch opened while the fan was running after ignition. This prevents overheating or unsafe combustion.

• Boiler overheating (e.g., due to poor circulation, air lock, or pump issue)
• Blocked flue gas system or faulty fan operation after ignition
• Faulty heat exchanger or flue gas temperature limiter
• Defective differential pressure switch or its associated wiring

Click steps to track your progress.

1. 1

   Turn off the boiler at the main power supply and wait for at least 30 minutes to allow the boiler to cool down completely. Then, restart the boiler.

2. 2

   Check that all heating system valves (flow and return) are fully open. Ensure there are no airlocks in the system by bleeding radiators if necessary. Check the pump for proper operation.

3. 3

   Inspect the flue gas system for any blockages or obstructions that could prevent proper exhaust. Ensure the fan is running correctly after ignition.

4. 4

   If the fault repeatedly occurs, it indicates a serious safety issue. Contact a qualified heating engineer to diagnose and replace faulty limiters, switches, or other components.