Blue light Slow flash (on then off) — Boiler running without central heating demand

What does Blue light Slow flash (on then off) mean?

The blue indicator light flashes slowly (briefly turning off from an on state), indicating the boiler is operating even when there is no active central heating demand from the external room thermostat or timer. This could lead to unnecessary heating.

• Issue with the external diverter valve
• External timer or cylinder thermostat fault
• Short circuit across ST8 (from cylinder stat/timer)
• Faulty fascia mounted timer, diverter valve malfunction, or control board problem

Click steps to track your progress.

1. 1

   Check the external timer or room thermostat to ensure it is not sending a continuous demand signal.

2. 2

   If fitted, inspect the fascia mounted timer for correct operation and settings.

3. 3

   Verify the external diverter valve is not stuck open, causing the boiler to run.

4. 4

   A qualified technician should check for a short circuit across ST8 or a faulty control board.