Blue light Fast flash — Hot water not working (central heating OK)

What does Blue light Fast flash mean?

The blue indicator light flashes rapidly, indicating that the boiler is not producing hot water, although the central heating system is functioning normally. This points to an issue with hot water demand or supply.

• Problem with the external diverter valve
• External timer or cylinder thermostat issue
• 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 cylinder thermostat settings to ensure there is a demand for hot water.

2. 2

   If an optional diverter valve is fitted, ensure it is directing water correctly for hot water production.

3. 3

   If a fascia mounted timer is fitted, verify its settings for hot water.

4. 4

   Contact a qualified technician if the issue persists, as it could be a diverter valve or control board fault.