20-04-20, 11:16 AM
Not a combi boiler bod, but I came across something similar on my own system recently and it was a faulty Pump Overrun control system. In my case it was the overrun thermostat.
This thermostat stat allows the pump to run for a while after the system turns off to cool the water in the heat exchanger
This thermostat stat allows the pump to run for a while after the system turns off to cool the water in the heat exchanger