Your boiler has detected that you have turned on a hot water tap and is currently working to provide hot water to your home.

DHW demand: Domestic hot water requirement via impeller sensor

What causes this fault?

This code appears when the flow sensor inside the boiler detects that water is running through the hot water pipe. It is the boiler's way of confirming that it has successfully registered your request for hot water and is firing up the burner to heat it. This is a normal operational signal rather than a malfunction of any internal components.