I have a Worcester 24I combi boiler, I have noticed when I run the cold tap then turn it off the boiler starts running, it doesn't fire up, just the pump starts by the sound of it.
The back pressure when you shut off the cold tap is triggering the dhw flow switch in the boiler. Have a plumber or gas engineer fit a mini expansion vessel onto the cold water supply pipe just below the boiler, this should be enough to cushion the back pressure and cure the fault.
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