After the boiler has stopped and the pump is over running the motorised valve is closed and the pump is noisier than normal. Should the valve be closed? I presume the pump is trying to force the hot water into the bypass. The system is S Plan with Potterton Profile 80 boiler, Grundfoss pump, Drayton ZA5 2 way valves and Horstmann H27XL programmer. All three boiler thermostats have been replaced. Thanks.