I have a potterton promax boiler in and s configuration with 2 - Zone valves. One for the Central Heating (ch) and one for the Hot Water (hw). When I switch the hot water on, the boiler does not start. The boiler Diagnostics identifies that the Flow Switch is not operating. If I switch on the Central heating, then both work OK. I have tested the HW Zone valve and it is definitely turning ON. I have even manually operated it, without sucess. However, when the CH is ON the discharge pipes of the CH and the HW both get hot simultaneously. So some water is flowing through the HW valve. I have taken it apart and cannot I/D any problem. Is there any data on the flow rate of these valves and the the Flow switch requirement? I have tried the Grundfos Pump in positions 1, 2 and 3, again with no success. Everything else is operational. I am at my wits end.