Hi guys. A friend has a prob with ( i think) his boiler. Open vented system, two two port valves. When calling for CH or HW, the boiler does not fire up. The boiler is a Potterton Prima F. The valves are opening when calling for CH or HW, the contact with micro switch is made and voltage is sent to boiler ( i checked in wire box) and boiler is getting the voltage too since i checked with multimeter. Does the prob lays within the boiler? Probably the thermistor??? or..? Btw, the pump runs. Thank u 4 your time.