hi, i agree with mike, but may i add that there is a micro switch that is operated by the diverter valve, when you turn on the tap the diaphram moves and trips the switch to start the boiler up, you should be able to here the switch click in and then see the boiler start,but if the diaphram is not moving very far ie about 4mm, then the switch will not operate.
bob roberts, January 2006