The boiler will not operate the gas valve unless it knows that water is flowing through the boiler (flow switch) and that that air is flowing (differential air pressure switch). It could be the pump, fan, or switches at fault (assuming that the system has water pressure). The pilot will stay lit for a while even if there is no electricity to spin fan to scavenge the combustion chamber, no electricity stops the boiler working too.
c, February 2008
