The gas and the electric supply to the boiler need to be on.
They usually have a timer / programmer to control on / off times of both hot water and heating. They have a user adjustable thermostat on the front - mine is set to 3 - to determine how hot the boiler heats the water. The timer needs to be "calling" for heat for the boiler to work.
When all conditions are satisfied the exhaust fan begins to run (you should be able to hear it) and when a pressure switch senses that the fan is working OK then the pilot light lights automatically (you can see it through the viewing glass) - when the electronics sense that the pilot light is working OK then the main gas valve opens and the boiler fires up - you will be able to see and hear it.
Good luck...
Peccavi, September 2009
