I have an Economy 50F boiler which will not fire. I think it may be the air pressure switch but in order to isolate the switch I need to know which two wires to pair. There are three wires red, yellow and black which come from the switch to the board.

Dave, January 2009

Don't, use a multimeter to determine if the switch is switching or not. You should have 240v at the normally open connector when the fan runs.

Iain, January 2009