The boiler won't fire up, the flue fan just keeps stopping & starting. Followed Faulting guide & swapped Air pressure switch & control board, checked all piping, also swapped fan but no change. Any idea's? Also what is the Air pressure switch actually "looking" for? Positive pressure down the red pipe, vacuum up the clear? If I disconnect both pipes to the switch the fan will run constantly but the boiler still doesn't fire up.