Modine unit heater. Burner short cycling. I check my pressure switch that closes at .91", and get an approx. .60 reading. Check flue, sure enough it is plugged up with a birds nest. Clean it out, and re-check pressure, I get above a 1" reading. But when burner fires, the pressure drops to about .80. The pressure switch is definately bad, because even after cleaning the flue with a 1" reading, the switch wouldn't close. I have one on order, but I am concerned about the drop in pressure after the burner fires. Is this normal? Or may I have heat exchanger problems. I didn't see any visible cracks, but it was bright as hell in this room when I was checking it. It is a hot house for flowers. Thanks in advance.
Rich