My rear furnace will blow air but no heat. I'm looking for suggestions as to what the problem could be. What can I do to check it out? Could there be a spider web clogging it or some else I can check?

Thanks

The furnace ignition circuit starts a spark and opens the gas valve only when the "sail" in the blower chamber has enough air pressure to trip the switch.

Your issue could be one of the following:

1. Low battery, so the blower's not blowing enough to trip the switch to the ignitor.
2. The ignitor isn't producing a spark; a thermocouple senses a spark and opens the gas valve. The thermocouple could be also be bad.
3. The gas line needs to be bled of air - bleed it out by lighting a stover burner.
4. Wiring to the sail switch, to the ignitor and thermocouple has an open.

A spider web's unlikely to cause the issue....

Furnace troubles been there!
I had a problem with my atwood 8531 1V furnace, would try to light, but the sail switch turned off. Problem was 2 styrofoam cups of sand out of the burner chamber as well as I had to replace the valve that was plugged by yes two tbl spoons of sand in it That was my problem !!
Dalton

1974 C Class 19'
30000 btu