Here is some info from Good Sam Insider that might help you out.
How A Furnace Works:
- A thermostat summons heat via a circuit board inside the furnace.
- After a short delay, the board activates a dual-purpose blower, injecting air into the combustion chamber while also moving air thru the ducts.
- A sail switch closes, signaling that the blower has enough air for combustion to occur.
- A gas valve is opened, admitting gas into the burner inside the combustion chamber, where an electrode assembly produces spark.
- Combustion occurs and gasses are vented outside.
- A limit switch halts gas flow if temperature is too high or if the blower becomes inoperative.
- The thermostat controls furnace run time.
Troubleshooting Malfunctions:
The most common malfunctions are generally related to one of two items:
- DC voltage and/or gas pressure.
- If the fuse panel is ok, use a 12v DC tester to check for power in the wires leading into the furnace. This will ensure that you have an uninterrupted power circuit.
- Check voltage to make sure it is adequate - at least 11 volts. Compare voltage at the battery and at the furnace. In weak circuits, voltage may drop as much as 1 volt between the battery and furnace, suggesting larger wiring is needed or battery is not capable of holding a charge.
- If the voltage is good, the problem may relate to the furnace circuit board, a relay, faulty blower motor, or another component such as the thermostat.
I recently ran across this article and thought I would pass the info on. Hope it helps you out.
Bob