Here is the basic ignition sequence for a furnace.
- Thermostat signals for heat.
- Exhaust blower creates a vacuum in the tube and causes the vacuum switch to operate.
- Pilot valve opens allowing gas to flow to pilot burner (if there is a pilot).
- Spark generator produces spark.
- Pilot lights and flame is rectified (if there is a pilot).
- Main burner valve opens.
- Pilot ignites main burner.
- Blower motor blows air across heat exchanger, and into conditioned space.
- Thermostat signals set temperature is reached.
- Gas valves close, main burner and pilot shut off.
- Exhaust blower motor shuts off.
- Blower motor shuts off.
If your exhaust motor is coming on, but the gas is not flowing. The first thing you should look at, is the vacuum switch. This switch is used to tell the furnace that the exhaust blower is on, and that the blower will clear any exhaust gases produced. If the switch does not close, the furnace will not light to prevent toxic gases from being spread through the conditioned space.
Check the tubing attached to the switch, to insure it's clear. Sometimes creepy crawlies like to make homes in things like this, so make sure there are no obstructions.
To check the switch...
- Turn off the breaker to the furnace (or the serviceman switch where applicable).
- Locate the vacuum switch (it will be a small metal bit, with a rubber tube attached to it).
- Disconnect the wires from the switch (make sure you mark them and remember where they're supposed to go).
- Set your multimeter to check continuity.
- Connect the probes of the multimeter to the wires on the switch.
- Check continuity (should be open).
- Suck on the tube (lightly).
- Check continuity (should be closed).
Professionals will likely use a different technique, but since it's not the safest practice I will not explain it here.
If the switch tests OK, you'll have to move on to the next part of the system. If the switch does not close, replace it and test the system again.
The next thing to check would be the pilot assembly, control module, and gas valves. However these checks are probably best left to a professional.