It sounds like a pretty basic failure. Possible faults:
1) Dead ignitor. If your furnace is less than 20 years old, it probably doesn't use a standing pilot light. More on that in a moment. Ignitors are either spark ignitors (a bit like a sparkplug) or a hot surface ignitor (HSI). An HSI is just like a...
Best answer: It sounds like a pretty basic failure. Possible faults:
1) Dead ignitor. If your furnace is less than 20 years old, it probably doesn't use a standing pilot light. More on that in a moment. Ignitors are either spark ignitors (a bit like a sparkplug) or a hot surface ignitor (HSI). An HSI is just like a light bulb: it gets so hot it glows nearly white-hot, and that ignites the flame. BUT - like light bulbs, HSIs eventually burn out, and need to be replaced. They're relatively cheap and due to their necessary occasional replacement, not too difficult to switch out.
2) Dead pilot light. If you have a standing pilot light, it's possible that it simply went out. Read the instructions on your machine and try re-lighting the pilot if you have one. If it lights and stays lit while you're holding the switch, but immediately goes out when you let go (hold it for a long count to 15), try replacing the thermocouple. The thermocouple is a simple device that holds the pilot's valve open as long as the pilot light is keeping the thermocouple's hot end hot. Like HSIs, thermocouples eventually fail. They are also fairly inexpensive and easy to replace.
3) Dead pressure switch. If your furnace makes a sound like a fan is spooling up, then quits, that may be a bad pressure switch. That first fan spooling up may be your draft inducer, and the pressure switch looks for a pressure change within the burner compartment to know that the inducer is working, which means there will be good airflow across the burners. That switch HAS to make before it'll allow the burners to fire. This is a somewhat more esoteric thing to test but again, the switches aren't terribly expensive and they're easy to replace.
4) Dead draft inducer blower. The flip side of the pressure switch not working is the pressure change not happening...because the blower doesn't work. The last one I replaced cost about $250 and was fairly typical of the breed, though there is a significant range and your mileage may vary. Again, replacement isn't too hard.
5) Failed limit switch. While this is possible, I would check the ignitor first. Some limit switches have a reset on them, in which case you should absolutely push that little button to reset the switch...then STAY there and watch while the furnace ignites, to see if you can detect why the limit switch tripped in the first place.
I'm sorry to say it but I don't see any way around clambering under the house to go look at the furnace. All of these things are ON the furnace itself.
I check back on my answers to see if askers need more assistance. Take good notes of what's going on, write down the manufacturer name and model number. If you have more questions, post them as Additional Information, and I'll help if I can.
Good luck with it.
3 days ago