Like all machines, aging furnaces can begin to regularly malfunction. This could be highly problematic in the middle of a cold night. Fortunately, you can troubleshoot many common forced-air furnace problems. Once you know what the problem is, you can determine if you need to have a qualified professional make a house call. Often, the fix is simple and can be done immediately yourself.
When troubleshooting an aging furnace, first check the thermostat. Make sure it is set to the “heat” position and that the desired temperature setting is higher—or warmer—than the current temperature. Next, check the furnace power supply. Sometimes an ordinary light switch is installed near a furnace for a quick power shut off. Also check the breaker box and make sure the furnace switch is in the “on” position. Once you’ve made sure the furnace is getting power, set the thermostat to the “off” position and wait 10 minutes.
After waiting 10 minutes, return the thermostat to the heat setting, look into the furnace and see if the igniter glows red. The igniter burns hot and is easily seen when functioning. You may have to remove the furnace access panel to see the igniter’s bright glow, but some furnaces are equipped with portholes for this purpose. If the igniter is functioning, the next thing to check is the gas flow. It’s easy to accidentally turn the handle on a gas valve and shut off the flow.
If the furnace ignites, produces warm air but prematurely shuts off, there could be several reasons that will depend on your particular furnace type. Also, if the gas flow is on and the igniter appears to function but does not fire up the furnace, it could be a control board problem.