Why Is My Gas Furnace Blowing Cool Air?

If your gas furnace is blowing cool air through your vents, it could be a simple fix – or something a bit more complicated. Here are three common causes for cool air coming from your furnace that you may be able to fix yourself.

1. Your thermostat is in the “ON” position – If your furnace is constantly blowing air, but that air is intermittently hot and cold, the problem could be your thermostat setting. If your thermostat is set to “ON,” the blower will continue to run even when your furnace isn’t heating the air.
   • Solution: Set your thermostat fan to “AUTO” rather than “ON.”
     
2. Your furnace is overheated – If your furnace suddenly stops blowing warm air after doing so for an extended period of time, it could be overheated (your furnace has a “limit switch” which turns the furnace burners off if things get too hot; the blower keeps operating to cool the furnace).What causes the furnace to overheat? More often than not, it’s a dirty air filter, which blocks air flow and causes the furnace to overwork to heat your home.
   • Solution: Clean or change the air filter, then give your heating system a rest for an hour or two before you try it again. DON’T IGNORE THE PROBLEM – repeated overheating can do costly damage the furnace’s heat exchanger.
     
3. Your pilot light is out – If your gas furnace is only blowing cold air, your pilot light may be out. Check to see if it is and try these solutions:
   • Solution 1: If you’re comfortable doing it, re-ignite your pilot light, following the manufacturer’s instructions carefully. If you’re not, don’t worry – we can help.
   • Solution 2: If the pilot won’t light, make sure the furnace’s gas valve switch is in the “on” position (parallel with the gas supply pipe).
   • Solution 3: If the pilot lights but won’t stay lit, your thermocouple (the copper rod that contacts the pilot flame) may need to be replaced ,