Furnace Blowing Cold Air or No Heat? What to Check

There are two very different problems hiding behind 'cold air.' The first is harmless: if the thermostat fan is set to ON, the blower motor runs constantly and pushes room-temperature air between burner cycles, which feels cold even though the furnace is fine. The second is real: the furnace isn't making heat at all, no ignition, no gas, or a safety has shut it down.

• Fan setting. Switch it from ON to AUTO so the blower only runs during an actual heat call.
• Thermostat, set it to HEAT and the temperature a few degrees above the room.
• Filter, a clogged filter overheats the furnace and trips a safety limit switch. Replace it if it's dirty.
• Power. Confirm the furnace breaker hasn't tripped and the furnace switch (it looks like a light switch nearby) is on.

If the basics are right and it still won't heat, the likely causes need a pro: a failed ignition or pilot, the gas supply shut off, a control-board fault, or, on a heat pump or dual-fuel system, a normal defrost cycle that briefly blows cool air and clears on its own. One thing we never want a homeowner to ignore is a possible cracked heat exchanger: if you smell odd odors or the furnace short-cycles on a safety, shut it off and have it inspected, because that's a carbon-monoxide concern.

Our winters are mild, so furnaces here run a fraction of the hours a northern furnace does, which means a fault can sit undetected for months and then surface on the first real cold snap, usually all at once across the city. A fall heating tune-up is how you find the cracked igniter or the weak limit switch in November instead of at 6 a.m. on the coldest morning of the year.

If your furnace is blowing cold air after the simple checks, or anything makes you suspect a safety problem, get it diagnosed. Our flat $85 diagnostic is credited to the repair, and we'll tell you honestly whether it's a quick fix or, on an older low-AFUE furnace, worth weighing a repair-or-replace decision.