Is My Thermostat Set to ON Instead of AUTO?
Start here, because this is the most common answer by far, and it isn't a broken furnace at all.
If your thermostat fan is set to "ON," the blower runs constantly, even between heating cycles. So during the stretches when your furnace isn't actively heating, that fan is still pushing room-temperature or cool air through your vents, and it feels like the furnace is blowing cold. Switch the fan to "AUTO" and it will only run when the furnace is actually producing heat.
While you're at the thermostat, make sure it's set to "Heat," bump the temperature up a few degrees to trigger a cycle, and if it takes batteries, try fresh ones. A surprising number of "cold air" calls end right here.
Is It Just Warming Up?
At the very start of a heating cycle, it's normal to feel air that isn't hot yet for a minute or so while the system comes up to temperature. If it warms up shortly after and heats fine, that's just how it works. It's when the air stays cold that something is wrong.
Have You Checked Your Filter?
Your filter again, yes. A badly clogged filter chokes the airflow through your furnace, and the furnace can actually overheat as a result. When it overheats, a safety switch shuts the burner off to protect the system, but the blower keeps running, so you get a furnace that's blowing cool air with no obvious reason why.
Pull your filter and check it. If it's dirty, replace it. A clean filter fixes this more often than people expect, and it's the cheapest thing you'll ever do for your system.
Could It Be the Ignition or the Flame Sensor?
This is where it usually crosses from something you can fix to something worth a call. On a gas furnace, if the burner isn't lighting, all you'll get is cold air. On older units, that can be a pilot light that's gone out, which you can sometimes relight yourself if you follow the instructions printed on the unit. On newer units, it's often a failed igniter or a dirty flame sensor, which makes the furnace light briefly and then shut the burner right back off. Those are jobs for a technician.
When Should I Call?
Call if you've checked the thermostat and the filter and it's still blowing cold, if the furnace keeps overheating and shutting off, if it lights and quits over and over, or if it won't ignite at all. If you have a high-efficiency furnace, a clogged condensate line can also shut it down, which is another one for us.
And the safety one, as always: if you ever smell rotten eggs near the furnace, that's a possible gas leak. Leave the house and call your gas company from outside.
The Short Version
Nine times out of ten, cold air from a running furnace is a thermostat set to "ON" instead of "AUTO," or a clogged filter overheating the system. Check those two first. If it's still cold, or the furnace is lighting and quitting, or it won't light at all, that's an ignition or safety issue worth a professional look, especially with the cold months ahead.
If the easy checks don't do it, we're glad to get you warm again. We're a family-run shop covering Nampa, Caldwell, Meridian, Boise, and Emmett, at 208-455-5158. Comfort Club members go to the front of the line when the cold-weather calls stack up.