Why Is My Furnace Blowing Cold Air? 8 Things to Check Before You Call
A furnace blowing cold air is usually something simple: the thermostat, a dirty filter, a clogged drain line or an ignition problem. Here is what to check yourself, what not to touch, and when to call, from the Smith family HVAC shop in Spanish Fork.
By Steve Smith, Cozi Air · Spanish Fork, Utah · November 9, 2026
If your furnace is running but the air coming out of the vents is cold, the cause is usually one of a handful of things, and several of them you can check yourself in a few minutes. Start with the easy ones below, in order. If none of them fix it, or anything smells like gas, stop and call.
1. Is the thermostat set to HEAT and the fan to AUTO?
If the fan is set to ON instead of AUTO, the blower runs all the time, including between heating cycles, so you feel cool air even when the furnace is working fine. Set it to AUTO and HEAT, and set the temperature a few degrees above the room. If the thermostat runs on batteries, a weak battery can cause strange behavior too.
2. Has it only been running a minute or two?
Most furnaces blow a little cool air for the first minute while the heat exchanger warms up. Give it two or three minutes before deciding something is wrong.
3. When was the filter last changed?
A clogged filter chokes the airflow, and many furnaces protect themselves by shutting the burners off when they run too hot, while the blower keeps going. That feels like cold air. Check the filter, and if you can't see light through it, replace it. In a Utah winter, most homes should check it monthly.
4. Is the furnace switch or breaker off?
There's usually a light-switch style power switch on or near the furnace, and a breaker in the panel. Either one can get bumped. Make sure both are on.
5. Is the condensate drain frozen or clogged?
High-efficiency furnaces make water, and it drains out through a small pipe. If that line clogs, or freezes where it runs outside, a safety switch shuts the burners off. If you see water around the furnace or a drain line that runs outdoors, that's a common winter culprit in Utah.
6. Is it trying to light and failing?
If you hear the furnace click and try to start, then shut down, the igniter or the flame sensor may be the problem. A dirty flame sensor is one of the most common winter service calls we get. This one is best left to a technician, because it means opening the burner compartment.
7. Is the gas on?
If other gas appliances are working, the gas supply is probably fine. If the gas valve at the furnace has been turned off, that will stop the heat. If you smell gas at any point, don't touch anything: leave the house and call 911 or the gas company from outside, then call us.
8. Is the air cold in some rooms but warm in others?
That's usually ductwork rather than the furnace: a disconnected duct in the attic or crawlspace, or a run that was never sized right. We build and repair ductwork in our own sheet metal shop, so we can fix the cause instead of just turning up the heat.
Still blowing cold? Call or text Steve at 801-361-4491. A real person at the family shop answers, and if it's an older furnace that keeps acting up, he'll tell you straight whether it's worth fixing or time to replace it.
When it's time to replace instead of repair
If the furnace is past fifteen years old, the repairs keep coming back, or there's any sign of a cracked heat exchanger, replacing it is often the better money. Every Cozi Air replacement comes with a free in-home estimate, good, better and best options, fittings built in our own shop, a full year guarantee on our work on top of the manufacturer warranty, and your first year of maintenance free.
Questions homeowners ask
Why is my furnace running but blowing cold air?
The most common causes are the thermostat fan set to ON instead of AUTO, a clogged filter making the furnace shut off its burners, a clogged or frozen condensate drain on a high-efficiency furnace, or an ignition or flame sensor problem.
Can a dirty filter make a furnace blow cold air?
Yes. A clogged filter restricts airflow, the furnace overheats and its safety switch shuts the burners off while the blower keeps running, so the air feels cold.
Why does my high-efficiency furnace stop heating in very cold weather?
Often the condensate drain line has frozen or clogged, which trips a safety switch. It's a common Utah winter problem and usually an easy fix for a technician.
Is it safe to keep running a furnace that blows cold air?
If you smell gas, hear unusual banging, or a carbon monoxide alarm goes off, turn nothing on: leave the house and call 911 or the gas company. Otherwise it's usually safe to check the thermostat, filter and switch, then call for service.
Who fixes furnaces in Spanish Fork and Utah County?
Cozi Air, the Smith family shop in Spanish Fork since 1938, serves all of Utah County, south Salt Lake County and down to Nephi. Call or text 801-361-4491.