Most furnaces that blow cold air have a simple cause: the thermostat fan is set to On, a clogged filter made the furnace overheat, or the burners never lit. You can check the first two yourself in a few minutes. On the first cold snap along the Front Range, a furnace that sat idle all summer has to run all night, and any weak spot shows up fast.
Below, we sort the causes by what you notice, tell you which ones you can handle, and mark the ones that need a technician. If you’d rather skip the checks, book a repair online.
Call for help first if you notice any of these
- A rotten-egg smell or a hissing sound near the furnace. Leave the house on foot, don’t touch switches or use your phone inside, and call your gas utility from outside.
- A carbon monoxide alarm sounding, or headaches and dizziness that ease when you step outside.
- Smoke or a sharp electrical burning smell. Turn the thermostat off and leave the furnace alone. Call us.
- No heat in freezing weather with young children or older adults at home. Match what you notice to the likely cause.
| What you notice | Most likely cause | Who fixes it |
| Fan runs nonstop, air feels cool, house never warms | Thermostat fan set to On | You |
| Thermostat blank, in the wrong mode, or set below room temperature | Thermostat settings or dead batteries | You |
| Warm air for a while, then cold, then the furnace shuts off | Dirty filter or blocked vents causing an overheat shutdown | You first, then a technician if it repeats |
| Cool air for a short stretch at startup, then it warms | Normal warm-up | No one |
| Furnace tries to start but the burners never light | Ignition problem | Technician |
| Burners light, then shut off quickly, again and again | Dirty or failing flame sensor | Technician |
| High-efficiency furnace, cold air, and water near the unit | Clogged or frozen condensate drain | Technician |
1. The thermostat fan is set to On
If the fan setting reads On instead of Auto, the blower runs nonstop and pushes room-temperature air through your vents even when the furnace isn’t heating. It’s the most common cause and the cheapest fix.
- Set the fan to Auto.
- Set the mode to Heat.
- Set the temperature a few degrees above the current room temperature.
- Wait a few minutes. Warm air should follow.
2. Thermostat settings or dead batteries
A thermostat in Cool mode, on a hold, or running on weak batteries can leave the furnace idle while the fan runs. Check that the display is lit and the mode says Heat. Replace the batteries if your thermostat uses them, and cancel any schedule hold that overrides your setting.
3. A dirty filter caused an overheat shutdown
A clogged filter restricts airflow, the furnace overheats, and a safety switch shuts the burners off. The blower keeps running to cool the unit, so what comes out of the vents is cold. If your system also runs your AC, the same filter has been working through the summer, so it may be loaded by the first cold day.
- Turn the thermostat to Off and let the furnace rest.
- Slide out the filter and hold it up to a light. If you can’t see through it, replace it.
- Install the new filter with the arrow pointing toward the furnace.
- Turn the thermostat back to Heat.
Need the right size? Our online shop carries filters. If the furnace shuts off again after a fresh filter, a technician needs to look at the limit switch and blower.
4. Closed or blocked vents and returns
Blocked airflow causes the same overheating as a dirty filter. Check that every supply register is open and that furniture, rugs or boxes aren’t covering a return grille. Leaky ducts in an unheated attic or crawlspace can also cool the air before it reaches the room, and a technician can test for that.
5. The ignition isn’t lighting the burners
If the furnace clicks, hums or starts its sequence but the burners never light, the fan can still run and blow cold air. The cause can be a failed igniter, an older pilot light that went out, or a gas supply problem. All of these are technician jobs.
All you can safely check is that the thermostat is calling for heat, the furnace power switch is on, and the breaker hasn’t tripped. Don’t open any gas valve or try to relight a pilot yourself, and if you smell gas, follow the box above.
6. A dirty flame sensor is shutting the burners off
The flame sensor confirms the burners are lit. When it’s coated or worn, the furnace shuts the gas off a moment after lighting, then tries again. You hear the furnace start, feel a little heat, and then cold air returns.
This is a cleaning or replacement job for a technician, because it means working inside the furnace cabinet. Repeated start-and-stop cycles are the sign to watch for.
7. The condensate drain is clogged or frozen
High-efficiency furnaces pull so much heat from the exhaust that it turns to water, and that water has to drain out. If the drain line clogs or freezes, a safety switch can stop the furnace. This shows up more in garages, crawlspaces and basements that aren’t heated, especially during a Front Range cold snap.
Look for water around the base of the furnace or a drain line that’s kinked or iced over. Don’t open the furnace. Call us and we’ll clear the line and find out why it backed up.
Furnace not working at all? Check these three things first
If the furnace won’t run, not just blow cold, start here before you call.
- Confirm the thermostat has power and is set to Heat.
- Find the furnace power switch, which looks like a light switch on or near the unit, and make sure it’s on.
- Check your breaker panel. If the furnace breaker tripped, reset it once. If it trips again, stop and call us.
When to call Comfort Pros
Call if the air is still cold after the checks above, if the furnace starts and stops repeatedly, or if the table above says technician. We trace the startup sequence to find where it stops, whether that’s the igniter, flame sensor, safety switches, blower, drain or venting.
We’re family owned, with 15 years in business and more than 40 years of combined team experience. Book a repair online or call (303) 945-4319. 5.0 from 678 Google reviews.
Why this shows up on the first cold day in Thornton, Northglenn and Westminster homes
Many 1960s to 1980s homes in Thornton, Northglenn and Westminster still have older furnaces, and a tired igniter or sensor often gives out the first time the system runs hard. Newer homes in Brighton, Commerce City and Broomfield are more likely to have high-efficiency furnaces, where the condensate drain is the weak point in an unheated garage or crawlspace.
Our shop is at 7270 Gilpin Way in north Denver, near Thornton and Welby. See our furnace and heating services for what we repair and install.
If your furnace is older and the repairs keep stacking up, read our guide to furnace repair vs. replacement in Denver. If replacement makes sense, our financing page explains the payment options.
Frequently asked questions
Is it normal for a furnace to blow cold air for a minute at startup?
Yes, briefly. The furnace has to warm up before the blower pushes heat out. If the air never warms up, that’s not normal.
Can a dirty filter make a furnace blow cold air?
Yes. A clogged filter can overheat the furnace, and a safety switch then shuts the burners off while the fan keeps running.
Why does my furnace blow warm air, then cold air, then warm again?
That pattern usually means the burners are starting and stopping. A dirty flame sensor or an overheating furnace are the usual causes, so start with the filter and call us if it continues.
Is a furnace blowing cold air dangerous?
The cold air itself usually isn’t. A gas smell, a carbon monoxide alarm, smoke or a burning smell are separate emergencies, and the box near the top tells you what to do.
Do I need a new furnace if it’s blowing cold air?
Not usually. Most causes above are repairs. Replacement becomes a question when the furnace is old and failures are piling up.
Still getting cold air?
Book a repair online or call us at (303) 945-4319. We’re Comfort Pros Heating and Air Conditioning, family owned, with 5.0 from 678 Google reviews.
