Short answer
First set the thermostat fan to Auto, since a fan set to On blows room-temperature air between cycles. Then replace the filter: a clogged filter makes the furnace overheat, the limit switch shuts the burners off, and the blower keeps running cold. If it still blows cold, the burners aren't lighting or staying lit, and it needs a tech.
A gas furnace that blows cold air is one of the most common winter calls I get. Sometimes nothing is broken at all. Other times the burners aren't lighting, or a safety switch keeps shutting them off while the blower keeps running. Here's how to tell which, in the order that finds it fastest.
If you have a heat pump instead of a gas furnace, the air works differently. See heat pump blowing cold air instead.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Set the thermostat fan to Auto
If the fan is set to On, the blower runs all the time, including between heating cycles, and you'll feel room-temperature air at the vents. Carrier calls this the most common cause of a furnace blowing cold air. Set the fan to Auto and the mode to Heat, with the temperature a few degrees above the room.
If the air is warm on the next cycle, you're done.
Give it a minute after it starts
Many furnaces wait about 30 to 60 seconds after the burners light before the blower comes on, and the first air out of the ducts can be cool. Wait a few minutes before deciding it's not heating.
Replace the air filter
A clogged filter traps heat inside the furnace. When it gets too hot, the high-limit safety switch shuts the burners off, but the blower keeps running to cool things down, so you get cold air. Pull the filter and hold it up to a light. If you can't see through it, replace it, then open any closed vents and clear the return grilles.
If it still blows cold with a new filter and open vents, something else is restricting airflow and it needs a tech.
Check that gas is reaching the furnace
The handle on the gas valve going into the furnace should run parallel with the pipe (open). If your stove or water heater runs on gas, check that they work. If you smell gas, don't touch anything: leave the house and call your gas company or 911 from outside.
Look for a full drain pan (high-efficiency furnaces)
High-efficiency furnaces make water and drain it through a PVC line. If that line clogs, a safety switch keeps the burners from lighting, and the blower may still run. Water in the pan or around the furnace is a sign. A stained ceiling or an overflowing pan means call us.
Read the error light before you reset anything
Through the little window in the lower panel, count the blinks on the control board light and write it down. Turning the power off often clears it. See how to read furnace error codes and what 4 blinks usually means.
Older furnaces: is the pilot light out?
If your furnace has a standing pilot (a small flame that stays lit), the burners can't light without it. Relight it only by following the instructions printed on the furnace, and never if you smell gas.
When to call a pro
- You hear the burners light, then shut off after a few seconds (often a dirty flame sensor).
- It still blows cold with a new filter, open vents and the fan on Auto.
- The error light keeps flashing the same code.
- You smell gas, or there's water around the furnace.
- A breaker or the furnace switch trips again after one reset.
Still not working? Same-day repair.
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
Why does my furnace blow cold air at the start?
Most furnaces light the burners first and start the blower 30 to 60 seconds later, and the air sitting in the ducts is cool. A short burst of cool air at the start of a cycle is normal.
Can a dirty filter make my furnace blow cold air?
Yes. A clogged filter lets heat build up inside the furnace until the high-limit switch shuts the burners off. The blower keeps running to cool it down, so you feel cold air. Replace the filter and open the vents.
What is a flame sensor?
A small metal rod near the burners that proves the flame lit. When it gets coated, the furnace lights for a few seconds and then shuts the gas off. Cleaning or replacing it is a quick job for a tech.
How much is a service call?
Frost Fire's diagnostic is $59, and it's waived if you go ahead with the repair. Same price on weekends.
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