If your furnace is blowing cold air, first check two things: whether the thermostat fan is set to ON instead of AUTO, and whether the air is cold only for the first minute or so of each cycle (which is normal). If the air stays cold, the most common causes are a dirty air filter causing the furnace to overheat, a dirty flame sensor, a failed igniter, a clogged condensate drain, or a gas supply problem.
Here are 8 causes and how to fix them, starting with the easiest.
Safety first: if you smell gas, leave the house immediately and call your gas utility or 911 from outside. Don’t bypass furnace safety switches.
Is it actually a problem?
Two situations are normal:
- At the start of a cycle, the blower may push 30 to 60 seconds of cooler air that was sitting in the ducts.
- At the end of a cycle, the blower keeps running for a minute or two after the burners shut off to pull remaining heat out of the heat exchanger. The air cools as it does.
If the air never gets warm, or turns cold in the middle of a cycle, keep reading.
1. The thermostat fan is set to ON
With the fan set to ON, the blower runs constantly, even when the burners are off between heating cycles, and pushes room-temperature air that feels cold.
Fix: set the fan to AUTO so the blower runs only when the furnace is heating. Also confirm the mode is HEAT and the setpoint is above room temperature.
2. A dirty air filter (overheating)
A clogged filter restricts airflow, the heat exchanger gets too hot, and the high-limit switch shuts off the burners while the blower keeps running to cool things down. The result: cold air.
Fix: replace the filter and open all vents and returns. See how often to change your air filter. If it keeps happening with a clean filter, have a technician check airflow and the limit switch.
3. A dirty flame sensor
If the flame sensor can’t confirm the flame, the furnace shuts off the gas a few seconds after lighting. On some furnaces the blower keeps running, blowing cold air.
Fix: clean the sensor. See how to clean a furnace flame sensor.
4. The igniter or pilot isn’t lighting
Modern furnaces use a hot surface igniter; older ones use a standing pilot light. If there’s no ignition, there’s no heat, though the blower may still run.
Fix: on older furnaces with a pilot, follow the relighting instructions on the furnace label exactly. If the pilot won’t stay lit, the thermocouple is often the culprit. For hot surface igniters, see furnace ignitor not glowing.
5. A clogged condensate line (high-efficiency furnaces)
High-efficiency furnaces drain water through a trap and line. A clog trips a safety switch that stops the burners, sometimes while the blower keeps running.
Fix: check for water inside or around the furnace and flush the drain line and trap. If you have a condensate pump, make sure it’s working.
6. Gas supply problems
No gas means no flame.
Fix: confirm the gas valve at the furnace is open and other gas appliances work. On propane, check the tank. Leave anything beyond that to the utility or a technician.
7. Leaky or disconnected ductwork
If ducts in an unheated attic, crawl space or garage are leaking or disconnected, warm air escapes before reaching some rooms, and those rooms may get chilly air.
Fix: check accessible ducts for disconnected sections. A professional duct test and sealing can solve it.
8. A control board or limit switch problem
A failing board or a faulty limit switch can stop the burners while the blower keeps running.
Fix: note the error code flashing on the board (see our furnace blinking light codes guide) and call a technician.
A note on heat pumps
If you have a heat pump, warm air will feel cooler than from a gas furnace: typically 90 to 100°F at the vents instead of 120°F or more. That’s normal. A heat pump also blows cooler air briefly during defrost cycles. See our heat pump vs. furnace guide for how they differ.
When to call a professional
- The furnace still blows cold air after you’ve checked the thermostat, filter and flame sensor.
- The furnace locks out repeatedly.
- You suspect a gas, igniter, limit switch or control board problem.
- You smell gas or burning, or your CO alarm goes off.
If your furnace has a status light, look up the code for your brand: Carrier, Goodman, Trane or Lennox.
For a step-by-step check that starts at the thermostat, see thermostat says heat on but no heat.
Burners shut off after a few minutes while the blower keeps running? See furnace high limit switch tripping.
Blower running nonstop even when the heat is off? See furnace blower won’t turn off.
Frequently asked questions
Why does my furnace blow cold air, then hot air?
Usually it’s normal startup: the blower pushes cool air from the ducts before warm air arrives. If it switches from hot to cold mid-cycle, the furnace may be overheating on its limit switch or losing flame.
Can a dirty filter make a furnace blow cold air?
Yes. It can make the furnace overheat, which shuts off the burners while the blower keeps running.
Why is my furnace blowing cold air after a power outage?
The furnace may be in lockout or the thermostat may have reset. Reset the furnace by turning its power off for 30 seconds and back on, and check thermostat settings.
For a complete troubleshooting guide, see Furnace Not Turning On: 11 Causes and Step-by-Step Fixes.
Disclaimer: this guide is for general information only. If you smell gas, leave your home and call your gas utility or 911 from outside. Contact a licensed HVAC professional for gas and electrical repairs.