Why is my furnace blowing cold air?
Cool air from the registers means the blower is doing its job and the burners aren't, at least not for long. Sometimes it is only a thermostat setting. Sometimes the furnace overheated and tripped a safety control, or the ignition system can't prove a flame. Work through the outside checks first. They solve a fair share of these calls.
Likely causes
The fan is set to On instead of Auto Safe to check
With the thermostat fan set to On, the blower runs nonstop, including between heating cycles. The air feels cool when the burners are off. DOE recommends the Auto setting for heat pumps for efficiency, and it is the normal setting on furnaces too.
A clogged filter tripping the high limit Safe to check
Restricted airflow overheats the heat exchanger, the limit switch shuts off the burners, and the blower keeps running to cool things down. That stretch of cool air is the symptom. Carrier says to replace a gray or dusty filter.
Ignition or flame sensing failure Pro job
Hot surface igniters crack and flame sensors get coated. The furnace tries to light, fails to prove flame, and locks out. Those parts sit in the burner compartment and need a licensed HVAC technician.
Gas supply interrupted Safe to check
A closed supply valve or a utility interruption leaves nothing to burn. You can confirm the valve position on the pipe feeding the furnace without touching the furnace's own control.
A clogged condensate drain on a high-efficiency unit Pro job
Condensing furnaces have a safety switch that stops the burners if condensate backs up. Water near the unit is the clue. A technician should clear internal traps.
Safe things to check first
- Set the thermostat to Heat, fan on Auto, and the setpoint a few degrees above room temperature.
- Replace the filter if it looks gray or you cannot see light through it.
- Look at the electrical panel and reset a tripped furnace breaker once.
- Confirm the gas supply valve on the pipe near the furnace is open (handle parallel to the pipe). Look, do not adjust the furnace's gas control.
- Check that the furnace door is fully closed. Many furnaces have a door switch that keeps the burners off when the panel is loose.
When to stop and call
Call a licensed HVAC technician if the burners still do not stay lit after these checks, if the breaker trips again, or if the furnace shows a flashing error light. Carrier warns owners not to diagnose or repair gas, combustion or electrical components themselves. If you smell gas, do not touch switches or try to restart the furnace. Get everyone outside and call the gas utility from there.
Repair or replace?
Start with its age. Type the serial from the label:
Replacing it: a new gas furnace (80,000 BTU, 95%+ efficiency), installed
National figures before permits and code extras. Prices for your ZIP code.
An igniter or flame sensor is one of the smaller furnace repairs, so on a mid-life unit it is an easy yes. DOE's average for a non-weatherized gas furnace is about 21.4 years, and a 2007 builders' survey listed gas warm-air furnaces at 18. Date yours with the age decoder above. Near those ages, with several calls already this season, compare the repair against the replacement range above.
Common questions
Is it normal for air to feel cool at the start of a cycle?
Yes. Most furnaces delay the blower until the heat exchanger warms, but you may feel a few seconds of cooler air from the ducts.
Why does it blow warm, then cold, then warm?
That pattern often points to the high-limit switch cutting the burners because of restricted airflow. Start with the filter.
Can I reset the furnace by turning off its power switch?
Cycling power can clear a lockout once. If it locks out again, call a technician instead of repeating it.