Short answer
Check the thermostat first: it should be on "Heat" with the fan on "Auto", not "On". On "On", the blower keeps running between heating cycles and the air feels cold. Replace a clogged filter, which can make the furnace overheat and shut off the burners while the fan keeps blowing. Then reset it once with the furnace switch. If the air is still cold, call a technician.
Safety first
If you smell gas (like rotten eggs), do not touch any switch: get everyone out and call 911 or your gas company from outside. Do not open the burner compartment or try to relight a pilot light yourself.
How to fix a furnace that blows cold air
1Cold air from the vents
The fan runs, but the air is cool. A minute of cool air when a cycle starts is normal. Longer than that, check these.
Read all the steps
- Cold air from the vents. The fan runs, but the air is cool. A minute of cool air when a cycle starts is normal. Longer than that, check these.
- Heat on, fan on Auto. Set the thermostat to "Heat", a few degrees above room temperature, and the fan to "Auto". On "On", the fan blows even when the burners are off.
- Replace a clogged filter. A dirty filter chokes the airflow. The furnace overheats, a safety switch turns the burners off and the fan keeps blowing cool air.
- Reset it once with the furnace switch. Turn the furnace switch off, wait one minute and turn it back on. That clears a lockout after the burners failed to light.
- Warm air in a few minutes. The burners light and the air warms up. Cold again by the next cycle? Write down the status light code and call a technician.
- Still not fixed? Call an HVAC contractor Especially if: the air is still cold after these checks, or it warms up and goes cold again every cycle.
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Why it happens
Besides the fan setting and the filter, a dirty flame sensor can shut the burners off after a few seconds. A failed ignitor or a pilot light that went out on an older furnace does the same. High-efficiency furnaces also stop when the condensate drain clogs or a vent pipe is blocked. Leaky ducts in an attic or crawl space can cool the air before it reaches you. A technician checks those.
What not to do
- Do not relight a pilot light or open the burner compartment. Call a technician.
- Do not keep resetting the furnace if it locks out again. One reset is a check; more is a job for a pro.
- Do not close vents in empty rooms to push heat elsewhere. Less airflow can make the furnace overheat.
When to call an HVAC technician
- The air is still cold after these checks, or it warms up and goes cold again every cycle.
- The status light blinks a code, or the furnace locks out again after one reset.
- An older furnace has a pilot light that went out.
- You hear clicking but the burners never light.
Frequently asked questions
Why is my furnace running but blowing cold air?
Most often the thermostat fan is on "On" instead of "Auto", or a clogged filter made the furnace overheat and shut off the burners. A dirty flame sensor, a bad ignitor or a blocked vent pipe come next, and those need a technician.
Is it normal for a furnace to blow cold air at first?
For a minute or so, yes, while the ducts warm up. If it keeps blowing cold after a few minutes, check the thermostat and the filter.
Should the furnace fan be on Auto or On?
Auto, for most homes. On "Auto" the fan runs only while the furnace heats. On "On" it runs all the time and blows room-temperature air between cycles.
Can a dirty filter make the furnace blow cold air?
Yes. The furnace overheats, a safety switch shuts the burners off and the fan keeps running to cool it down. A new filter often brings the heat back.
Can I relight the pilot light myself?
Call a technician instead. A pilot light that goes out usually points to a part that needs repair, and the burner compartment is no place for guesswork.