Why is my furnace not blowing hot air?
Short answer: Often the thermostat fan is set to On instead of Auto, so the blower runs between heating cycles. Also check Heat mode, batteries, the filter, the breaker and furnace switch, vents, and outdoor PVC vent pipes for snow. If you smell gas, leave the home and call your gas utility or 911 from outside.
Stop first: when not to troubleshoot
| Situation | What to do |
|---|---|
| You smell gas | Leave the home. Call your gas utility or 911 from outside. |
| CO alarm is sounding | Get everyone into fresh air and call 911 from outside. See where CO detectors belong. |
| Gas valve, burners or control board | Never DIY. Call a licensed HVAC technician. |
| Opening the cabinet or relighting a pilot | Only if your owner’s manual says the homeowner may. |
Carrier says to remove the cabinet door “only if your owner’s manual specifically permits homeowner access,” and not to repair the control board yourself.
Is my thermostat set wrong?
- Mode. It should say Heat, not Cool or Off.
- Fan On vs. Auto. On runs the blower nonstop, even between heating cycles, and that air feels cool. Carrier and Lennox both advise Auto, so the blower runs only while heating.
- Set point. It must be above the current room temperature.
- Schedule. Carrier notes a schedule can shift after a power outage or daylight saving change.
- Batteries. Lennox says battery-only thermostats need fresh batteries.
Is it just the startup delay?
Carrier says some furnaces delay the blower about 30 to 60 seconds after the burners ignite, so a brief puff of cool air can be normal. Your manual lists your model’s delay.
Could a dirty filter cause cold air?
Yes. A clogged filter restricts airflow and can make the furnace overheat. Carrier says this trips a safety limit switch that shuts off the burners while the fan keeps running.
Carrier recommends replacing 1-inch filters every 1 to 3 months, more often with pets or dust. See how often to change an HVAC filter.
Does the furnace have power?
- Power switch. Many furnaces have a switch that looks like a light switch near the unit. Make sure it is on.
- Breaker. Check the furnace breaker in your panel. If it trips again after a reset, call a pro.
- Cabinet door. If the door was recently removed, make sure it is seated properly.
Are vents and return grilles open?
Carrier advises opening all supply vents. Keep return grilles clear of furniture and rugs. If that does not help, have a pro check the ductwork.
Could snow be blocking the PVC pipes outside?
High-efficiency furnaces often vent through white PVC intake and exhaust pipes on an exterior wall. New Castle County, Delaware, says they typically sit 12 to 24 inches above the ground.
- Snow and ice. A blocked intake or exhaust can trigger a safety shutdown. Clear snow with a soft brush, not a shovel.
- Recheck after storms. Drifts can re-block the pipes.
- CO risk. The county warns a blocked exhaust can push carbon monoxide indoors. Test your CO detectors.
What about the condensate drain?
Lennox says a clogged condensate line can shut off the burners. Carrier lists malfunctioning condensate lines as a job for a professional. Unless the clog is obvious and easy to reach, call a pro.
When to call a technician
Call if the checks above fail, or if you notice:
- Burners that will not light or stay lit
- A breaker that keeps tripping
- Blinking error lights (note the pattern for the tech)
- A pilot that keeps going out, which Lennox links to a faulty thermocouple
If other gas appliances also fail, Carrier says to contact your utility.
Quick answers
Why is my furnace running but blowing cold air? Usually the fan is set to On, or a dirty filter caused an overheat shutdown. Set the fan to Auto and replace the filter.
Should the thermostat fan be on Auto or On for heat? Auto, so the blower runs only while the furnace is heating.
Can snow stop my furnace? Yes, on furnaces with outdoor PVC vents. A blocked pipe can trigger a safety shutdown.
Is it safe to relight my pilot? Only if your owner’s manual gives homeowner instructions. Lennox says most furnaces made since 2010 have no pilot. If you smell gas, leave and call from outside.