What to Do If Your Furnace Is Blowing Out Cold Air

Your heater has turned on, but somehow your house feels colder the longer the heater runs. You hold your hand up to an air vent, and, sure enough, cold air is coming out, almost as if the air conditioner is running!

Several possible problems can cause your furnace to blow out cold air. The good news is that you can solve the simple problems on your own. However, for more complicated issues, you will need to involve an HVAC technician.

If your furnace is producing cold air, follow these troubleshooting steps.

1. Give your furnace enough time to heat up.

Your furnace is not going to produce hot air immediately after turning on. Similarly to how it can take a tank water heater a moment to deliver warm water to your faucet, it can take a moment for a furnace to bring warm air to your air vents.

Also, keep in mind that the warm air travels through ductwork, and if the ductwork is cold, it can “steal” the warmth from air passing through it. In short, give your heater a minute, and if the air is still cold after that, follow the steps below.

2. Check the thermostat settings.

Ensure that you've set your thermostat to "auto." If your thermostat is set to “on,” then your HVAC system’s fan is going to keep running even when your furnace is not producing warm air.

If the thermostat is acting strangely or not responding, it may need new batteries. If this doesn’t solve your thermostat’s behavior, a technician might need to repair or recalibrate it.

3. Check the air filter.

Dirty air filters are one of the top causes of HVAC issues. Even if you changed your filter recently, it’s still worth it to check the filter for any problems.

4. Check the pilot light.

If you own an older gas furnace, it most likely has a “standing” pilot light that is supposed to stay lit all the time so that it can ignite the burner whenever the gas turns on. If that pilot light goes out, the burner won’t light—meaning your furnace can’t produce heat. If you need to relight the pilot light, it's safest to refer to your furnace's user manual.

If either an old or a new furnace’s pilot light cannot ignite or stay lit, it’s best to involve an HVAC technician. In some cases, the technician might only need to clean the burners. Additionally, there might be an issue with the thermocouple, a safety device that will shut off the gas to your furnace if the pilot light goes out or if the igniter fails.

A gas supply issue could also be the culprit. Sometimes gas companies will shut down gas lines during emergencies or maintenance. Check one of the other gas appliances in your home to see if it will turn on. If it won’t, then it’s safe to say that your home is not being supplied with gas.

If you smell gas:

  1. Don’t turn anything on.

  2. Open windows.

  3. Evacuate your home right away.

  4. Call your gas company.

5. Check the condensate line.

If your high-efficiency furnace’s condensate line (drain line) gets blocked, water will back up into your furnace and cause it to shut down. Several things can block the condensate line, including dirt, dust, mold, and occasionally ice, if the area around your furnace gets cold enough.

Inspect your furnace to see if there’s any water puddling around it since that is a common sign of a condensate line clog. A broken condensate pump might also cause water to pool. The condensate line itself is usually a pipe made of PVC.

While it’s possible to unclog the condensate line yourself, you may want an HVAC technician to do this for you so that they can also inspect your furnace for any water damage at the same time.

I’ve checked all of this. What next?

If you’ve discovered an issue with your furnace that you cannot repair on your own, or if you still cannot locate the source of the problem, it’s best to involve an HVAC technician. Morrison Plumbing, Heating and Air has been providing prompt and professional heating services to Belleville and the surrounding areas for over 30 years. We are available Monday through Friday, 7 AM until 7 PM, and Saturdays from 8 AM until 4 PM, and we offer 24/7 emergency service: (618) 352-5599.