Help, My Furnace is Blowing Cold Air!

January 28, 2021

AC Troubleshooting

Furnaces typically produce air between 140° and 170°. If your furnace isn't quite hitting those benchmarks, there could be an issue with the furnace itself.

That being said, a few of the “issues” that cause furnaces to blow cold air are small problems that can be easily resolved by a homeowner and don’t require a professional’s help.

A few of the most common reasons for a furnace to blow cold air include:

  • A clogged air filter
  • Wrong thermostat setting
  • Holes or leaks in your ductwork
  • Pilot light issues
  • Inadequate gas supply

Below, we’ll walk through each of these issues in greater detail, so you can get a better idea of why your furnace is blowing cold air and what you can do about it.

If you’d prefer to enlist the help of a professional, we’d be happy to step in! You can learn more about the furnace repair services we offer or schedule service with us!

Clogged air filter

One of the most common reasons furnaces blow cold air is due to a clogged air filter. The air filter works as the “bouncer” between the air in your home and your furnace. When your furnace cycles on, it pulls in cold air from your home to be heated, and that air is pulled through your air filter.

However, if your air filter is clogged, your furnace won’t be able to draw in as much air. If the airflow to your furnace is stifled, your furnace can overheat. In some cases, an overheating furnace will simply shut down. But prior to shutting down, it can do something surprising, like blow cold air into your home!

Under these circumstances, if a furnace is not getting enough airflow, it will attempt to avoid overheating by pulling in more cool air from your home to keep its internal parts at manageable temperatures. It won’t actually heat this air. Instead, it just blows the same cool air out of the vents.

If you haven’t changed your filter in a while (1 month+), this is likely your issue. Replace your air filter and give your furnace some time to cool down. If your furnace is still blowing cold air 1 hour or so after you’ve replaced the filter, there’s likely another issue with your system.

Wrong thermostat setting

If your thermostat is on the wrong settings or there is an issue with your thermostat, it can cause your furnace to blow cold air.

The most common issue we see is a thermostat that’s simply on the wrong settings. Your thermostat fan setting should be set to AUTO not ON. If the fan setting is on ON, your system will blow air throughout your home all the time, even if that air has not been heated.

If your thermostat is set to AUTO, it will only blow air that has been heated by your furnace.

Check your thermostat fan setting and ensure it’s set to AUTO. If it is, keep reading. You’re likely dealing with a duct or furnace issue.

Holes or leaks in ducts

If there are holes or leaks in your ductwork, it’s very probable that cold air could be coming from your vents.

There are two ways that duct leaks can cause cold air to come from your vents:

  • Loss of warm air to holes on the supply side
  • Input of cold air due to holes on the return side

Loss of warm air on the supply side:

Your ductwork typically travels through unconditioned areas of your home, like attics or crawl spaces. If there are holes in your ductwork on the supply side, air that your furnace has heated can be lost through these holes into unconditioned spaces, like your attic. By the time the existing heated air reaches your vents, it will still be hot, but there won’t be as much heated air as there should be, which can make it feel like cool air is coming from your vents.

Input of cold air on the return side:

Another reason you may feel your air is cold is if cold air gets sucked into your ducts through holes on the return side. Your return ducts are ducts that pull in air from your home to your furnace to be heated. So, if there are leaks in your return ductwork, cold air from unconditioned spaces can get sucked in with the air from your home. While this air will pass through your furnace and become heated, it can make the overall temperature of the heated air colder, which is why you may feel cool air coming from your vents.

Pilot light is out

If you have an older furnace that actually produces a pilot flame, your furnace could be blowing cold air if the pilot light is out.

The pilot light is responsible for igniting the gas in your furnace. If the pilot light is out, your furnace can’t heat your home’s air.

To check your pilot light or to relight it, you’ll need to follow the instructions on your furnace. If you’re having issues relighting your pilot light or you have relit it and it goes out again, reach out to a professional.

Note: If your furnace has a pilot light, it’s probably time to install a new system. We would suggest reaching out to a professional who can provide you with recommendations on what kind of system would be best for you and when you should consider replacing your current system.

Inadequate gas supply

If you have a gas furnace and your furnace is blowing cold air, it could be because your furnace isn’t getting the right supply of gas.

A gas furnace runs on electricity but uses gas to produce heat. So, if your gas furnace is running but blowing cold air, this is likely because it’s not getting enough gas to produce the heat it needs to warm your home.

To see if this is your issue, check the gas valve to your furnace and to your home and ensure they’re both open. If the line is open, the valve will be parallel to the gas line. Another way to check your home’s gas supply is by turning on another gas appliance. For example, if you have a gas stove, turn that on to see if your gas supply is on and working.

If your gas line is open but your furnace is still blowing cold air, we’d advise you to reach out to a professional.

Still having issues with your furnace? Contact the best pros in Florida, Red Cap!

If you’ve tried the DIY fixes above without success or have one of the issues that requires the help of a professional, feel free to reach out to us! We know how important it is to have a working furnace during the winter, and our skilled team of pros will do everything we can to get your system safely running as soon as we can.