Troubleshooting Your Furnace And Why It’s Blowing Cold Air

In the middle of a frigid winter day, cold air blowing into your home is the last thing you want. There’s only so much warm clothing can do, especially when you’re supposed to be comfortable at home. The sudden change of cold air from your furnace can be completely unexpected, even more so when it was working just fine recently. When this happens, many homeowners ask the question, “Why is my furnace blowing cold air?”

There are many different reasons why your furnace may suddenly start blowing cold air, many of which you can easily find with a look at your system. As it is with many repairs, some of the causes we’ve listed below can be done by yourself with the appropriate tools. 

If you need help with repairs or can’t find the issue, our River Valley Air Conditioning team is ready to help whenever you call us.

A Clogged-up Condensate Line

Frozen. Sad latina female sit on couch at freezing cooled studio flat in warm cap and blanket shiver tremble with cold. Unhappy young lady spend time at home feel bad suffer of heating system problems

One of the possible answers to “why my furnace is blowing cold air” is a blockage somewhere in the unit’s condensate line. As a furnace runs, water can be created on or in the unit, especially when the hot air that’s created comes into contact with external cold air. Typically, many different models of furnaces have a specific way to get rid of that excess water; the condensate line.

However, if something clogs up the line – usually dirt or some debris – the water will start to back up into the furnace. The water backup causes a shut-off switch to flip to prevent any damage or risks. If your furnace doesn’t have the switch or something’s wrong with it, the water can damage the system. The result though is the same in both situations; the furnace won’t light and blows cold air before shutting down.

If this is your home’s answer to “why is my gas furnace blowing cold air,” you might be glad to know that you can easily solve this problem at home. Unclogging your furnace’s condensate line is as simple as using vinegar and water. All you need to do is find and remove your condensate line’s drain cap and pour a 50:50 mixture of water and vinegar into the line. And, if the clog is denser than expected, a drain snake can work just as well. 

The Pilot Light or Igniter Is Damaged

Another possible answer to “Why is my furnace blowing cold air” is an issue with the furnace’s pilot light. Responsible for creating a tiny spark to make the gas fuel a flame, any small issues with the pilot light can cause the air coming from your furnace to be cold. If the pilot light – also called the igniter – often goes out, your furnace won’t start at all, and only cold air moves through the system.

This is only an issue common to older furnaces, though. Many newer furnace models use a solid-surface igniter that automatically raises its temperature to light the gas. The best way to check if the pilot light is the issue is to access it through the furnace itself. The part is typically located at the base of your furnace and close to the front behind a cover or window. You can also check your owner’s manual if you can’t find it.

If your furnace does have a pilot light and the small flame is gone, there is a safer way to restart it. With proper access to the light, turn it – or its corresponding switch – to the “off” position before waiting five minutes to avoid any danger. Then, hold a long lighter to the end of the pilot light tube and press the igniter button, being careful to leave a little space between the two openings. 

Hold the igniter button down for 30 to 60 seconds until the pilot light’s flame stays lit. Then simply turn the switch back to “on.” If the pilot light won’t stay on at any point during the relighting process, contact our team as soon as possible!

You Have a Dirty Air Filter

Possibly one of the simplest answers to “Why is my furnace blowing out cold air”, a dirty air filter can be surprisingly simple whenever you suspect a far more complicated cause. A dirty air filter can cause the furnace to start overheating, which, in turn, can cause cold air to blow out instead. 

To prevent this problem, in addition to any damage that can happen from overheating, check the state of your furnace’s filter at least once a year. A dirty air filter can be the cause of quite a few other furnace-related problems as well, so you end up stopping quite a lot of worry! 

The Flame Sensor Is Dirty

As mentioned, newer furnace models often aren’t manufactured with the same pilot light or igniter as an older one. However, newer furnaces have a flame sensor to ensure that the furnace has lit the gas and turned the other parts of the system on. 

From its place close to the flame’s path, the flame sensor is essentially a safety feature to prevent gas from leaking into your home. The sensor shuts down the furnace whenever it fails to sense any fire. But, like with any other part of the system, dirt, and grime can build up on the sensor’s surface, shutting down the furnace even though there are flames.

Like the pilot light, a flame sensor can easily be cleaned, though it requires more care. Locate the flame sensor by manually checking inside the furnace or the owner’s manual. Then carefully disconnect it and clean off any stuck-on grime or dirt with a gentle emery cloth or fine steel wool. Simply reconnect it after and turn the furnace on.

Your Home’s Ducts Are Damaged or Leaking


Out of the causes of “why is my furnace blowing cold air,” damaged ducts are possibly one of the more severe possibilities. If you’ve only just moved into an older home before noticing the cold air from your furnace, there’s a chance the ductwork got damaged due to age. All types of things can lead to leaking ducts, so it’s important to check what you can.

The damage can be as simple as loosened bolts and gaps in the best possible scenario. The solution, though, depends on where the leaking air is. If it’s somewhere you as a homeowner, have access to, you might easily be able to repair or replace a section of the ducts. Hidden leaks will require a technician’s help, however.


If your furnace is blowing out cool air, there are a few possible things you can do to troubleshoot the problem. Take a look at the listed causes and check all visible parts of your furnace before calling a technician. Some of these causes can be handled by yourself, but if you can’t find the cause or if you need a professional’s help, just contact us at River Valley Air Conditioning today.