Expert advice on how to troubleshoot a furnace that is blowing cold air through air vents. Includes both gas and electric furnaces. 

Have you ever turned up the thermostat on a cold day only to get an icy blast of cold air? Why is there no heat? In this step-by-step guide, we’ll give you basic furnace repair advice for troubleshooting this frosty problem.

What Causes A Furnace to Blow Cold Air?

A blower pulls air from rooms, sends it through ductwork to the furnace, and then cycles it back to rooms. As the air goes through the furnace, it’s heated by a gas burner in a gas furnace or electric elements in an electric furnace. If the burner or elements don’t work, the air doesn’t get heated—so it returns to the rooms as a cold breeze instead of warm air.

Cut-away diagram of a furnace's internal and external parts, including filter, blower, and heat exchanger.
Furnace Parts Diagram (Gas Furnace) Don Vandervort, HomeTips © 1997 to 2023 | HomeTips

In gas furnaces, the gas-fed flame must be ignited by a spark or pilot light. If this ignition doesn’t take place, the flame will not ignite and the air won’t get warm.

An electric furnace creates heat more like a toaster—as electricity flows through heating elements, they get red hot. It’s these elements that heat the air as it passes through. If these elements are damaged, are worn out, or have electrical problems, they won’t heat the air passing through the system. The result? Cold or room-temperature air being blown through air ducts and into your rooms.

In both cases, the failure of the heat source is what prevents the air from being properly heated—and in both cases, the furnace blows cold air.If you have a furnace blowing cold air and you want to try to solve the problem yourself, follow these steps.

Note: If you’re unsure about troubleshooting or doing DIY furnace repairs, call a professional HVAC technician.

1. Check the thermostat

First, look at your thermostat settings. Make sure it is set to “HEAT” mode and that the desired temperature (“set temperature”) is higher than the current room temperature. Quite often, a simple thermostat adjustment is all it takes to restore warm air circulation. If the thermostat is set to “COOL” or to “FAN ONLY,” the system will not heat.

thermostat temperature
Make sure your thermostat is set to HEAT and the set temperature is higher than your room temperature. © Dewitt |

2. Check pilot light and ignition system

For gas furnaces, a malfunctioning pilot light or ignition system could be the culprit. Look for the pilot light through the little window beneath the heat exchanger and make sure it’s lit. If it’s out, follow the manufacturer’s instructions to relight it safely. If you haven’t done this before, don’t let it intimidate you. It’s a simple task on all modern heaters. For more about relighting the pilot and checking the pilot light assembly, ignition system, or an issue with the gas supply, See Furnace Not Working—No Gas Flame.

Man lifting an air-handler cabinet's door.
Remove the air handler’s door (often by lifting it) to access the pilot light. Lennox

3. Inspect the flame sensor

A gas furnace has a flame sensor that shuts off the gas if the flame goes out. This is a safety feature that keeps gas from leaking into the space around the furnace. Sometimes a dirty or faulty flame sensor can shut down the flame. Locate the flame sensor (usually near the burner assembly) and gently clean it using a soft cloth or fine-grit sandpaper. For more detail, see Furnace Heating and Recycling Problems. Note: This step requires fairly advanced DIY skills—if it’s over your head, call a pro.

4. Inspect air filters

Though it shouldn’t eliminate the heat entirely, a clogged air filter can significantly impact your furnace’s performance when heating your home. Inspect your furnace’s air filter for accumulated dust, debris, or blockages. If it’s visibly clogged with dirt and debris or hasn’t been replaced in 6 months, it’s time for a quick filter change. A clean filter allows for proper airflow, preventing your furnace from working harder than necessary. For more, see How to Replace a Furnace, AC or Heat Pump Filter.

A hand placing a new air filter in a wall register, with airflow arrows pointing toward the ductwork.
Change filters to improve heating (and cooling) efficiency. ©Don Vandervort, HomeTips

5. Verify air vents (registers)

Blocked or closed vents and registers will disrupt the airflow, causing uneven heating or even a complete loss of warm air. Of course, in this case, you won’t feel the system blowing cold air through a particular register—it won’t be blowing any air at all. Make sure all vents and registers are open and not blocked by furniture or rugs.

A hand using the lever on a heating register to open the vent.
Be sure the registers are open to allow free flow of air. © Don Vandervort, HomeTips

6. Check the ductwork

Leaky or damaged ductwork can cause warm air to escape, leading to inadequate heating and cold air blowing through your vents. Inspect your ductwork for any visible signs of damage, such as loose joints or gaps. Use duct tape or mastic sealant to seal any leaks or call a professional duct repair service or HVAC technician.

old gas furnace with ductwork
Unless you see obvious separations in the ductwork, have an HVAC pro inspect it. Amy Walters /

Schedule Regular Maintenance

Prevention is better than having to find a cure. Regular maintenance plays a vital role in keeping your furnace running smoothly for heating your home. We recommend scheduling professional maintenance at least once a year to ensure optimal performance, identify potential issues early on, and avoid surprises like cold air blowing from your furnace.


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About Don Vandervort
Don Vandervort has developed his expertise for more than 30 years as a remodeler and builder, Building Editor for Sunset Books, Senior Editor at Home Magazine, author of more than 30 home improvement books, and writer of countless magazine articles. He appeared for 3 seasons on HGTV’s “The Fix,” and served as MSN’s home expert for several years. Don founded HomeTips in 1996. Read more about Don Vandervort