As the chill of winter settles in, there’s nothing quite as unsettling as discovering that your furnace is blowing cold air instead of the cozy warmth you crave. Before you reach for the phone to call in the professionals at Indoor Comfort Team, take a moment to explore some troubleshooting tips and tricks that could potentially save you time, money, and discomfort.

At Indoor Comfort Team in St. Louis, we understand the urgency of ensuring your home is comfortable during the coldest months. With over 45 years of dedicated service, we’ve seen it all when it comes to HVAC issues, and we’re here to share our expertise to help you stay comfortable.

In this comprehensive guide, we’ll walk you through common reasons why your furnace might be blowing cold air and provide step-by-step solutions that you can try on your own. From simple fixes like checking your thermostat settings to more complex issues such as pilot light malfunctions, we’ve got you covered.

So, grab a cup of hot cocoa, cozy up, and let’s dive into troubleshooting your furnace to bring back the warmth and comfort you deserve.



Here are the 9 most common reasons your furnace is blowing cold air.


  1. Thermostat Issues
    The first most common cause of your furnace blowing cold air is your thermostat. Since your thermostat controls your home’s temperature, incorrect configuration can lead to cold air coming through your vents. You’ll want to verify the settings and your thermostat to make sure that someone hasn’t changed them. You want to ensure the thermostat is set to ‘heat’ and not ‘cool, make sure the fan is set to ‘auto’, and make the temperature is set where you would like it. A furnace with dead or dying batteries, can also cause the thermostat to malfunction, so changing your batteries may be helpful as well. If any of these settings are incorrect, make the appropriate changes and your furnace should begin blowing warm air. Keep in mind it takes about 15 minutes for a furnace to heat up, so please be patient and allow it time to warm up.


  1. Dirty Air Filter
    If your furnace is still blowing cold air, the next place to check is your air filter. Your air filter traps dirt, dust, and debris within your system and if not changed routinely can lead to restricted airflow in your system. When the airflow is restricted, the cold air entering the furnace from the return ducts is limited and leads to your furnace not having sufficient air to heat. Further, a dirty air filter and cause your furnace to overheat and go into safety mode. Depending on household factors like pets and allergies, it’s recommended that your furnace filter be changed every 30 to 90 days. If it’s been a while, putting in a fresh air filter just might do the trick.


  1. High Limit Switch Issues
    If your furnace filter was dirty and replacing it didn’t help, the issue may be a faulty high limit switch. This switch measures the temperature of the air that passes over the heat exchanger within your system and shuts down the furnace if it overheats. Lack of airflow is a very typical cause of your furnace overheating. If you experience frequent tripping of the high limit switch, it can cause the switch to fail. If your blower continues to run between cycles or cycles on and off, this typically indicates a faulty limit switch. Give the professionals at Indoor Comfort Team a call at (314)230-9542; we’d be happy to get this issue taken care of for you.

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  1. Fuel Issues
    A lack of fuel can be another common cause of a furnace blowing cold air. Based on your type of furnace, it runs on either electricity or fuel.
    1. If you have an electric furnace, it won’t function without electricity. You will want to ensure that the switch in your utility room that powers your furnace is in the ‘on’ position. You will also want to check at your breaker box and make sure that the breaker is in the ‘on’ position and has not tripped.
    2. If you have a gas furnace, it won’t heat up if the gas supply has been shut off, the gas valve has been turned off or has seized, or if the propane/oil tank is empty. You will want to check to make sure there are no issues with your gas supply and that the appropriate gas valve is open.

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  1. Clogged Condensate Line
    Another common cause of your furnace blowing cold air is a clogged condensate line. Condensation from your furnace drains away via the condensate line, which is a white PVC pipe that runs down the furnace’s outside and empties into a floor drain. If the pipe becomes clogged, the condensation will accumulate in the drip pan and trigger the float switch. The float switch is a safety feature that shuts down the furnace when the condensation doesn’t properly drain to prevent water from spilling over the drip pan.


If you find a clogged condensate line is your issue, the clog must be cleared for your furnace to function properly. As a homeowner, you can try to take on this maintenance task, but the professionals at Indoor Comfort Team have specialized tools to make the job easier and more efficient –

    1. Before trying to clear the clog, you will want to make sure you turn off the power to your furnace. You can do this by flipping the switch (resembles a light switch) on the furnace or by turning off the breaker for your furnace at your circuit breaker.
    2. Inspect the end of the condensate white PVC pipe for debris. If you find any, they can be removed with a brush or a small screwdriver.
    3. If you didn’t find any clogs at the end of the pipe, you can remove the cap near the top of the pipe and flush the line with a solution of equal parts white vinegar and hot water. You can repeat this process several times if needed.
    4. Once you have cleared the clog, restore power to your furnace and give it time to warm up.
  1. Ductwork Issues
    Leaking or incorrectly sized ductwork can also lead to issues with your furnace blowing cold air. Call in the experts at Indoor Comfort Team to properly check and repair your ductwork to ensure optimal heating performance of your furnace.

    1. If your ductwork is leaking, it can cause warm air to leak out as well as allow cold air to enter through the openings in the ducts. Leaks are caused when ducts become disconnected or cracked.
    2. If your ductwork is incorrectly sized, it can cause issues with the heating performance of your system. If your return ductwork is too small, it won’t be able to handle the volume of air that’s needed to properly heat your home. This will need to reduced airflow through the system and will cause your furnace to blow cold air. Ductwork that is too big causes the air pressure to significantly decrease and therefore the airflow doesn’t make it to all the parts of your home.


  2. Flame Sensor Issues
    A dirty or malfunctioning flame sensor can be another common cause for your furnace blowing cold air. The flame sensor is a safety device that will turn off the gas supply to the burners in your furnace if they don’t ignite. These sensors can become coated with dirt, soot, or other carbon deposits and make them incapable of sensing the flame.

You can access and try to clean off the flame sensor or call in the professionals at Indoor if you don’t feel comfortable or if you think the sensor needs replaced. Call us anytime at (314)230-9542.

First, make sure you turn off the gas to the furnace and then remove the front panel where you will find the ignitor or ignition sensor near the burners. If the sensor or ignitor is dirty, you can remove it carefully with a screwdriver and gently clean it – be careful not to break it. Once cleaned, replace it back how you found it, put the front panel back on, turn on the gas, and then turn your furnace on.

  1. Electric Ignition / Pilot Light Issues
    A faulty electric ignition or an issue with your pilot light can also cause your furnace to blow cold air. If your furnace was manufactured prior to 2010, they generally were still using pilot lights. More modern furnaces will use an electronic ignition.

    1. If your furnace has a pilot light, check to see if the flame has been blown out by a strong draft. If the flame has blown out, you will want to consult your furnace’s manual for instructions on re-lighting the pilot light. If the flame continues to go out, there may be dust or debris built up on the pilot light causing a bad connection.
    2. If you have a more modern furnace with an electric ignition, these electronic ignitions can fail and would require a call to the professionals at Indoor Comfort Team at (314)230-9542.


  1. Incorrectly Sized Furnace
    Another common cause of a furnace blowing cold air is an incorrectly sized furnace for the home. Size matters when it comes to your furnace system. If you’ve recently installed a new furnace and find yourself puzzled by its failure to product hot air, chances are it’s undersized for your home. An improperly sized furnace can result in the circulation of cold air and can lead to issues with both effectiveness and efficiency.

    When your furnace is too small for your home, it struggles to accurately gauge the internal temperature, often resulting in the distribution of cool air. Conversely, an oversized furnace may cycle on and off unexpectedly, causing disruption to your comfort and potentially leading to higher energy bills.

    Our HVAC professionals are expertly trained to help your select the right size furnace for your house and your individual comfort needs. You can request an appointment anytime –


There are a handful of more serious issues that could be causing your furnace to blow cold air and would require a call to the professionals at Indoor. Our technicians are expertly trained to diagnose and resolve these issues and have you back up and running quickly! Call now at (314)230-9542!

  • Malfunctioning Gas Valves
  • Cracked Heat Exchanger
  • Blocked Burner
  • Clogged Coil
  • Faulty Circuit Control Board


Opting for professional furnace maintenance with our VIP Club Plan through Indoor Comfort Team is a proactive step towards safeguarded your home comfort. Our skilled technicians offer a comprehensive inspection of your heating system, proactively identifying potential issues before they lead to your furnace blowing cold air. By addressing these concerns, our team not only boosts your furnace’s efficiency but also guarantees its safe and dependable performance.

Our trained experts can pinpoint and resolve underlying issues that might escape the untrained eye, preventing unforeseen breakdowns and sparing you from expensive repairs down the line. With Indoor Comfort Team’s VIP Club Plan, you can rest assured that your furnace is in optimal condition, ready to keep you warm and cozy all season long. Learn more here –


Whether you’re in need of a repair or a whole system replacement, Indoor Comfort Team is the trusted professionals in St. Louis for more than 45 years! Remember that your comfort and peace of mind are our top priorities. By following the tips and tricks outlined in this guide, you’re equipped with the knowledge to tackle common furnace issues and restore warmth to your home.

However, if you find yourself still facing challenges or prefer the expertise of professionals, don’t hesitate to reach out to us at (314)230-9542 anytime. As the trusted HVAC company in St. Louis, Indoor Comfort Team is here to ensure your HVAC system operates efficiently and reliably, keeping you comfortable throughout the year.

We offer a variety of specials and great financing deals to make sure you are comfortable in your home for many years to come –


Contact us today!

Indoor Comfort Team has been a trusted name in the greater St. Louis area since 1979. Whether you live on the Missouri side of the river or in Illinois, our skilled and licensed HVAC technicians are ready to deliver the services you need.

We provide complete heating and cooling installation and repair as well as water heater and indoor air quality services.

Need a repair today? Call (314) 230-9542 or (618) 688-0707.

Whether you need repair, maintenance, or new installation of your furnace or air conditioner, contact us today to let us get our comfort specialists right to work on your home or business.

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