Reasons for a Broken Furnace & Troubleshooting Tips to Restore Heat

furnace blowing cold air troubleshooting tips

The middle of a frigid St. Louis winter is the worst time for your heater to break down or a furnace blows cold air. Waiting in a freezing home with a furnace blowing cold air until a repair technician shows up is always something you want to avoid. But unfortunately, every winter our Indoor Comfort Team receives dozens of calls from homeowners across the St. Louis area who are exactly in this position.

While we work around the clock to restore heat to all our customers, we also want to help homeowners know the various factors that can cause a furnace to break down – and the troubleshooting steps that may help it!

With this article on one of the biggest furnace issues that can happen in cold weather – a furnace blowing cold air. Read about the various reasons why it happens and learn troubleshooting steps so you can try to get the heat back on by yourself!


What to do first when your furnace is blowing cold air

While discovering cold air coming out of your vents may be distressing, you do not immediately need to call for a repair. While some issues do require a trained professional to fix your problem, there are a handful of minor issues that can cause a furnace to blow cold air. And some of these issues can be fixed by the homeowners themselves.

Need a Furnace Repair Right Away?
Indoor Comfort Team is here whenever you need a professional heating technician to service your furnace. We are available 24/7.
Just call us at (314) 230-9542 or you can contact us for service online.

Furnace Blows Cold: Reasons & Troubleshooting Steps

Before calling for a professional repair service, look over these reasons that can cause a furnace to blow cold air. You may discover the cause of your problem and fix it yourself!


Cold Furnace: Check Your Thermostat Temperature

thermostatMake sure that your thermostat is set for a temperature higher than the current temperature in your home.

You also should make sure the thermostat is set to HEAT (instead of being set to COOL or OFF). As basic as it sounds, this situation happens more often than you may think. It is possible someone else in the home lowered the setting without you being aware. While that may put a kid in timeout or a spouse in hot water, this is the easiest problem to fix, at least technically.

Often people who have programmable thermostats disregard this possibility. You have your thermostat set to be exactly how it should be at all times of day, and so checking your thermostat setting is not as common of an occurrence. If this is the problem, simply setting the temperature up several degrees than it is now should quickly cause your furnace to blow warm air again.

Thermostat Fan Setting

Next you will want to check your fan setting. Is it set to ON or AUTO? If the fan is set to ON, it is running constantly and pushing air through your vents 24/7. That means air is coming out even when the ambient temperature matches the desired thermostat temperature.

What happens when your inside air has already been warmed to the desired temperature, the furnace isn’t working to heat the air that is coming out of your vents. So, by comparison, blowing air may feel cool – some may even think their air conditioner is running. Try turning your fan setting to AUTO. That way the air will only come through your vents when the furnace has heated the air first.

Need a Furnace Repair Right Away?
Indoor Comfort Team is here whenever you need a professional heating technician to service your furnace. We are available 24/7.
Just call us at (314) 230-9542 or you can contact us for service online.

Broken Blower Fan

Your furnace works with a blower fan that pushes the air through your ductwork and your vents. It does not do much good for your furnace to heat the air without that air getting pushed out the vents throughout your home. However, on the reverse side, a blower fan running that is not pushing out warm air is a problem.

As just mentioned above, changing a fan from being set to ON to AUTO may remedy the problem of a blower fan running when the air isn’t getting heated. However, sometimes this blower fan can break down and cause problems that can’t be fixed by changing the setting at your thermostat. If that is the case, you will need to call for a professional furnace repair.

Furnace Repair Tech

Thermostat: Failure to Communicate

Your thermostat is your line of communication to your furnace. It is responsible for telling your furnace to put out heat or your air conditioner to put out cool air. If something interrupts its normal functions to tell the furnace how hot you want it, your thermostat could be sending the wrong message – or NO message – to your home’s heating and cooling system.

Thermostat Error causing furnace to blow cold airAs great as smart thermostats are, glitches can happen. Just as a computer may need a restart to get it operating again, a smart thermostat that is failing to communicate to your furnace may also need a restart.

One potential issue is having a thermostat that is no longer connected to your home’s internet. You may want to try reconnecting your thermostat’s internet connection. This may require utilizing the app your smart thermostat works with to reconnect it.

Fortunately, smart thermostats will typically provide you with an error message when there is a communication failure. If restarting the system does not resolve the problem or you are not sure how to reset or restart your thermostat, just call for a professional repair. The furnace repair technicians on our Indoor Comfort Team are trained in handling every type of heating system as well as every type of thermostat. We can figure out what is causing the communication issue and get it fixed.


Thermostat Broken or Batteries Dead?

Your thermostat is your line of communication to your furnace. It is responsible for telling your furnace to put out heat or your air conditioner to put out cool air. If it is damaged or not working correctly for any reason, it could be sending the wrong message – or NO message – to your home’s heating and cooling system.

Dead batteries are a common problem that can cause a furnace to stop working. It is amazing to consider that something as small and cheap as a couple of batteries could put a stop to your home’s entire heating system, but it happens! Even if you see the normal display on your thermostat, your batteries could be low enough to cause a problem with your furnace. Change your batteries and see if that fixes the problem.


Even a low battery can cause a problem like a furnace blowing cold air. And a low battery is easy to fix. Some thermostats work with your home’s central power system, but other ones work off battery unit that is inside the thermostat unit.

Even if you see the display lit up as usual, you may still have batteries at low enough power to cause problems. If you have a unit with batteries inside, those are going to decline (and eventually die) over time. A battery does not need to be completely dead to cause problems in your thermostat.

Changing Thermostat

Bad wiring or improperly installed thermostat can lead to a furnace blowing cold air. If new batteries do not fix the problem, but the issue is with the thermostat, call a professional HVAC technician to diagnose and fix the problem. Call Indoor Air Comfort Team anywhere in the St. Louis area at (314) 230-9542.

There are a lot of great thermostat options that can help you have more control over your home’s comfort and energy efficiency. A programmable thermostat can help ensure you keep your home warm when you need it while also not wasting the energy to heat a home that is empty when everyone is gone at school or work.


Thermostat Installed Wrong

While not a common cause to a furnace blowing cold air, a thermostat that was improperly installed could cause this to happen.

Is this the first time you needed your heat to come on since a new home HVAC system or thermostat was installed? If so, something may have been installed correctly. Contact a professional HVAC technician to check your home’s system.

While there are a lot of amazing smart thermostat options available to buy at Amazon or elsewhere, not every type works with every time of heating and cooling system. If you had a smart thermostat installed by someone who is not a licensed HVAC technician, you put your system at risk of having such problems as causing a furnace to blow cold air.

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Check Your Air Filter

Batteries are not the only easy and affordable part that can wreak havoc with a home HVAC system. An air filter is a vital component of the entire system working. Having a dirty filter can cause heating problems including a furnace blowing cold air.

A dirty furnace filter can actually block air from flowing through your ductwork. This blockage can cause the heat exchanger to overheat, which can possibly lead to hazardous problems like a carbon monoxide leak. Some heating systems will automatically shut off its burners when there is a potential problem with the heat exchanger as a safety precaution. If you hear your furnace frequently turning on and off again over and over (also known as short cycling), this is a sign that your furnace is engaging in auto-shut off procedures.

Some homeowners think their system will simply stop running if their air filter is too dirty. That is not the case. Your HVAC system could still be operating but operating quite poorly with a dirty filter. But a dirty filter can cause a lot of problems.

Change a dirty filter, even if it is ahead of schedule for your regular changing. It may need to be changed more frequently than you realize.

To change your air filter, follow these steps:

  1. Turn off your furnacedirty Air Filter can cause furnace blowing cold air
  2. Find the filter compartment in your furnace
  3. Open the access door (some units require removing the door)
  4. Slide out the old air filter
  5. Insert a new filter in, making sure it goes in the exact same direction as the old filter was
  6. Close the access door and dispose of the old filter
  7. Turn your furnace back on



  • If your furnace does not start back up, call a repair technician to reset the limit switch.
  • If you notice your air filter is very dirty (you can’t see much light coming through it), you should reconsider your current air filter schedule. You are likely not replacing it often enough.

A dirty air filter is not the only problem you could have. A filter installed in the wrong direction can also cause problems like that of a furnace blowing cold air. There is a definite right and wrong way a furnace filter needs to go in.

An air filter can be put in wrong. If you are not sure which direction your air filter goes in, refer to the markings on the filter frame to show you how to put it in. You can also refer to your unit’s manual for instruction on what air filter to install and the correct way to install it. You will always be having the arrow facing TOWARD the unit and AWAY from the return duct.


Is the Furnace “Light Switch” Off?

One of the easiest fixes our technicians make can be quite embarrassing for a homeowner. Their furnace switch was turned off.

While this is a common feature on most furnaces, it is not one that every person is aware of. This is not, in fact, a light switch. It is your furnace power switch.

Typically, right on or near your furnace there is a switch to activate the furnace. It can look exactly like a light switch and so get mistaken with disastrous results. If someone went to the basement or wherever your furnace is and turned off power switches they mistook as light switches, they could have mistakenly turned off power to your furnace.

Nothing you do at your thermostat or anywhere else will remedy the problem. That switch needs to be on for your furnace to work.

As this is the easiest, simplest issue any homeowner can fix, it is worth checking. Go to your furnace and look for a light switch. Make sure it is in the on position.

Furnace Switch

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Is Your Pilot Light Out?

Most modern furnaces do not have pilot lights that need to be lit, so if you had a furnace installed within the last 10-15 years, this likely shouldn’t be an issue for you. But if you have an old gas furnace with a pilot light, you should check to see if it is still lit. Check where your burners are to make sure a flame is still burning.


Too Many Vents are Blocked or Shut

Many people believe you should close vents in rooms you are not using to save energy. That is a common misconception, but it is not good advice.

The ductwork connecting your furnace to your vents was designed to work altogether. That means if you shut off too many vents, the air does not flow as the system was designed and therefore does not operate well. Closing a few vents usually does not cause a problem, but closing too many can be an issue.

blocked vents can make it seem furnace is not blowing hot airBlocking vents can also cause the same problem. Vents that are opened by covered by furniture or other items will effectively block them from contributing to proper air flow.

To troubleshoot this potential problem, go throughout your home and look for any vents that may be closed or blocked. When all are open and/or unblocked, see if that resolves your issue of cold air coming from your furnace.

If you have a vent that is in an inconvenient place, annoyingly blowing air on you often, you need to find other solutions than closing or blocking that vent.

Air vent diverters (aka vent deflectors) are great devices you can buy and place over a vent to help. It will direct the air to still come out of the vent but in a direction that is not causing problems. Air diverters can also often help direct air coming from a vent that is partially covered by a chair or other furniture.


Bad Fan Limit Switch

Is the air coming out of your vents blowing hot sometimes but cold at other times? That could mean there is a problem with the fan’s limit switch. This switch controls when the fan goes on and off during heating cycles.

An error with this switch can cause the fan to run when the furnace is not working to heat the air. It can cause your system to constantly start and stop, an issue called short cycling. Ongoing issues of this happening can damage your system, even causing it to completely break down.

A limit switch is not typically a component of an HVAC system that homeowners need to be aware of. It cannot be handled easily with DIY troubleshooting work.

A qualified heating repair technician will be able to easily handle this issue. They may be able to simply adjust the setting for this limit switch so that the fan will not blow air when it is not getting heated. But sometimes this issue is larger, and the component will need to be replaced.

Indoor Comfort Team Heater Repair in St Louis home

Need a Furnace Repair Right Away?
Indoor Comfort Team is here whenever you need a professional heating technician to service your furnace. We are available 24/7.
Just call us at (314) 230-9542 or you can contact us for service online.

Furnace is Overheating

Furnaces are going to be warm, but big problems can make them too warm. As mentioned earlier, with issues caused by a dirty air filter, a furnace can overheat and break down.

Even worse, an overheated furnace can cause problems that can lead to such hazards as a carbon monoxide leak. For that reason, any signs that a furnace is overheating should be dealt with by a professional HVAC technician. You should call for a repair right away and may even need to turn off your furnace to prevent dangers to your home or family.

Other than a dirty filter, a furnace could overheat for various reasons including:

  • A build-up of dirt inside the unit can harm how well components work
  • Mechanical failures can occur over time. This is more of a concern for older furnaces.
  • An old furnace that cannot function sufficiently. If you have had a lot of repairs over recent years, it may be time to replace it with a modern furnace.

A Puddle Near the Furnace

If you see water pooling around your furnace, you may have a clogged line in your system. HVAC systems produce condensation. Most furnaces either contain a condensate line or it is connected to a central AC system with one. A condensate line is a narrow tube that runs typically to a drain pan.

Furnace Water LeakThis moisture is supposed to move out through the condensate drain line, but if there is a clog in that line, the water can create a puddle around your furnace. If dirt or debris (or even sometimes ice) is blocking that condensate line, water backs up. That can trigger a safety shut-off of your system and/or your furnace to not light. A broken condensate pump can also cause an issue with the line and create a water leak around your furnace.

You can call Indoor Comfort Team to handle a problem with standing water around your furnace. Send us a message for service online or by calling us at (314) 230-9542.


Broken Control Panel

Among the problems that can cause a furnace to blow cold air is a problem within the furnace’s electronic control panel. Most modern furnaces operate in a specific sequence of functions controlled by an electric control board. If your furnace is blowing cold air, you might have a problem within this control panel.

Some furnaces have diagnostics built in and will indicate there are potential issues by blinking in distinct patterns. You don’t need an electrician to handle this problem. A licensed HVAC technician is trained in fixing the electronic components inside your furnace.

Sometimes fixing this issue is as simple as resetting the system. You can turn the power off to the furnace at the light switch (mentioned above). Wait a few minutes before turning it back on.

If that doesn’t fix the problem, you will need to call for a repair. A trained furnace repair technician will be able to correctly diagnose the problem and find a solution.


Leaks in Duct Work

A rip somewhere along your ducts could cause a lot of problems with your airflow including causing it to feel like your furnace is blowing cold air. If you are able, inspect any duct work that is visible. Look for any rips or separation that can cause heated air to leak out.

A lot of a home’s duct work can be hard to see or access. A professional furnace repair technician will be able to fix any problems with the duct work.


When To Troubleshoot vs Calling for Professional Repair

While there are several minor issues that can be resolved by a homeowner that may completely fix the problem, often a qualified HVAC technician is needed. When a furnace is blowing cold air in the middle of winter, you may not be in a position to wait and see if you can fix it yourself.

In the middle of a cold St. Louis winter, a broken furnace is a risk to you and your family’s safety. If none of the tips we provided here help you get hot air blowing again from your furnace, call on a professional furnace repair company like the Indoor Comfort Team to fix your furnace and make sure it is operating safely.

You can call us for a furnace repair at (314) 230-9542. Indoor Comfort Team is available for 24/7 emergency furnace repair throughout the entire St. Louis metro area.

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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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