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Troubleshooting Common Causes of Furnace Blowing Cold Air


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If your furnace is blowing cold air in your home, this is a sign of an issue with the flame detector, gas supply, air filter, pilot light, or condensate lines. Many of these issues you can quickly solve on your own, especially since winters in Canada can be freezing cold and may drop as low as -30 to -40 degrees celsius. 

During this cold weather, you must know what to do when your furnace is not working as it should. So, here are five common causes your furnace is blowing cold air and tips on how it can get fixed:

1. Worn-Out Flame Sensors or Flame Limiter

The flame limiter system will shut off the furnace when the flame sensor is worn out. If this happens, it can lead to the furnace becoming blocked in the on position, which can be bad news if you’re cold and rely on your furnace for heating. For example, your furnace could run full-time and have an internal motor burn out due to excess heat. 

How to Fix It: You may be able to get your furnace fixed quickly by replacing the sensors. However, you should contact a professional to get a more permanent fix.

2. Faulty Gas Valve

The gas valve will shut off gas flow to the furnace if there is no flame. If the gas valves are faulty or too old, this will cause your furnace to blow cold air.

How to Fix It: Turn the gas off at the furnace to ensure you do not get any dangerous gas leaks. You may also need to call a professional to turn the gas back on or ensure the gas lines are all connected properly.

3. Blocked Air Filter

A blocked air filter can lead to your furnace not working as well. A furnace needs an adequate amount of airflow to keep the fire burning. If it does not have the right airflow, it can lead to the furnace blowing cold air.

How to Fix It: Some people find it easier to take the air filter off and clean it themselves. If you do this, make sure to replace it back in the exact spot where it was before you removed it.

4. Old or Faulty Furnace

Furnaces are a major investment, so if your furnace is old, you will likely need to replace it. Call an expert repairman right away if you notice any of the following:

  • The furnace is too noisy.
  • The furnace is not blowing enough air out of the vents.
  • Condensation is leaking out of the furnace.

How to Fix It: You can hire a professional to come into your home to inspect your furnace and work on it if needed.

5. Blocked Gas Lines

If your furnace is not working, the problem could be blocked gas lines. This will keep the furnace from getting the gas it needs to keep running.

How to Fix It: If you see any blockages, you will need to contact a professional to have them removed. You may also need to call in a professional to have the entire gas line checked.

6. Defective Thermostat

If your thermostat is defective, it can lead to your furnace not pumping enough air. The furnace will then try to get the air going by blowing frigid air out of the vents, but since the thermostat is not working, it will never turn on.

How to Fix It: You can get a new thermostat from your local hardware store. You may also need to contact a professional to come and fix the thermostat if you cannot do it on your own.


Furnace troubles are the last thing you need in the winter months. If your furnace is not blowing out hot air, it will likely be one of the most unpleasant home repairs you can have to deal with. More often than not, it will be better to call for professional help because they can identify problems immediately and get your furnace running fast, saving you the trouble and extra cost of ineffective DIY efforts.

For furnace repair in Pickering, give us a call at Climate Experts. We install, repair, and maintain all of the major brands of air conditioners and furnaces, serving the Durham region, GTA, and neighbouring areas. Call us to get 24/7 emergency support from our team of experts.

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