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Why Is Your Furnace Blowing Cold Air?

Your heating system seems to be working well, but why is your furnace blowing cold air?

The winter time calls for heavy use of your furnace on a daily basis. You may turn on your heating system expecting warm and and find that your furnace is blowing cold air instead. This is never ideal, especially when you’re relying on your furnace to keep you and your family warm during the cold Canadian winter. If this is the case, it’s not always necessary to call a technician right away as you may be able to find a simple solution to the problem by taking our advice.

Furnace blowing cold airDirty Air Filter

If your heating system has overheated, it is common to find your furnace blowing cold air. In the case of an overheating furnace, your heating system has a safety function called a limit switch which will turn off the burners when the system’s temperature is above a safe level. In turn, the heating system’s fan will be running and blowing cold air through the ducts in order to prevent any serious damage from occurring, which is why you might find your furnace blowing cold air.

If your furnace’s air filter has not been changed in a while it is certain that there will be an excess buildup of dust, dirt, and other debris which will restrict the airflow in your heating and duct system. Restricted air flow means that your furnace has to run for a longer period of time to reach your desired temperature, and thus it overheats. Changing your furnace’s air filter once a month will ensure that you do not face this problem and avoid the hefty repair costs that come with an overheated furnace and a broken heat exchanger.

Check the thermostat

The solution to your furnace blowing cold air could reside in your thermostat settings. Make sure that your thermostat is not set to ON and is instead set to AUTO. When the thermostat is set to ON, there will be cold air being blown through the ducts  constantly by the fan in your heater. If your thermostat settings were the problem, setting your thermostat to AUTO should fix the problem.

Ignition system not working (Furnace blowing cold air)

If you own a gas furnace and are experiencing your furnace blowing cold air, it is possible that the problem has to do with the pilot light. If your pilot light is out that means that the heating system is not lighting the fuel and is therefore not heating your home.

The manual that comes with your gas furnace should come with instructions on how to relight the pilot light. If you relight it and the flame dies down once again, then there may be a bigger problem. If this is the case you may need to book a repair appointment with a trained HVAC technician and they will repair or replace the necessary components and bring your furnace back in working order. If you relight the pilot light and it stays lit then you have successfully fixed the problem and your gas furnace should now be warming up your home.

If you are unable to relight the pilot light whatsoever, it may be a problem related to the gas valve. Check to make sure that the gas valve is turned on. If the gas valve is turned on, but the pilot light is off then you may have a dirty pilot light. Clean the pilot light and the problem should be solved.

Poor Gas Supply

Your home’s furnace has a safety function which causes it to shut down when there is not enough gas to warm the air. If the gas supply is turned on and this is the problem then you have a severed gas line. This problem would make it crucially important for you to reach out to a trained technician for further assistance.

Blocked Drain Lines

Condensate drain lines are built into furnace systems in order to remove ant built up water that is a result of the heating procedure. If this drain lines becomes clogged then your furnace system will not allow the burners to turn on and warm the air.

Flame Sensor Problem

If you find that your furnace has a dirty flame sensor, this may be why your furnace blowing cold air. A dirty furnace flame sensor would cause your furnace unit to consistently start and stop running, which would not allow your gas burner to stay on and would cause for cold air to circulate through your duct system. Cleaning the flame sensor would solve this problem.

Damaged Air Ducts (Furnace blowing cold air)

If your home’s duct system has connection issues or leaks in it then that would allow cold air from your attic or other poorly insulated places to enter into the system and make it seem like it’s your furnace blowing cold air. This does not mean that your furnace isn’t working properly, it just means that your ducts need to be sealed or repaired. A damaged duct system would result in your home’s heating system needing to work much harder than necessary to achieve results and would cause you to pay more money on your monthly utility bill.

Not all possible reasons for your furnace blowing cold air have been exhausted in the list above, but the ones listed are what is most common to find as the cause for this problem. If you have followed our advice and have still not found the solution to your problem, your next step should be to contact a trained HVAC technician to diagnose the issue and provide the best possible solution.


If you have any further questions about your furnace blowing cold air or any of your other HVAC needs, don’t hesitate to contact DeMark Home Ontario to find the answers. We provide quick and efficient furnace service and replacement performed by our highly experienced and trained technicians. Give us a call at 647-847-2998 or fill out online request form and we will be happy to assist you with your request.

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