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Why Is My Furnace Blowing Cool Air?

Posted on: November 29, 2017

When a furnace first turns on, it is relatively common for cool air to be blown out of the ductwork for a few seconds, until it is replaced by heated air. If your furnace continues to blow cool air, however, there may be a problem with your thermostat or the furnace itself.

Common Reasons Why a Furnace Will Blow Cool Air

Cool air can be caused by numerous problems, from incorrect thermostat settings to duct leaks. The most common include:

  • Incorrect fan settings – If the fan switch on your thermostat is set to “On” instead of “Auto,” the fan will run continuously, drawing cooler air between heating cycles.
  • Improper thermostat settings – If your thermostat is not set to the correct temperature it could blow cool air from the air conditioner, instead of warm air from the furnace. Check the thermostat’s settings, especially if the batteries are low or have been replaced recently.
  • Damaged thermostat – A defective thermostat could cause problems with the furnace or the fan. Check it for problems and have it replaced, if necessary.
  • Limit switch problems – The limit switch for the furnace turns the blower fan on only once the air has been heated thoroughly and keeps it running for a while after the furnace turns off to make use of any remaining heat. This improves the furnace’s efficiency, and prevents the heat exchanger from overheating and cracking. A damaged limit switch could allow the fan to run before the air is heated properly, or after the air has already cooled.
  • Ignition failure – A damaged electronic igniter or a pilot light that has blown out can prevent the burners from lighting when required, resulting in cool air from the vents.
  • Thermocouple failure – In a furnace with a pilot light, a thermocouple is used to ensure that the gas is only on when the pilot is lit, preventing dangerous gas accumulations when the pilot goes out. A damaged thermocouple will prevent the pilot light and burners from functioning properly, resulting in cool air.
  • Flame sensor problems – A dirty or damaged flame sensor on newer furnaces could cause the burners to shut off prematurely, resulting in cooler air if the fan keeps operating.
  • Furnace overheating – If the furnace does not receive enough airflow, the heat exchanger may become too hot, and the burners will shut down to prevent damage to the heat exchanger. If the fan keeps running, cooler air may be expelled from the vents. Poor airflow can be caused by a clogged air filter, duct obstructions, or blocked return vents.
  • Duct leaks – If you have ducts that are not thoroughly sealed, cool air can be drawn in from other areas of your home. Likewise, air ducts that are not insulated properly can allow the heated air to cool, especially if they are in cold areas like a crawl space.

If an errant thermostat setting, a blocked vent, or a clogged air filter is causing your furnace to blow cool air, you may be able to correct it yourself. For more serious problems, however, you will need professional assistance.

Furnace Repairs in Lakeland, Florida

When you are having furnace problems in your Lakeland home, you can count on our team at Iceberg Cooling for fast, reliable repairs. We have been serving the Lakeland and Polk County area for over 70 years, and our HVAC professionals are available 24/7 for emergency repairs.

To schedule an appointment or request an estimate, contact our team at (863) 223-1849 today!