3 Possible Reasons Why Your Furnace is Blowing Cold Air

3 Possible Reasons Why Your Furnace is Blowing Cold Air

When you turn on your furnace, cold air isn't what you're looking for or expecting. A furnace that doesn't put out quite enough warmth for your comfort level may simply be a case of a thermostat that's set too low. However, any time a furnace is blowing outright cold air something else is amiss: maybe something simple, maybe something major. For safety reasons, a gas-fired heating system isn't the place for DIY repair attempts, so calling in a qualified HVAC professional may be necessary. In the meantime, here are three possibilities to help you at least narrow down the problem.

  • Improper thermostat setting. To get heat your thermostat must be in the "Heat" mode and the switch should be set to "Auto" not "On." The desired temperature setting should be at least 5 degrees below the room temperature. If the thermostat switch is in the "On" position--instead of "Auto"--only the blower fan runs and the burners do not ignite. Cold air is the result.
  • Overheated furnace. Safety switches inside a furnace detect dangerously high temperatures and automatically shut off the burner. From that point on, only the system blower runs and cold air comes out of heating vents. An overheating furnace presents a definite safety hazard so diagnosis and repair must be left to qualified professionals. Some of the causes include insufficient air circulation through the ductwork, causing the burners and combustion chamber to overheat.
  • Ductwork issues.Defective or deteriorating ducts in your home may be allowing cold air from an uheatead zone such as the attic or crawl space into your system airflow. This may lower the temperature of air reaching individual rooms, making it feel like the furnace is blowing cold air. The duct system is carefully balanced so that all rooms in the house receive an equal volume of hot air, no matter how far away they are from the furnace. Defects in the ductwork can upset that balance and some rooms may receive cold air. This requires professional inspection and diagnosis.

For fast service to resolve furnace cold air issues, contact Valley Heating, Cooling Electrical.


Our goal is to help educate our customers in the South Bay Peninsula and Santa Cruz areas of California about energy and home comfort issues (specific to HVAC systems).