Why Your FurnaceBlows Cold Air

Winters are mild, yet about 10 nights a year fall to freezing (National Weather Service). When the desert air turns cold, your gas furnace works hard to keep your living space comfortable. If you walk past a supply vent and feel a stream of cold air blowing out, the heating system is failing to do its job. Finding the cause is the first step toward getting the equipment running correctly again.

Checking the Thermostat

Before assuming the furnace is broken, check the thermostat on the wall. Sometimes the issue is a simple setting. If the fan setting is switched to the on position, the blower motor will run constantly. The furnace only produces heat during a heating cycle, so when the cycle ends, the fan will continue to circulate room-temperature air. This air feels cold coming out of the vents. With the fan on auto, the blower runs only while the furnace is heating.

Smart and programmable thermostats can also cause confusion if the schedule is set incorrectly. Verify that the system is set to heating mode and that the target temperature is higher than the current room temperature. If the screen is blank, the batteries might need to be replaced, or the thermostat might have lost its connection to the control board.

Air Filter Restrictions

Airflow is critical for a gas furnace to operate properly. The blower pulls return air from the house, pushes it across the heat exchanger, and sends the warmed air back through the supply ducts. The air filter sits between the return duct and the furnace to catch dust. If the filter is clogged with dirt, airflow is severely restricted. ENERGY STAR advises checking the filter every month and changing it at least every three months.

When airflow drops, the heat exchanger cannot transfer its heat to the house. The temperature inside the furnace rises until it trips a safety limit switch. The limit switch shuts off the burners to protect the furnace while the blower keeps running to cool it. The result is cold air blowing from your vents while the burners remain shut off.

Ignition and Flame Sensor Failures

Modern gas furnaces use electronic ignition rather than a standing pilot light. When the thermostat calls for heat, the igniter glows red hot to light the gas. If the igniter is cracked or burned out, the gas will not ignite, and the furnace will blow cold air. After a few failed attempts, the control board will lock out the system for safety.

If the gas does ignite, a metal rod called a flame sensor confirms that a fire is present. The flame sensor sits directly in the burner flame. Over time, carbon buildup and soot can coat the sensor. If the sensor cannot read the flame, the control board shuts the gas off after a few seconds as a safety step. The blower continues to run, distributing cold air. Cleaning the flame sensor is a standard part of routine maintenance.

Gas Supply Issues

A gas furnace needs a steady supply of natural gas from Southwest Gas to generate heat. If the gas valve on the side of the furnace was accidentally turned off, the burners will not light. Similarly, an issue with the gas meter outside or a service interruption in your neighborhood can stop the flow of fuel. If your other gas appliances, such as the stove or water heater, are also failing to work, the problem is likely with the main gas supply.

Never adjust the gas valve inside the furnace yourself. Incorrect pressure can cause poor combustion, leading to soot buildup and carbon monoxide risks.

How We Can Help

Kestrel Heating & Air provides gas furnace repair across the Las Vegas valley. When a technician arrives, they inspect the burners and ignition, check the blower and filter, and test the safety limits to find the cause of the cold air. We provide a written price before any repair work begins. ENERGY STAR suggests it may be time to consider replacement of a furnace older than 15 years, and we never recommend replacement on age alone; the Kestrel Comfort Club includes a fall heating tune-up that inspects the burners and ignition and cleans the flame sensor.