Why Is My Furnace Blowing Cold Air?

If your furnace is blowing cold air, start at the thermostat. The fan setting is an easy fix you can check yourself. If that is not the problem, we look at airflow, ignition, furnace controls, and other parts that can stop the furnace from heating.

Check Your Thermostat Fan Setting First (A 30-Second Fix)

Make sure your thermostat is set to HEAT and the fan is set to AUTO. If the fan is set to ON, it keeps running after the burners shut off and sends room-temperature air through your vents.

Switch the fan to AUTO. If the air warms up with the next heating cycle, you found the problem.

8 Reasons Your Furnace Is Blowing Cold Air

After servicing furnaces in the Lawrence community for decades, Dunco has found eight common causes for furnaces blowing cold air. While some of these reasons are simple fixes, others require the assessment of an HVAC professional.

1. Thermostat Fan Set to ON

If the fan is set to ON, it keeps running after the burners shut off. Set it to AUTO so the blower runs with the heating cycle.

2. Clogged Filter Causing Overheating

Pull out the filter and take a look. If it is packed with dust, replace it.

A dirty filter blocks airflow. The furnace gets too hot, and the high-limit switch shuts the burners off. The blower may keep running, leaving cold air at the vents. You may notice furnace short cycling too.

3. Pilot Light or Ignition Failure

If the furnace cannot light the burners, the blower has no heat to move through your house.

For an older furnace with a pilot light, follow the lighting instructions printed on the equipment. If the pilot will not light or stay lit, stop there. We need to check it. Newer furnaces use electronic ignition that requires testing when it fails.

4. System or Control Problem

If the thermostat and filter look right but the burners still will not stay on, a furnace control may be stopping the heating cycle. We need to test the controls instead of having you keep resetting the system.

5. Dirty or Failing Flame Sensor

If the burners light and shut off a few seconds later, the flame sensor may not be confirming the flame. We need to check the sensor and ignition system before you keep running the furnace.

6. Leaky or Uninsulated Ductwork

Your furnace may be making hot air, but some of that heat can be lost before it reaches the room.

Leaks in ducts running through a cold attic, crawl space, or other unheated area can make the air at the vent feel cooler, especially during very cold Kansas weather.

7. Condensate Line Issues (High-Efficiency Furnaces)

High-efficiency furnaces produce condensate while running. If that water cannot drain, the furnace may shut the heat off to protect the equipment.

If you notice cold air and find your furnace leaking water, the two problems may be connected. We check the condensate drain, trap, and other drainage components to find where the water is backing up.

8. Thermostat Wiring or Settings

If the thermostat is set to HEAT and AUTO but the furnace still blows cold air, the thermostat or its wiring may not be sending the right signal.

Check the settings and batteries. If those are not the issue, call a professional and leave wiring work to us.

What's Safe to Check Yourself

Before you call, check the thermostat, filter, open vents, and accessible outdoor intake and exhaust pipes. If you have an older furnace with a pilot light, follow only the lighting instructions printed on the equipment.

If you smell gas, leave the house and call the gas utility from outside. Do not try to relight the furnace first.

When to Call a Lawrence Furnace Pro

If the thermostat, filter, vents, and outdoor pipes look right but the air stays cold, schedule furnace repair in Lawrence. We can test the furnace and find why the burners are not staying on or why the heat is not reaching your rooms.

If your furnace smells at startup, pay attention to what you smell. If it smells like gas or rotten eggs, leave the house and call the gas utility from outside. If you smell electrical burning, shut the furnace off and have it checked before you run it again.

A furnace tune-up gives us a chance to catch dirty components, worn parts, and airflow problems before heating season. Schedule an appointment online or call us today for furnace assistance in Lawrence, KS. For problems that happen after hours, Dunco offers 24/7 service booking by phone or online.