Why Is My Furnace Running But Blowing Cold Air? (3 Simple Checks)

Key Takeaways (TL;DR):

• Thermostat Settings: Ensure your fan is set to "AUTO," not "ON," to prevent it from running without heat.
• Airflow Blockage: A dirty air filter is the #1 cause of a furnace blowing cold air. It triggers a safety feature that shuts off the burners.
• Pilot Light: On older gas furnaces, a pilot light that has gone out prevents the system from igniting.
• Safety First: If you smell gas, hear loud banging, or suspect a major failure, turn off your system and call Fairview immediately.

It’s a chilly morning in Brentwood or Oakley. You hear your furnace kick on and expect warmth, but ten minutes later, the house still feels cold. You put your hand to a vent and feel a steady stream of... ice-cold air.

Don't panic. This is one of the most common furnace calls we get.

Why is my furnace blowing cold air? The most simple reason is that your blower fan is running while the burners are off. This usually happens because of an incorrect thermostat setting or a safety limit switch that has been triggered by a dirty air filter. In many cases, this is a simple fix you can do yourself.

Here is the 3-step checklist to try before you call for repairs.

3 Steps to Fix a Furnace Blowing Cold Air

Step 1: Switch Your Fan from "ON" to "AUTO"

Before you go to the furnace itself, check your thermostat. Look at the "Fan" setting. Is it set to "ON"?

When set to "ON," your furnace’s blower fan will run 24/7, even when the heating cycle is finished. This means it will constantly circulate room-temperature air, which feels cold coming out of vents. Switch the setting to "AUTO." This ensures the fan only runs when the furnace is actively heating the air.

Step 2: Check for a Dirty Air Filter

This is the most common culprit. A clogged, dirty air filter blocks airflow over the furnace’s heat exchanger. Without air moving over it, the heat exchanger rapidly overheats.

Your furnace has a safety device called a "High Limit Switch." When it detects overheating, it shuts off the burners to prevent a fire or damage to the unit. However, the blower fan will keep running to cool the system down—resulting in cold air blowing from your vents.

• The Fix: Locate your furnace filter. If it looks gray and clogged with dust, replace it immediately with a new one.

Step 3: Check Your Pilot Light (Older Gas Furnaces)

If you have an older furnace, it may rely on a standing pilot light to ignite the burners. If a draft blows it out, or a faulty thermocouple shuts the gas valve, the main burners will never light. The fan may still turn on, but it will only blow unheated air.

• The Fix: Look through the small window on the front of your furnace burner compartment. Do you see a small blue flame? If not, you can try to relight it following the manufacturer's instructions on the unit. If you aren't comfortable doing this, or it won't stay lit, give us a call.

When is Cold Air a Sign of a Major Failure?

Sometimes, cold air is a symptom of a deeper mechanical issue.

Question: What if my furnace blows cold air for a few minutes then shuts off? Answer: This is often a sign of a failing Flame Sensor. The sensor’s job is to confirm that fire is present. If it’s dirty or broken, it won't detect the flame, and it will shut off the gas valve as a safety precaution after a few seconds.

Question: Why is my heat pump blowing cold air? Answer: If you have a heat pump instead of a furnace, know that they naturally blow air that is cooler (around 90-95°F) than a gas furnace (around 120-140°F). It may feel cool to your hand, but it is still warmer than the room air and is heating your home. However, if it is blowing ice-cold air during winter, it could be stuck in "defrost mode" or have a refrigerant leak.

Dangerous Symptoms: Do Not DIY

If you’ve checked your filter and thermostat and the problem persists, you likely have a mechanical failure that requires a professional. Do not attempt to fix internal components yourself, as they involve gas lines, high voltage, and carbon monoxide risks.

Call a licensed technician immediately if you notice:

• Smell of Gas: A "rotten egg" smell indicates a gas leak. Evacuate immediately and call PG&E and Fairview.

• Loud Banging or Popping: This could indicate delayed ignition or a cracked heat exchanger.

• Soot Around the Furnace: A sign of improper combustion, which can lead to carbon monoxide leaks.

Don't let a small issue turn into a no-heat emergency during a cold snap. If you live in East County and can't get the heat back on, our team offers fast, reliable furnace repair in Brentwood, Antioch, and Oakley.

Frequently Asked Questions About Furnaces