Furnace Blowing Cold Air? Here’s Why…

2015 Jan 20
Posted in: Heating

During winter, people often ask us this question: “Why is my furnace blowing cold air?”

What this means: Your furnace’s blower is still working, but the furnace gas burners have stopped. 

Here are 4 common reasons a furnace starts blowing cold air:

  1. Thermostat fan setting is set to “ON”
  2. Pilot light is out
  3. Furnace overheated
  4. No gas being supplied 

Let’s explain these issues in detail and what you need to do to get your heat back.

Thermostat fan setting is set to “ON”

What this means: Does your furnace blow hot air sometimes and cold air other times? If so, your thermostat is probably set to “ON.” That means the furnace blower continually runs...even when the burners are not running.

Solution: Turn the thermostat fan setting to “AUTO.” Now the blower should only run when the burners are on. 

If the blower still runs continually, there is another issue that an HVAC professional will need to solve.

Pilot light is out

What this means: Older furnaces use a permanently lit pilot light to ignite the gas for the burner. If the pilot light goes out, then you won’t get any heat.

Solution: Relight the pilot light. Instructions on your furnace should show you how to do this. If the pilot light keeps going out, you have another issue that a professional needs to solve. 

Side note: Old furnaces waste tons of gas to keep the pilot flame lit. Consider upgrading to a newer furnace that does not use a pilot light or to a heat pump that uses electricity.

Furnace overheated

What this means: The furnace’s heat exchanger (the part that heats the air) got too hot. So the furnace turned off the burners and kept the blower running to cool off the heat exchanger. Thus why you’re only getting cold air. 

Anything that limits airflow over the heat exchanger causes it to overheat. 

Solution: A dirty air filter is a common cause of restricted airflow. Change the filter and see if that fixes the problem. 

No gas being supplied 

What this means: No gas to the burners=no flames. No flames=no heated air.

Solution: Ensure the gas valve lever is parallel/in line with the gas supply pipe (aka, in the “on” position.) 

Know when to call a professional for help

Try the above DIY solutions first. If none of those work, you need a furnace repair person’s expertise.

If you live in the Phoenix metro area, schedule a repair with George Brazil Air Conditioning & Heating.