Why Is My Furnace Blowing Cold Air?

So you’ve turned on your furnace, but it’s giving you the cold shoulder by blowing cold air.

What’s wrong? It could be multiple issues… Here are a couple of reasons why.

1. The Thermostat’s Fan Setting

Does your furnace blow hot air sometimes, but cold air other times? Your thermostat’s fan setting may be set to ON.

The fan setting controls the blower, the part that circulates air throughout your home, Setting it to ON means the blower will run 24/7—regardless of whether the furnace is heating the air or not, thus why you get cold air sometimes.

Do this: Ensure your thermostat’s fan setting is set to AUTO, not ON. AUTO ensures that the blower will run only when the furnace heats the air. 

2. The Furnace’s Air Filter

Your furnace may be blowing cold air because the filter is too dirty.

A dirty air filter blocks airflow over the furnace’s heat exchanger, causing it to overheat. When overheating, your furnace can trip a high limit switch, causing the furnace burners to shut off so that the heat exchanger does not crack.

Do this: Turn off your furnace at the thermostat, and check the furnace filter. If it’s dirty, change it. You may need to call a technician to help you reset the furnace.

3. The Pilot Light

Do you have an older furnace with a standing pilot light? If the pilot light isn’t lit, then the furnace’s burners won’t light, meaning no heat.

What to do if the pilot won’t light or stay lit
At this point you should call a furnace technician for help. You may have a malfunctioning thermocouple (an inexpensive fix) or other issues that cause the pilot light to burn improperly. 

4. The Condensate Line

Do you have a high-efficiency furnace? Do you see water pooling around the furnace? Then the furnace’s condensate line (usually a PVC pipe) may be blocked, which causes the furnace to shut down. When high-efficiency furnaces run, they create water (condensate), which is emptied out a drain line. However, if that line gets blocked, water backs up into the furnace, causing an overflow kill switch to shut down the furnace to prevent water damage.

Common causes of condensate blockage are:

  • Dust
  • Dirt
  • Mold
  • Ice (only during cold snaps and if the line runs through an unconditioned space)

Besides blockages, the condensate overflow may also be caused by a broken condensate pump. In that case, you’ll need a professional the fix the pump.

Need a furnace repair in Los Angeles, CA?

Why not give Coolray a call? (818)308-8388

Go Air Heating and Air Conditioning is your comfort expert with specialists in heating, air conditioning, air quality and ventilation. Have more questions? We’d be happy to help – just contact us

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