Troubleshooting
Furnace Blowing Cold Air: Troubleshooting Guide
6 min read••
Nothing is worse than expecting warm air and getting a cold blast instead. Here are the reasons your furnace might be blowing cold and what to do about it.
When temperatures drop in the Bay Area—and yes, it does get cold here, especially at night—the last thing you want is a furnace that blows cold air. Let's troubleshoot.
Check These First
Thermostat Settings This sounds basic, but check that your thermostat is set to HEAT mode and the fan is set to AUTO, not ON. When set to ON, the fan runs continuously even when the furnace isn't actively heating.
Give It Time Modern furnaces have a delay before blowing warm air. The blower doesn't start until the heat exchanger reaches the proper temperature—usually 30-90 seconds.
Common Causes
Pilot Light or Ignition Problems Older furnaces have pilot lights that can blow out. Modern furnaces use electronic ignition, which can fail. If you don't hear the ignition clicking or see a flame, the furnace can't produce heat.
Dirty Flame Sensor The flame sensor tells the control board that the burner is lit. When it gets dirty, it can't detect the flame, causing the furnace to shut off for safety.
Overheating If the furnace overheats, the limit switch shuts down the burners but keeps the fan running. This often happens due to restricted airflow from a dirty filter.
Gas Supply Issues Check that your gas valve is open and that you're receiving gas. If other gas appliances aren't working either, contact your utility company.
Ductwork Problems Cold air infiltration through leaky ducts can make it seem like the furnace is blowing cold when it's actually warm air mixing with cold attic air.
Safety Warning
Never try to repair gas furnace components yourself. If basic troubleshooting doesn't resolve the issue, call a qualified technician. Gas furnaces involve combustion, and improper repairs can create dangerous situations.