My furnace recently started blowing cold air at random times. If you listen when the thermostat kicks the heat on, it will try to light three times; then if it does not, it blows cold air. Other times, it lights on the first try and is fine. It seems to not light correctly at night. Any suggestions?

Answer

Without being there, I'd say you have a dirty flame sensor or intermittent issue with the pressure switch. The blower coming on after three attempts is a default when going into lockout.

Updated on April 12, 2018

Original Article:

Troubleshooting Common Furnace Problems With an HVAC Expert
By Dan Robbins

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