Gas furnace blows warm air 1st half of running time then 2nd half starts blowing cold air?

Mine does the same thing. It could be that what you're noticing is the fan pushing the last of the hot air out of the furnace and the duct work after the burner has finished producing what your thermostat asked for in the way of heat. It always runs for a minute or so after the burner has shut off. The first time I heard it, I got as upset as you are, called the repair guy, and was totally embarrassed when he explained there was nothing wrong.

On the other hand, when I caught it running without producing heat for long periods, that turned out to be a leak in the gas line valve outside the house, and I had to have the company fix it. Listen to it carefully, and if it's truly running for a very long time (more than a minute by the clock) without producing heat, then call and have it checked.

Take the cover off the burner compartment of the furnace. Obtain the make and model of the furnace off the rating plate in the burner compartment and post it here.


How long does the furnace fan continue to run after the burners shut off. Use a watch to measure the time.

Seattle is heading in the right direction so try to answer his questions. I run my blower 24/7/365 and it helps keep all levels of the home a more uniform temp using only slightly my electrcity than having my tstat set on AUTO.