Ken, Its almost always the thermostat. They are made so cheaply now that it usually takes 4 or 5 replacements until you get one that works correctly. Replace it again. Bubba
There are 2 pictures. The second one is a picture of a thermostat (the furnace fan-control thermostat) that's mounted to the heat exchanger inside a furnace (in this picture the cover was removed from the thermostat). Do you this on your furnace?
The round plate with a slot in it rotates based on the temperature inside the heat exchanger. It's what turns your furnace fan on and off, and also has the hi-limit switch.
Your problem could be that your fan is cooling down your heat exchanger to the point that your fan is turned off even if your burners are still going. This could be caused by an incorrect Fan-Off setting, or maybe you need a new furnace fan-control thermostat.
Alternatively, you could be hitting the hi-limit, which means that the temperature inside the heat exchanger has reached the hi-limit setting of the fan-control thermostat. When this happens, the burners are shut off, but the fan will (or should) keep running until it reaches the Fan-Off setting.
What you should do next is go and watch your furnace (take the front cover panel off) and see if your fan turns off while the burners are still on, or if the burners turn off (but the fan is still running).
It's possible that a safety of some sort is being tripped. There are many other possibilties. If you're not really handy you probably should call someone to take a look at it
Ummm, would you really want a thermostat made by a company that makes such a high defect rate like that even if the one you got happened to work? A thermostat is something where you should go out of your way to get a good one (not whatever they happen to have on ths shelf at Wal-Mart or Home Depot that's cheap).
I also suspect it's a limit switch inside the furnace.
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