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What would make the fan on a furnace turn off and immediately back on while running?

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It depends on the furnace. Older ones had 'temperature initiated' and 'temperature terminated' cycling controls for the blower motor in the heat mode. Newer ones energize the blower through a tiny black or white relay soldered into a circuit board. Those types are "time initiated" and "time terminated" and it could be that the board is going a bit wonky cycling the fan relay.

As described, it just about cannot be the motor cycling on its internal winding thermostat because it has to cool down several minutes or longer before the internal overload resets. And they never fail in a way to cause the motor to do what you are saying.

Flip the thermostat's fan switch from 'auto' to the 'on' position and see if it still does it like you described above. If it does not, the relay board is most probably the culprit. The 'on' position USUALLY uses a different relay on the circuit board and USUALLY runs the blower on a higher speed. There are exceptions though.

A 'far out' possibility is a faulty wiring connection that makes and breaks the circuit due to the vibration....but that's a slim chance. After a few times, the ends of the stranded wires begin to 'arc' and melt then they rapidly become too short to come into contact.

Posted on Nov 24, 2014

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Need more info about your unit

Posted on Nov 24, 2014

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Hi,
The chances are that the furnace was running when the power went out. This will cause the high limit switch to cutout and needs to be manually reset.
You will find that switch in the blower compartment near the blower. Usually somewher near the highest point of the cabinet.
Push the little red button firmly till it clicks.
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If you have a electric tester you can also follow the low voltage circuit till you find the open switch. Ther is usually a wiring schematic on the door cover of most furnaces.

I hope that this will help you to solve your problem!

Thanks for using Fixya!!

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