Fan cycles on/off after the burner stops

Old 10-12-09, 10:02 PM
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Fan cycles on/off after the burner stops

My furnace fan cycles on/off/on/off/on/off after the burner stops. One minute after my furnace thermostat cycles the furnace burner off and the fan stops, the fan comes back on again for about 10 seconds, shuts off, then repeats this two more times. What causes this?
Old 10-12-09, 10:47 PM
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For what it's worth, mine does basically the same thing. A few minutes after the fan shuts off it will come back on once or twice. But if I hold my my hand to the register it will tell me it is blowing warm air. Which tells me there is still some heat left in the combustion chamber which is rising and triggering the 'thermostat' for the fan.
Old 10-13-09, 06:12 AM
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Residual heat in the heat exchanger causes this. The fan switch may need adjusted to run longer after the heat cycle to cool down the heat exchanger completely.
Old 10-14-09, 07:43 PM
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My furnace is doing the exact same thing. Is adjusting the fan switch easy to do? Can I do it, or do I need to call in a Pro?
Old 10-14-09, 07:53 PM
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This is a problem normally associated with counter-flo furnaces (heat from the floor). As said in previous post the off temp may be set a little to hi or some of your supply registers closed. Check and see what type of fan control you have. May be one that shows "off" temp only or "on" temp only or shows both temps plus limit setting. Report back with your findings. Also jot down the numbers you find.
Old 10-14-09, 07:59 PM
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It's a simple adjustment if your furnace has a fan limit.


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