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Old 10-16-2012, 10:53 AM   #16
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Fan Keeps Running


In my area, those pictures are what I'd expect from a furnace from the early 70's.
All I'm hearing is a fan/limit control that needs it's fan temp nibs adjusted properly to stop the fan from short cycling.

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Old 10-16-2012, 10:55 AM   #17
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Fan Keeps Running


Quote:
Originally Posted by how View Post
In my area, those pictures are what I'd expect from a furnace from the early 70's.
All I'm hearing is a fan/limit control that needs it's fan temp nibs adjusted properly to stop the fan from short cycling.
Fan isn't short cycling. Its running to long after heat call is over. According to the OP.
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Old 10-16-2012, 11:11 AM   #18
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Fan Keeps Running


Nope! Try reading that opening op post again.

It sounds like a classic example of miss adjusted temp nibs of a fan limit control or a bimetal fan/limit control in need of replacing. Those older units require re adjustments over time.

Either the temp "off" nib is too high, allowing the exchanger heat to cause another fan run after the furnace shuts down or the temp on the fan "on" nib is low enough to allow the normal exchanger temp to catch it again after the furnace has shut down, for another blow.

Last edited by how; 10-16-2012 at 11:17 AM.
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Old 10-16-2012, 11:25 AM   #19
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Fan Keeps Running


Still sticking. if it just needed re adjusting, it ouldn't work correctly sometimes.
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Old 10-16-2012, 11:32 AM   #20
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Fan Keeps Running


I guess an op picture of the fan/limit control with its cover removed and a corresponding small adjustment to the temp nibs would be a fast and easy thing to check.
What he reports of what he sees with that temp dial position when the fan comes back on for the short little run will help us all diagnose the problem.
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Old 10-16-2012, 01:30 PM   #21
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Fan Keeps Running


sorry to ask but where would the fan/limit controls be? I'm guessing in the cavity where the fan is located, so while waiting for a reply, i'll take a look.
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Old 10-16-2012, 05:49 PM   #22
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Fan Keeps Running


Follow the wires back from the blower motor.
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Old 10-16-2012, 06:25 PM   #23
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Fan Keeps Running


I did do some tracking and it will require i take off a panel, screwed on, to get to it, it's between 2 removable small panels. however, all it appears to be is a reset type button toggle switch. I'm sure it does a whole lot more, but it is very small. outside of that, the wires run from the blower motor to this switch, a couple wires, with the majority running to the red unit i took a picture of and from there branches to the black as well as a switch panel, one switch that shuts down the fan.
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Old 10-16-2012, 08:32 PM   #24
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Fan Keeps Running


The fan control, because it's a temp control will usually be situated at least a foot above the burners. On some older furnaces it can be found immediately above the furnace on the hot air plenum.

It measures the temp of the exchanger and is therefore never found in the fan compartment.
Beenthere's suggestion of tracing where the blower motor wires lead to is the easiest way to find the fan controls.


Last edited by how; 10-16-2012 at 08:41 PM.
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