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Old 02-17-2011, 08:56 PM   #1
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furnace fan will not shut off


the fan on my 12 year old sears dc80 natural gas furnace will not shut off. fan switch is set on auto. the only way to get it to shut off is flip the switch on the side of the furnace. what is the likely cause?

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Old 02-17-2011, 08:59 PM   #2
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furnace fan will not shut off


is it just the fan running or fan and heat? I've had thermostats cause heat and fan to stay on, some furnaces run continuous fan when there is a problem.

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Old 02-17-2011, 09:04 PM   #3
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furnace fan will not shut off


fan only. no heat
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Old 02-17-2011, 09:40 PM   #4
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furnace fan will not shut off


My first recommendation would be to check the ignitor. The ignitor is typically below the far right burner (not stage right, your right), it glows orange to ignite the gas. If it does not glow when you turn the power on and call for heat I would try to get a visual of it and see if there is a crack in it. Here's what a cracked ignitor looks like, start there
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Old 02-18-2011, 06:21 AM   #5
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furnace fan will not shut off


Also check Fan limit. If you had dirty filter or a/c coil plugged it will cause it to break and making fan run only.
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Old 02-18-2011, 03:14 PM   #6
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furnace fan will not shut off


If you turn the thermostat to off and the fan switch to auto then reset the power switch on the side of your furnace does the fan come back on immediately? If so then the problem is probably not either of the above posts. Remove the green thermostat wire (or which ever wire is connected to your "G" terminal on your control board and see if the fan shuts off. If it shuts off then you have a bad T-stat or thermostat wire. If it keeps running then turn off the power and remove the highest speed wire from the control board then turn power back on,(depending on your motor: basic colors for motor leads are black = high, Blue = med, red = low). Any change? If the motor does't run when the wire is removed then the control board has a stuck relay. You can remove the lower speed wire to determine if the other relay is stuck. If it turns out to be the low speed then double check your manual reset safetys (roll-outs) before changing the control board. Hope some of this helps.
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Old 02-19-2011, 04:27 PM   #7
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furnace fan will not shut off


i left out some important info. the furnace will no longer come on. if the thermostat is set to heat and the fan switch is set to auto the fan continues to run. the only way to get it to shut off is to turn off the switch on the furnace. i unhooked the green wire to the thermostat but it continues to run. if i unhook the black wire on the control board the fan shuts off. is it a bad relay or is the control board bad?
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Old 02-21-2011, 06:29 AM   #8
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furnace fan will not shut off


If the heat will not run then it will be a safety. if one of your safeties is out then the control board will turn on one of the fans. Check the items mentioned in the posts by the other two, ignighter, blower comaprtment high limit cutout (usually mounted on the blower housing), High limit, ect...
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Old 02-21-2011, 11:30 AM   #9
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furnace fan will not shut off


i have checked the ignitor and it is fine, it is only a year old and looks new with no breaks. i have found two relays one on each side of the box that houses the burner tubes. they both check out good. i cannot find any in the fan compartment. i'm sure that i am overlooking the obvious but i don't see what looks like a relay to me. any suggestions?

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Old 02-22-2011, 06:10 AM   #10
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furnace fan will not shut off


The blower relays are built into your control board and are not repairable. I'm not sure I can help much from here, there should only be 4 to 5 safety switches in that circuit. If they all check out and removing the wire from G terminal doesn't work, and the ignighter is good then I would conclude then control board needed to be replaced. Last question? Did you use a meter to determine if the safeties were good or did you bypass them? Some of the troubleshooting methods we use in the field to troubleshoot I wouldn't ever recommend for a home owner to use and could be very dangerous if left in that condition by accident. Make sure that all components are put back to their original configuration.
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Old 02-22-2011, 08:58 AM   #11
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furnace fan will not shut off


I unhooked the wires from the relays and used a meter to test them. thanks for all your help.
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Old 03-23-2011, 05:24 PM   #12
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furnace fan will not shut off


so if i replace my thermostat and the fan is still continuously running where do i go from there?
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Old 03-25-2011, 01:35 AM   #13
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furnace fan will not shut off


Quote:
Originally Posted by gtshorty8611 View Post
so if i replace my thermostat and the fan is still continuously running where do i go from there?
Most modern furnaces will run the fan when there's a problem, you'd be better off paying for a service call then throwing parts at it
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Old 07-28-2013, 12:42 AM   #14
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furnace fan will not shut off


... after the A/C runs a lot when hot, then when it cools down, & we try heat, it keeps going. Just noticed in late this afternoon/early evening (bad timing ... Saturday night; Sunday calls are more than weekdays!).

Not sure what's up, but do believe this is a repeat.

Sad thing is, I have to go out Monday ~~ forgot to check driver's license, & that's the last day I can get it without it being expired the next day! ~~ & seems we should not let it run TOO long like this (would it ever get too hot, etc?). If I leave my 87yo mother here, who doesn't hear well, not sure what will occur. Really feel I should be here, but then there's that license thing ...........
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Old 08-01-2013, 10:58 PM   #15
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furnace fan will not shut off


Sounds like a high temp limit kicked. Most furnaces will go to fan only when 1 trips. Look on the inside of your door for wiring diagram. Might be more than the 2 you found (those were roll out switches in case gas ignites in cabinet due to clogged heat exchanger). There should also be 1 that inserts into the heat exchanger on the same circuit line. This one "pops" in and out depending on temp. Sounds like it needs replaced. Only reasonable explanation for all your info as given.

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