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Furnace Fan Won't Stop Running! AllAboutCollies 01-12-2008
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Posted by on January 12, 2008, 11:16 am
I have a very old furnace, 40-50 years old, at least. The fan won't
stop running even when the thermostat is turned off, or when I turn
off the electric switch to the furnace. The only way I could stop it
running is to turn off the breaker to it. It seems very simple: pilot
light, burners, blower, thermostat, and that's about it. No A/C or
humidifier. I have to fix it myself. Does anyone have any ideas?
Thanks so much for your help. Peggy

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Posted by RBM on January 12, 2008, 11:40 am
The fan is probably connected to a "fan limit switch". This would be a
junction box with a tubular shaft sticking into the plenum and would have
one or two cables attached to it. It turns the fan on when the air in the
plenum is hot enough, then turns it off once the air cools down, and also
shuts off the burner if the air gets to hot. Here is a link to a picture of
one:
http://www.amazon.com/dp/B0009784NY?smid=A2KWCD4IAZU01T&tag=nextag-tools-tier4-20&linkCode=asn






>I have a very old furnace, 40-50 years old, at least. The fan won't
> stop running even when the thermostat is turned off, or when I turn
> off the electric switch to the furnace. The only way I could stop it
> running is to turn off the breaker to it. It seems very simple: pilot
> light, burners, blower, thermostat, and that's about it. No A/C or
> humidifier. I have to fix it myself. Does anyone have any ideas?
> Thanks so much for your help. Peggy



Posted by CJT on January 12, 2008, 12:37 pm
RBM wrote:
> The fan is probably connected to a "fan limit switch". This would be a
> junction box with a tubular shaft sticking into the plenum and would have
> one or two cables attached to it. It turns the fan on when the air in the
> plenum is hot enough, then turns it off once the air cools down, and also
> shuts off the burner if the air gets to hot. Here is a link to a picture of
> one:
>
http://www.amazon.com/dp/B0009784NY?smid=A2KWCD4IAZU01T&tag=nextag-tools-tier4-20&linkCode=asn
>
>
>
>
>
>
>
>>I have a very old furnace, 40-50 years old, at least. The fan won't
>>stop running even when the thermostat is turned off, or when I turn
>>off the electric switch to the furnace. The only way I could stop it
>>running is to turn off the breaker to it. It seems very simple: pilot
>>light, burners, blower, thermostat, and that's about it. No A/C or
>>humidifier. I have to fix it myself. Does anyone have any ideas?
>>Thanks so much for your help. Peggy
>
>
>
I agree with that, but it seems to me the furnace fan should be
controlled by the service switch. Maybe they did it differently
50 years ago, but having the fan "live" while the service switch
is off seems to defeat the purpose of the switch.

Also, it might be useful for the OP to know that the fan limit switch
generally controls a relay rather than control the fan directly, and
that limit switches are adjustable -- if the shutoff temp is too low,
the fan might never shut off.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

Posted by RBM on January 12, 2008, 12:48 pm
My guess is that Peggy is not going to dig into any wiring on the unit. If
she can find the control, she may find an adjustment on it that was touched
by someone, causing it to run continuously, and correct it





> RBM wrote:
>> The fan is probably connected to a "fan limit switch". This would be a
>> junction box with a tubular shaft sticking into the plenum and would have
>> one or two cables attached to it. It turns the fan on when the air in the
>> plenum is hot enough, then turns it off once the air cools down, and also
>> shuts off the burner if the air gets to hot. Here is a link to a picture
>> of one:
>>
http://www.amazon.com/dp/B0009784NY?smid=A2KWCD4IAZU01T&tag=nextag-tools-tier4-20&linkCode=asn
>>
>>
>>
>>
>>
>>
>>
>>>I have a very old furnace, 40-50 years old, at least. The fan won't
>>>stop running even when the thermostat is turned off, or when I turn
>>>off the electric switch to the furnace. The only way I could stop it
>>>running is to turn off the breaker to it. It seems very simple: pilot
>>>light, burners, blower, thermostat, and that's about it. No A/C or
>>>humidifier. I have to fix it myself. Does anyone have any ideas?
>>>Thanks so much for your help. Peggy
>>
>>
>>
> I agree with that, but it seems to me the furnace fan should be
> controlled by the service switch. Maybe they did it differently
> 50 years ago, but having the fan "live" while the service switch
> is off seems to defeat the purpose of the switch.
>
> Also, it might be useful for the OP to know that the fan limit switch
> generally controls a relay rather than control the fan directly, and
> that limit switches are adjustable -- if the shutoff temp is too low,
> the fan might never shut off.
>
> --
> The e-mail address in our reply-to line is reversed in an attempt to
> minimize spam. Our true address is of the form che...@prodigy.net.



Posted by on January 12, 2008, 12:55 pm

>RBM wrote:
>> The fan is probably connected to a "fan limit switch". This would be a
>> junction box with a tubular shaft sticking into the plenum and would have
>> one or two cables attached to it. It turns the fan on when the air in the
>> plenum is hot enough, then turns it off once the air cools down, and also
>> shuts off the burner if the air gets to hot. Here is a link to a picture of
>> one:
>>
http://www.amazon.com/dp/B0009784NY?smid=A2KWCD4IAZU01T&tag=nextag-tools-tier4-20&linkCode=asn
>>
>>
>>
>>
>>
>>
>>
>>>I have a very old furnace, 40-50 years old, at least. The fan won't
>>>stop running even when the thermostat is turned off, or when I turn
>>>off the electric switch to the furnace. The only way I could stop it
>>>running is to turn off the breaker to it. It seems very simple: pilot
>>>light, burners, blower, thermostat, and that's about it. No A/C or
>>>humidifier. I have to fix it myself. Does anyone have any ideas?
>>>Thanks so much for your help. Peggy
>>
>>
>>
>I agree with that, but it seems to me the furnace fan should be
>controlled by the service switch. Maybe they did it differently
>50 years ago, but having the fan "live" while the service switch
>is off seems to defeat the purpose of the switch.
>

I fully agree. The furnace switch is supposed to shut off EVERYTHING
on the furnace. I'd start by getting that switch checked and/or
rewired.

On your thermostat, there could be a small switch (usually on the
bottom), that says FAN. Did someone bump it to the ON position?
That switch is there so you can run the fan all the time, or turn it
on in the summer to circulate the air in the house.

As others said, the Fan-Limit switch would be your cause if none of
the above apply. They're pretty each to replace and not costly. Of
course you got to get one that will fit. Those devices also may have
a switch on them to turn the fan on manually. If someone changed a
furnce filter or something, did they bump that switch?


>Also, it might be useful for the OP to know that the fan limit switch
>generally controls a relay rather than control the fan directly, and
>that limit switches are adjustable -- if the shutoff temp is too low,
>the fan might never shut off.


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