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?...0&linkCode=asn






wrote in message
...

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.

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