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Old 12-17-2009, 08:58 PM   #1
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why do I have two switches

I moved into this old house last year. The furnace is about 55 years old but
the fan motor was replaced two years ago. Last winter the furnace seemed
to be fine but last week I noticed the fan motor was staying on all the time
or shutting off only after a long time. I noticed when I tapped on the
fan limit switch, it would shut off so I replaced it with a camstat l59-3b.
This has a slide on the extension for adjusting between 150 and 250 degrees.

If I don't set the switch to the lowest possible temp, the motor cycles on
and runs for only a few seconds and then shuts off; then repeats this
continuously. It never comes on and stays on for more than a few seconds.

At the lowest temp, the fan seems to come on and run normally but in my
opinion it shuts off too quickly after the burner shuts off. Also, it cycles
on and off a couple of times briefly until it stops running. I'm not
happy, but at least it doesn't run continuously.

If I set the switch to 150, wouldn't the temp have to reduce to 150 before
shutting down? Could this be the wrong switch for this furnace? I found
an old part somebody left in the furnace compartment that has a lower temp range
(camstat model f14 -- up to 110 degrees).

There is another thermostat switch (AT013) next to the fan control switch.
There are two wiring diagrams on the furnace; one for the fan control
and one for the thermostat. It looks like this switch is connected
to the thermostat and the gas valve. The temp adjustments go from
175-215 degrees. I also replaced this switch, but I don't understand
what this switch does or what the setting should be. Can anyone
explain what this does, and how to determine the temp setting?


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Old 12-17-2009, 09:08 PM   #2
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Thats a limit switch. its a safety device to prevent over heating of the heat exchanger, and burning the house down.

You may not have the correct probe length for the fan switch.
So it probably isn't able to sense the temp like it should.


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Old 12-18-2009, 07:41 AM   #3
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When the thermostat calls for heat the burners light, after it heats up the fan switch at a set temperature starts the fan, the fan runs until the burners are shut off by the thermostat, when the set temperature shutoff is reached the fan shuts off.

Now to back up to the part where the burners light and heat up the fan switch starts the fan....(if it don't start the fan it keeps getting hotter then the limit switch shuts down the burners.

Just a little info for you.
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