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justplumducky 09-16-2010 09:50 AM

Blower Overheat Switch?
I'm working on what is probably a Miller or Nordyne furnace in a single wide mobile home, approx. 14'x 70'. I said Miller or Norydyne because I was told the part no. off the Fan snap-disc switch was a Miller part no., and it's my understanding that Miller was bought by Nordyne or Intertherm? It's a down-flow, propane-fired furnace, standing pilot.

From what I have just been told, the two side-by-side snap disc switches turn the fan/blower on and off (F115-30F) and provide a safety cut-off, in series with the gas valve 24vac circuit, for excessive heat (L140-40F).

There is another two-wire switch. I'm assuming it's a switch - haven't seen the part that sticks into the heat exchanger compartment - maybe it's a temp sensor with no mechanical action? This one is mounted into the top of the heat exchanger compartment, to the left of the base of blower assembly.

I was told this switch is to protect the blower motor from overheating from filters not being changed enough. Do you agree with this? Seems odd that the sensing part of this switch would be sticking into the heat exchanger compartment, if it's supposed to protect the motor from an overheat condition. If it's not a blower overheat switch, can you tell me it's function?

Thx much for any help :thumbsup:

kenmac 09-16-2010 12:21 PM

If it's sticking in the heat exchanger. It would kill power to the gv incase the blower fails or cracked heat exchanger

yuri 09-16-2010 04:39 PM

On downflow/counterflow furnaces there is always an extra limit switch with a manual reset button usually mounted on the fan housing or nearby. This could be it. Provides protection if the fan fails as heat rises into it. Code requirement.

justplumducky 09-17-2010 10:25 AM

Thx so much for you replies, Gentllemen.

Would also be very grateful if you could confirm the following explanations I was given for the operation of the other two switches on this particular furnace.

These are the two switches, followed by the explanations, and they are mounted side by side in what I call the control box of this old, propane gas fired, standing pilot furnace (the box housing the furnace master switch, transformer, etc):
Switch in left position: L140-40F
Switch in right position: F115-30F
"The fan switch activates at 115 degrees and opens no later than 30 degrees below that. The Limit opens at 180 and closes at 140."

kenmac 09-17-2010 02:57 PM

Which limit was open ? Is it for the combustion chamber Or outside the chamber ?

hvaclover 09-17-2010 05:10 PM

mobil home furnace are a science onto themselves....especially the stand pilot type.

justplumducky 09-17-2010 06:01 PM

Neither one was open, but at this point, I would just like to know if what I was told is correct, about the operation of the switches.

The switches are inside the control box - it's really not a box however; just a 4-sided enclosure up against the vertical wall of the heat-exchanger compartment, with a removable front access panel. Inside this "control box" , the switches are found mounted into the wall of the heat-exchanger compartment.
Switch in left position: L140-40F
Switch in right position: F115-30F

I know the switch position doesn't matter - they are both sensing the heat-exchanger compartment. Just helps me remember that when I checked voltage on the right switch (the FAN switch), it was 120v feeding that switch.
The following (in quotes below) is what I was told about the operation of these switches. Just wanting to confirm if this correct. In other words, the FAN switch activates/closes at 115 degrees (as stated below), instead of the opposite: closing at 85 & opening at 115 degrees?
"The fan switch activates at 115 degrees and opens no later than 30 degrees below that. The Limit opens at 180 and closes at 140."

Is this the correct operation sequence of those switches?

Thx much.

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