Is this my gas furnace's limit switch?


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Old 11-25-15, 07:55 AM
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Is this my gas furnace's limit switch?

Hey all,

Just trying to learn a little more about my furnace on our house (an old Nordyne natural gas/forced air furnace, model CRFYUD-100A16E). I've not been able to find any references to this model online (I did come across one post with someone referencing a RFYUD-100A16E, but that's it), so not looking good in terms of getting a manual.

Anyway, I'm wondering if anyone could tell me if the device in the picture (below) is the furnace's limit switch?

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If this is the limit switch, is it normal for it to be attached to the exhaust stack?

Thanks,
Kristin.
 
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Old 11-25-15, 01:43 PM
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No. That's not what would be called a furnace high temperature limit switch.


The high temperature limit switch is a temperature probe that extends into the spacfe between the heat exchangers. If something interferes with the flow of room air through the furnace, the furnace will overheat since the furnace can't get rid of all the heat produced by the burners.

If that happens, the temperature probe detects that overheating condition and open an electrical switch, which shuts off the burners.

The high temperature limit switch is commonly located in the middle of the furnace a few inches above the burners.

What you have appears to be checking on the temperature in the vent pipe. If there is a hole in the vent pipe in front of the snap disc limit switch, it's probably there to detect if the chimney is plugged with debris. If it is, the combustion gasses would start coming out the hole , exposing the limit switch to the hot combustion gasses which would shut off the burners.
 
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Old 11-25-15, 01:54 PM
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Thanks! That makes a lot more sense (considering it's hooked up to the vent pipe and there is a hole). Do you think it's an issue that the face of the snap disc limit switch is tarnished? Hard to tell in the photo, but the face of it (the part directly facing the hole i the vent pipe) looks "rusty". Something I can clean, manually, like the flame sensor?

Anyway, thanks again, and back to the furnace room to find the high temperature limit switch. Man, if I had the manual for this furnace, it'd make things a heck of a lot easier. <G>

k.
 
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Old 11-28-15, 01:59 PM
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It's unlikely that slight corrosion on the limit switch is a problem.

This safety limit switch may be identified as the draft safety switch or DSS on the circuit diagram. Usually it's a manual reset switch that must be reset by pushing a button if it opens.

You can test that this switch works by heating it up with a gas match or other way of applying some heat. You will probably hear and see the manual reset button pop out when it gets hot enough.

But frankly, it's very unlikely that the switch is a problem, unless the switch has been opening when the furnace is operating.
 
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Old 11-28-15, 03:44 PM
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Are you sure CRFYUD-100A16E is the correct model number ? It doesn't look correct.
 
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Old 12-01-15, 09:34 AM
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Well, here's a shot of the label on the inside of the furnace...maybe I'm reading it wrong, but beside MODEL NO it says "CRFYUD-100A16E"?

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