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Old 10-15-2009, 08:24 PM   #1
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Main limit switch - correct settings?

This is our first winter in the house so I'm not very familiar with our 20+ year old natural gas furnace. It says "Kenmore 90" on the front panel but I didn't find an actual model number anywhere.
I had a guy over today to do an annual cleaning/servicing before the winter season. One of the things he told my wife was that the "main limit" was set at 150 degrees but they are normally set 2 to 3 times higher than that. He then told her that he set it up where it should be.

My question is, did the guy know what he was talking about and will the change he made hurt this ancient beast? My own online searching found main limits set anywhere from 140 to 300 degrees so I'm not sure what to think. I'm wondering if the switch was set to 150 on purpose for some reason. What reason might the previous owners have had for setting it lower?

A pic of the switch is attached. It has 3 "prongs?" set at 100, 150, and 250 degrees. Any comments would be welcome.


Here are more details if they're relevant:
32 year old house, 20-ish year old furnace-a/c
furnace is in an attached garage
house is 2200-ish sq ft.
furnace has worked fine since he left earlier today, though it's only in 40s, not really cold yet.
Attached Thumbnails
Main limit switch - correct settings?-000_0058.jpg  


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Old 10-15-2009, 08:31 PM   #2
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With the age of the furnace I would set the 250 down to 220-225.

I have seen furnaces set as low as 190 and as high as 240. This setting will turn off the gas when the temp of the exchange excedes this temp

The 100 turns off the fan and the 150 turns on the fan. I would turn the 150 down to 140

I'm sure others will chim in soon this just my $.02


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Old 10-16-2009, 12:02 AM   #3
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That fan/limit control has a timer to turn the fan on, hence the two low voltage wires directly below the dial. The high limit should not have been changed. The design of some of these furnaces didn't allow the fan switch to sense the heat properly, this is why they use the timed on fan switch. The heat exchanger could possibly be damaged if the blower fails because the switch may not turn the burner off in time. It needs to be set back to where it was.
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Old 10-18-2009, 06:20 PM   #4
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Thanks guys.

I moved the high setting back down to just above the middle one.
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main limit switch kenmore

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