Checking High Limit Switch

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  #1  
Old 12-02-15, 10:06 AM
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Checking High Limit Switch

I would like to check the limit switch (L150-20F) to see if it is opening too soon, mostly curiosity.

Can I put this in water and check continuity using an ohm meter and digital meat thermometer or would dunking it in water be a bad idea? New one will be here in a few days.

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Old 12-02-15, 10:15 AM
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Dunking it in water is a bad idea.
 
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Old 12-02-15, 10:27 AM
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I was just about to do it then thought being electrical it could be bad, I'll wait for the new one then do it.

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Old 12-02-15, 11:25 AM
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You might try a heat gun if you are that curious.
 
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Old 12-02-15, 01:56 PM
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That's okay, I'll wait till the new one arrives.
 
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Old 12-02-15, 02:14 PM
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You can do a fairly good job of checking on the limit switch by drilling a small hole in the warm air plenum and inserting a thermometer to measure the temperature when the fan circulating air through the furnace is running.

The warm air temperature allowed is governed by the rated heat rise for the furnace, which is given on the rating plate in the burner compartment of the furnace.

The heat rise is the outgoing air temperature from the furnace minus the incoming temperature.

Measure the heat rise in your furnace by drilling a second hole in the return air duct near where it enters the furnace.

Lets suppose that incoming air temperature is 60 degrees F and the outgoing air temperature is 150 degrees. The heat rise would then be 150-60= 90 degrees.

A typical furnace might have a rated heat rise of 60-80 degrees. The 90 degree heat rise would be excessive and require that the furnace and ductwork be inspected to remove defects and obstructions that are causing the excessive heat rise. An overfired burner, plugged filter, too low fan speed and lots of other causes could cause that kind of issue.

Ideally you'd be aiming for a heat rise somewhere near the middle of the rated heat rise.
 
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