I have a Goodman GMCE075-3 forced air furnce. This morning I came home from vacation and the house was 59 degrees. The circuit board indicated an open limit switch, however, upon testing the switch it was not open. There are three additional aux limit switches wired in series around the burner enclosure. I manually reset these. Now the furnace runs but periodically the burners will shut off. They will come on perhaps five to ten minutes later. During this time the fan continue to run at high speed as if the burners were operating.

Question: Can the thermistor comprising the limit switch be out of kilter such that it would not test open but periodically fail depending on temperature?

What exactly are the auxiliary limit switches testing?

Thoughts?

TIA -
Eric (the homeowner)

A limit switch can go out of calibration and cause this kind of problem.

The main limit switch controls the heat over the burners by the main heat exchanger, as a safety switch... an over heat condition such as a dirty air filter, bad blower motor, over fired burners, fan speed too low, or feed / return vents blocked or closed will trip this switch to the open position and shut down the unit. ( blower will run to cool off the unit) The aux. limits are located in other areas of the furnace to sense the same high heat conditions, you can test them by checking across both terminals with a volt meter on the connunity setting with the two wires removed ( and power off to the unit) and another test that is performed by HVAC techs is to place a temperature thermistor at that location to check the actual temp vs what is the temp rating of the safety limit... I have seen bad limit switches, which open on a much lower temp that the proper temp to operate at. NOTE: NEVER bypass these safety devices to run your funnace!!

Quote:

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Originally Posted by tsa7man

NOTE: NEVER bypass these safety devices to run your funnace!!!!!

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I must admit that during the initial panic phase that exact thing did cross my mind. I have had a lot of construction going on in the house. I will check to ensure that all of the vents are open and the filter is clean (I have been replacing it weekly due to sawdust in the air). I do in fact have a thermistor for my fluke multimeter. What plenum temp would I expect to see? Also, for a given limit switch (mine main is the Texas Instruments B13709-10) how might I find the open/close specs?

Thanks for your answer, by the way!

The specs should be on the limit switch itself. So I would replace it with one with the same specs, if that proves to be the problem.
Maybe you should check the limit switch for continuity on one of the occasions when the furnace has acted up to see if it is open.