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Thread: bad fan limit switch?

  1. #1
    DIY Junior Member travjudd's Avatar
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    Default bad fan limit switch?

    I have a 1990 ICP gas furnace model# NUGE100BG01 with honeywell controls. The furnace comes on fine but after 5 minutes the flame goes out due to the fan limit switch rising above 200 degrees. I have a new filter and opened all of the supply vents. I checked the temperature at the first supply vent and it was a consistent 122 degrees. I pulled out the temp probe on the limit switch and immersed it into boiling water to gauge if it is accurately depicting a temperature. It was actually lower than 212 degrees. I put a 4" temperature probe in the spot where the limit switch was and it only registered a 141 degrees. I did some research and some people with the same problem stated that it was from having to big of a furnace to their house. Can these limit switches temperature probes go bad or is there something that be causing the probe to actual see that high of a temperature.

  2. #2
    DIY Senior Member guy48065's Avatar
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    They do go bad but most fail outright so it's probably rare for one to be "off" so much. It's more likely the limit switch is positioned closer to the heat exchanger than your probe was. Try drilling a hole in the limit mounting plate & position your temp probe so it's touching the face of the limit, then re-insert the limit and see what your reading is then.
    Have you had your furnace cleaned lately? Your blower could be gummed up with clinging dust, maybe you have many registers or dampers closed or blocked with furniture. The furnace needs to breathe or the plenum temperature will rise.

    That said--I have that issue (inadequate flow) and don't know what to do to correct it.
    Romeo and Atlanta, MI

  3. #3
    Jack of all trades DonL's Avatar
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    Air Flow can cause problems.

    Wrong Blower speed caused by a blower Motor problem or low voltage. A incorrectly programed or wired t-Stat could affect blower speed.

    It sounds like you need to increase the air flow.

    The limit sounds like it is doing its job.

    How long is your Gas Valve ON before the Blower kicks on ? How long does the Blower run after the call for heat is gone ?


    Good Luck.
    Theory only works perfect in a vacuum.

    Cyber Security Protection for Windows C:\ > WWW.WinForce.Net

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The furnace will have a means to adjust the fan speed, it may be switches, jumpers, or require moving a belt on the pulleys (if it isnt' direct drive). Based on the BTU, it will have a recommended speed. It may be a range...setting it outside of that range will generate problems.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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