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Fan Limit Switch Tripping Open

Frank2644 Member Posts: 1
I have a lennox gas furnace where the flame is cutting out due to the fan limit switch opening. I tried a new fan limit switch, but it did not help. The blower motor seems to be running normally and the air filter is clean and all heating ducts in the house are open. Although the ducts seem to be passing air I suppose there still could be a partial clog. I'm guessing that the air flow may be limited due to a clogged A/C evaporator, but I have no access to clean it or even see it. Should I cut a hole in the ductwork for cleaning and inspection and then patch it somehow?

Any other thoughts/suggestions?

Thanks in advance


  • Tim McElwain
    Tim McElwain Member Posts: 4,597
    That is a possibility

    I would have a professional take a look at it, in the long run it may be cheaper. They can test for airflow with a meter and tell if it is plugged.
  • Lance
    Lance Member Posts: 257

    Good try, as I just fixed a lennox furnace with a bad hi limit.

    The cause of limit tripping is either a bad limit or it is doing its job. If you are referring to a hi limit it may be doing its job and protecting against overtemperature. Look at cause of lack of air flow or too much fuel. Check the combustion,  make sure it is not overfired. Check airflow, blockages due to clogged coils are a common cause, especially with poor filter situations. Is the blower wheel dirty? If so the coil can be matted. The most difficult job to clean is the indoor coil since it may have to be removed and duct cut to access. So it generally gets no attention. It will cost more money to ignore it so it must be cleaned. Possible blower speed problem?

    Temperature difference between return air and supply air should not exceed 60-70F.  70F in 140F out. 

    If you measure the temps you can make sure the new part is not bad. Sometimes even new parts are no good and are the last thing we want to verify.
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