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  1. #1
    Join Date
    Jan 2003
    Posts
    73

    Unhappy New Lennox Gas Furnace Constant Trips Limit Switch

    Can someone give some idea as what to look for?? As the installer seems to be lost.I had a new House Built last year The General Contractor had a HVAC Contractor install A lennox elite 90 plus propane gas furnace and A/C Unit It appeared to operated fine last winter But This winter it continues to trip the limit switch and the furnace shuts down before the temp is reached in the house I also have a honeywell ditigal set back thermostat installed. Here is what has been done by the installer so far. He has replaced the Limit switch (2) times he has changed the fan speed to operate on high and has also replaced the thermostat this still has not corrected the problem he stated he does no know what else to do.The Thermostat is set to operate at 62 degrees after 11.00 pm at night and back to 68 at 6.00 am I believe the thermostat has some type of memory as to how long it takes the furnace to reach its set temp in the morning The furnace now comes on at about 4.30am trying to reach the 68 degree temp by 6.00 am but as always it reaches approx 66 degrees the the furnace shuts off and I must manually reset it to get to run again.The house also seems to heat very unevenley compared to last year, Any suggestions??? Thanks Dan

  2. #2
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,763
    How often do you change the air filter.

    Dirty air filters can cause this.
    So can running the unit without a filter for a prolonged period of time.
    Are any of your returns or supplies blocked, or shut off. If so, open them.

    If, after all that is checked, or corrected, it still does it.
    Then tell your contractor that you think he should call Lenox tech support to help him.

    Good luck.
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    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    Jan 2003
    Posts
    73

    limit switch

    Sir the filter is changed every 30 days thanks

  4. #4
    Join Date
    Jan 2008
    Posts
    85
    And what type of air filter are you using? Some of the filters with higher MERV ratings are actually bad for a furnace because they reduce the airflow too much.

  5. #5
    Join Date
    Jan 2003
    Posts
    73

    lennox limit switch

    1 more question Is there a way I can check for proper air flow??? How?? Thanks Dan

  6. #6
    Join Date
    Mar 2008
    Location
    Northwest Ohio
    Posts
    17
    Lets look at the basics......A limit will trip if there is not enough air moving thru the furnace, the gas pressure is set to high, or the condensation is not being removed from the secondary heat exchanger.

    Things to check.
    What type of filter do you have and is it stopped up? If poor filters are used the a/c coil can become restricted with debris/dirt the filter didn't catch. Check the temperature rise. The difference between the return temp vs supply temp. Match it to the nameplate data. If it is higher than rated then the fun begins. Gas pressure should only be checked by a qualified tech. If you have internally insulated duct, a piece of insulation may have come loose and is restricting air flow. A piece of insulation internal of the furnace may have come loose and is restricting air flow. Make sure the condensate from the furnace itself is draining well. If these obvious things have been checked than I would suggest a service tech to come have a look rather than an installer. Installers are great but not all installers are trained to service the equipment. Service techs are trained.

  7. #7
    Join Date
    Jan 2003
    Posts
    73

    lennox limit switch

    Sir using just a basic furnace filters

  8. #8
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,763
    Not real well.

    You can check the return air temp and the supply air temp at the closest regs to the furnace.
    They should be check at the furnace. But that can be harder then you think.
    And since the installer has changed out the limit twice altready.
    You need someone to come there that knows what they are doing.
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  9. #9
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,594
    If your house is built like they do around here, probably way to small of a duct system

    A good tech would check static pressure and see just how bad it is. In the past, old stuff was oversized. Not as common today but could be.

    Cycling in limit is inefficient and those limits will fail open leaving you cold pretty soon.

  10. #10
    Join Date
    Jan 2003
    Posts
    73

    lennox limit switch

    Thanks for the info How do I check the Temp Rise?? Is this measured at the closest supply?? or farthest Also do I check the return at the furnace?? Also the furnace was installed using all metal duct inc supply as well as returns As I stated prior My house seems sometimes a lot hotter than the stat shows Would setting the fan speed to high have any effect on proper heating?? Thanks

  11. #11
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,763
    Can cause uneven heat.

    You can check at the return and supply regs closest to the furnace. That will be acturate enough for your needs.

    Your not going to fix this yourself.
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  12. #12
    Join Date
    Jan 2003
    Posts
    73

    lennox limit switch

    Thanks for everyones response Sounds like I may have an air flow problem I would have thought the installer would have thought that as well and would have performed some type of air flow test instead of just replacing parts I will contact lennox Thanks Again Dan

  13. #13
    Join Date
    Mar 2008
    Location
    20141
    Posts
    1,153
    The bottom line sir, is that something is not right. If you change your filter regularly, (and you should be using a pleated filter I might add because they offer more surface area to trap dirt, etc.) then the problem more than likely is with some or a component of your furnace, ductwork, or thermostat. Considering that a limit switch is there to protect first of all you, and second of all the furnace you shouldn't try to do anything but pick up a phone and call your contractor.

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