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  1. #1
    Join Date
    Dec 2000
    Posts
    61

    Carrier rooftop limit switch

    Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.

  2. #2
    Join Date
    Dec 2005
    Posts
    1,921
    Quote Originally Posted by jollygreen60 View Post
    Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.
    Air flow problem.

  3. #3
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,340
    Quote Originally Posted by jollygreen60 View Post
    Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.
    Which limit are you referring to? You have one that sits on a plate above the burner area and one that sits on the base plate between the blower and the heat exchanger.

    A good rule for all carrier rtu's, is to check the heat exchangers every visit. They seem to have gotten better, but a 48tm008 pull that cover off the heat exchanger and check it out. Also I know the configuration is crap, but pull the burner rack and clean it up.

  4. #4
    Join Date
    Dec 2009
    Location
    Southern US
    Posts
    504
    Any chance its supplying a zone system with a bypass damper?
    I'm good at making things cold...You can ask my first two wives!

  5. #5
    Join Date
    Dec 2006
    Location
    Middle Tennessee
    Posts
    11,347

    *

    Quote Originally Posted by BKS60 View Post
    Any chance its supplying a zone system with a bypass damper?
    if it is, he don't have the DATS hooked up



    .

  6. #6
    Join Date
    Jan 2010
    Posts
    6,618
    Rollout switch cutout temp 195*f
    Temp rise range
    TMD 20-50
    TME 35-65
    TMF 45-75
    Still looking for high limit spec.

  7. #7
    Join Date
    Dec 2009
    Location
    Southern US
    Posts
    504
    Quote Originally Posted by Airmechanical View Post
    if it is, he don't have the DATS hooked up



    .
    Or an old Parker with a bad bypass stuck open or the whole thing crapped out.
    I'm good at making things cold...You can ask my first two wives!

  8. #8
    Join Date
    Dec 2007
    Posts
    26
    close to same problem i had. rtu had bad filter in ceiling. not enough return to have enough air to push the heat off the heat exchangers (limit open). I always ask what temp to they leave it overnight. some people expect the it to be 62'f and hit 75'f in 10min. just an idea.

  9. #9
    Join Date
    Dec 2007
    Posts
    26
    i'm probably thinking off the wrong model yall are talking about. lmfao. my bad.

  10. #10
    Join Date
    Dec 2008
    Location
    Independence, KY
    Posts
    18
    Quote Originally Posted by just_opinion View Post
    Air flow problem.
    Or motor problem. Gotta love those motors that take 2 hrs to overheat and fail out.

  11. #11
    Join Date
    Apr 2009
    Location
    surrey, b.c
    Posts
    3
    agree with air flow problem. is it possible evap coil icing up (T-stat settings,low gas,etc)then calls for heat and trips limit.

  12. #12
    Join Date
    Feb 2010
    Location
    OMAHA, NE
    Posts
    130
    May be worth taking some static presures on the air flow.

  13. #13
    Join Date
    Jun 2005
    Location
    DuPage County, IL
    Posts
    151
    My thoughts...

    What's your actual temp rise?
    What's your actual gas pressure at the gas valve?
    Are your returns clean and not restricting airflow?
    Are your supplys too close to your retuns, short cycling heated air?
    Is this an 8 or 10ft ceiling, stratisfied heat returning to unit?
    Is the heat ex cracked or corroded at the seams?

    With a 172°F measured temp at the limit, assuming your return temp is approx 75 to 80°F, you're right around a 100° rise. That a bit high. Check name plate for rated temp rise
    Figman

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