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  1. #1
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    Sep 2008
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    Carrier RTU 48PGN Heat Exchanger Delta-T?

    I have a carrier 48pgn RTU that keeps tripping off in the high limit fault when the 2nd stage heat is enabled. I inspected the heat exchanger, checked all the basics (belt, filters, gas pressure, and duct work), I also swapped limit switches with another RTU. I'm taking some temperature readings across the heat exchanger and was wondering what an expectable delta would be. I'm currently reading 67degf return and 167degf supply before it trips.


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  2. #2
    Join Date
    Apr 2014
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    Wisconsin
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    I would be looking into airflow if everything else is to Carriers recommendation..

    Either not enough return/ supply/ plugged evap coil etc.

    any other info regarding past history or repairs?

  3. #3
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    Sep 2008
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    New England
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    Yea I checked everything that I could on the air flow side such as blocked grills, evap, smoke dampers, exc... I'm having one of our air balancers come by tomorrow to see what this things putting out for CFM.


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  4. #4
    Join Date
    Feb 2003
    Location
    Madison, WI
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    Allowable temperature rise should be on the unit data tag. 35-75 degrees F. is typical but could be different for your unit. If out of range it is air flow or over firing or hole in heat exchanger.

  5. #5
    Join Date
    Jan 2013
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    Batavia, New York
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    100 degree delta t way to high. Having checked air flow, dampers ect, what is your duct static? Manifold pressure? Orifaces correct for said unit?

  6. #6
    Join Date
    Apr 2014
    Location
    Wisconsin
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    Also another thing I'd also check that screwed me once, double check blower motor, and both shives.

    Had one RTU once that someone put too small of a motor in because it's what "grainger" had

    Also had someone put the wrong blower sheave and belt on for an emergency repair that never got changed out.

    Best of luck!

  7. #7
    Join Date
    Jan 2010
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    Never seen a RTU's name plate show a 100* rise.
    Your model number sounds more like ICP (International Comfort Products) verses Carrier.

  8. #8
    Join Date
    Nov 2006
    Location
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    The 2nd stage manifold pressure is on the label. Check it.

    As mentioned above, compare the motor and sheave to another unit, perhaps the (identical?) unit you used to swap the limit.

    100 is indeed too high.
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  9. #9
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    Sep 2008
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    Quote Originally Posted by timebuilder View Post
    The 2nd stage manifold pressure is on the label. Check it.

    As mentioned above, compare the motor and sheave to another unit, perhaps the (identical?) unit you used to swap the limit.

    100 is indeed too high.
    As I stated in my previous post I confirmed the gas pressure settings. I'm leaning towards air flow as well, which is why I have a balancer coming in to check the CFM & Static.


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  10. #10
    Join Date
    Nov 2006
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    Quote Originally Posted by alliedtech12887 View Post
    As I stated in my previous post I confirmed the gas pressure settings. I'm leaning towards air flow as well, which is why I have a balancer coming in to check the CFM & Static.


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    I saw your post.

    Friend, I have an expectation of seeing a number in a post that was the result of that check. Not picking on you, but I see a lot of posts that say something has been checked. I'm sure some would look at the flames and say, "that pressure looks good." Not you mind you, but I tend to ask for more specifics than others.

    Is the variable sheave in the other unit set to the same diameter?
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