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  1. #1
    Join Date
    Jun 2011
    Location
    Central Indiana
    Posts
    612

    Carrier RTU odd pressures

    I encountered a RTU today that puzzled me. I can diagnose most issues with a refrigerant circuit but this one got me.

    So here is the jist

    Carrier rooftop unit in a strip mall, above a vacant store. One unit for the whole store. Property owner goes to show the store to a perspective leaser, calls in the no cool after unit runs for 3 days and still off temp

    R-22 unit, 77 DB indoors 97 DB ambient
    Suction pressure 80 psig, Head pressure 440 psig. So obviously the head pressure immediately set off alarms in my head. Super heat in the teens (I cant remember the exact number) subcool in the singles. So I cleaned the coil and got the badly hail damaged fins in decent order and fire it back up. No change. So I start thinking over charge as the target superheat according to the charging chart is higher than actual so I pulled some refrigerant off of it. Get the head to 400, but now my super heat has skyrocketed.

    So I have hit a blank here and I am hoping someone might point me in the right direction for my trip back tomorrow. Sorry for the vagueness but I have seen way to many numbers today

  2. #2
    Join Date
    Jan 2010
    Posts
    6,056
    Whats the model # for that Carrier RTU ?

  3. #3
    Join Date
    Feb 2009
    Location
    Wisconsin
    Posts
    310
    Take the top off the unit, split the condenser and wash it again.
    You have to fight through some bad days to earn the best days of your life.

  4. #4
    Join Date
    Jun 2011
    Location
    Central Indiana
    Posts
    612
    I don't have the model but I do have serial handy, 2196G20400. And it doesn't have dual layer coils, already checked that.

  5. #5
    Join Date
    Apr 2005
    Location
    where the beer flows like wine
    Posts
    2,871
    non-condensables

  6. #6
    Join Date
    Jun 2011
    Posts
    14
    washed the condenser with cleaner and a hose? if you havent you have to start there. even if it looks clean, its amazing what cleaning a coil that even looks clean can do for your head pressure. and even more so if its that hot outside. condenser fans look like they been replaced? maybe someone threw in the wrong motor or blades? both good places to start... rtus are nice because you can always remove the gas, pull a vac, and weigh in the charge so you know its correct.

    thats a good place to start if you havent

  7. #7
    Join Date
    Sep 2007
    Location
    edmonds wa
    Posts
    208
    at 440 lbs this should have been tripping hp switch and or comp bypass internal.pull the top and check the od coil for two coils tied together at end with screws and or clips. indoor coil metering holes could be plugged and extra 22 put in, pull and weigh the charge.

  8. #8
    Join Date
    Oct 2004
    Location
    Central WA
    Posts
    1,468
    ... You did put the door back on after you gauged up, right?

  9. #9
    Join Date
    Jul 2002
    Location
    Yuma, AZ
    Posts
    2,361
    Quote Originally Posted by buford View Post
    at 440 lbs this should have been tripping hp switch and or comp bypass internal..... indoor coil metering holes could be plugged and extra 22 put in, pull and weigh the charge.
    If this is an old system with capillary tubes, restricted cap tubes is highly likely. Subcooling should be 10 to 20 maximum regardless of metering device type.
    "I have never let my schooling interfere with my education."
    Mark Twain
    More at: http://www.quotationspage.com/subjects/education/

  10. #10
    Join Date
    May 2011
    Location
    New York
    Posts
    307
    Quote Originally Posted by 5thRoot View Post
    I encountered a RTU today that puzzled me. I can diagnose most issues with a refrigerant circuit but this one got me.

    So here is the jist

    Carrier rooftop unit in a strip mall, above a vacant store. One unit for the whole store. Property owner goes to show the store to a perspective leaser, calls in the no cool after unit runs for 3 days and still off temp

    R-22 unit, 77 DB indoors 97 DB ambient
    Suction pressure 80 psig, Head pressure 440 psig. So obviously the head pressure immediately set off alarms in my head. Super heat in the teens (I cant remember the exact number) subcool in the singles. So I cleaned the coil and got the badly hail damaged fins in decent order and fire it back up. No change. So I start thinking over charge as the target superheat according to the charging chart is higher than actual so I pulled some refrigerant off of it. Get the head to 400, but now my super heat has skyrocketed.

    So I have hit a blank here and I am hoping someone might point me in the right direction for my trip back tomorrow. Sorry for the vagueness but I have seen way to many numbers today
    No model number, but what size is the unit ??
    How many tons ?
    How many and what kind of compressors ?

    Many possibilities here, OFM running backwards (is it 3 phase ??)
    How many OFM's on the whole unit ??
    Did you pull amp readings on the OFM's ??
    Extended dehydration is the key

  11. #11
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    17,802
    Shouldnt this be moved?
    "Politicians are the lowest form of life on Earth. Liberal Democrats are the lowest form of politician"

    - General George S. Patton

  12. #12
    Join Date
    May 2011
    Location
    Oklahoma
    Posts
    105
    Are you sure the fins are in decent enough shape to get proper DP reading?
    Could someone else have put 410A in the system?
    What is the static psig when the unit is off?

  13. #13
    Join Date
    Aug 2006
    Posts
    2,583
    We had a Gemini split system, only one of the two condensor coils was brazed into the circuit. This was a factory mistake. BTW I looked at it for two days and didn't see it till I got some help and pulled the top. It had been that way for 2 1/2 years, they took refrigerant out in the summer, put it back in the fall. Someone had run a trend on it, had a 6 F split across the Evap.

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