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  1. #1
    Join Date
    Aug 2011
    Location
    Eastern South Dakota
    Posts
    28

    Heat pump with low suction pressure

    I am working on a ruud heat pump MN# upne-048jaz with an electric air handler inside mn#uhla-hm48 with R-22 coil mn#rcsa-hm4824au. The home owner called with a complaint of freezing up on hot days with 85 degrees and up. Unit is 4 years old and this is the first hot summer we have had since it's install (not an install of mine). With 80 degree ambient outdoor it has 180# head, 50# suction with 50 degree line temp. We sucked it down and purged with nitrogen and vacuum and weighed in a new charge. Checked the pressure drop across filter drier and all seems to be well there. Replaced txv and no change with that either. Have cycled unit into heat mode in hopes that if there was oil in distributor tubes when in heat mode i do have a slight pressure difference from each port. Any ideas?

  2. #2
    Join Date
    Jan 2008
    Location
    Clermont, Florida
    Posts
    140
    Check airflow and ductwork I had one beginning of summer rtn duct would suck shut when running
    Its not hard to stop a Trane.. Just give it a Chinese run cap.

  3. #3
    Join Date
    Apr 2010
    Location
    Minnesota
    Posts
    517
    Check static pressures in ductwork, for starters.

  4. #4
    Join Date
    Aug 2011
    Location
    Eastern South Dakota
    Posts
    28
    I should have mentioned I have done that, that was the first thing we did. I am starting to lean towards a check in the outside txv that is not opening, also on this ruud there are small strainers braized in on either sid of the txv outside. possibly a issue there but i dont have a noticiable temperature change from one side to the other of the strainers, but maybe i am wrong in thinking i should. trying to decied what the next move should be or if im missing something.

  5. #5
    Join Date
    Oct 2008
    Location
    Nashville ,TN
    Posts
    284
    Txv? Sure its not undercharged? , check subcool and delta T that will let you know if its refrigerant problem or airflow. Need to know subcool?

  6. #6
    Join Date
    Aug 2011
    Location
    North West FL
    Posts
    97
    I would think an under charged system would freeze when outdoor temps are lower. Like at night. Although a/c's run 24/7 here all summer. I had one that would freeze up sometimes. When I got there and thawed it out it would run fine for weeks. I did not even look at the contactor. I managed to catch it stuck on. Such a simple fix and it took me a few return trips to check. I was so sure it was a charge or air flow issue, I skipped the basics. That was a fun one to explain to my boss. lol

  7. #7
    Join Date
    Sep 2008
    Location
    Cedar Rapids, IA
    Posts
    486
    What's your superheat at the expansion valve bulb? If its good or low then you have an airflow problem, or a plugged evaporator coil. Might also be a improperly sized coil, but your model #'s seem to indicate its matched.

    If you are getting a high superheat then the txv isn't feeding enough refrigerant causing the low suction. (and consequently the low head pressure)

  8. #8
    Join Date
    Dec 2009
    Location
    Fort Worth, Texas
    Posts
    151
    need info unless you want a bunch of guessing and part changing.
    whats your superheat and whats your subcooling.
    if you want refrigerant diagnosis this is the info you will always need to give.
    if you want a guess then check return air duct and grill sizes, coil dirty, low charge or wrong condenser fan motor. ie 1175 when an 820 rpm motor is oem.

  9. #9
    Join Date
    Aug 2011
    Location
    Goose Creek ,SC
    Posts
    252
    I used to work for a Rheem dealer , I have seen the out door txv cause the same symptoms you are talking about .
    "I just think things should work properly"
    Freon is a brand name-word


  10. #10
    Join Date
    Sep 2008
    Location
    Largo
    Posts
    78
    I agree to check static pressure. If it's 80 degrees out with only 50/180 it seems to be undercharged. Once temps drop at night, that would cause it to freeze up. Everyone else gave good input too. Check sub and superheat...and split before replacing more parts. Good luck!

  11. #11
    Join Date
    Jan 2007
    Location
    Stongsville Oh
    Posts
    895
    when you said you weighed in the charge, where are you getting the data?
    ckartson
    I didn't write the book I just read it!

  12. #12
    Join Date
    Jun 2008
    Location
    NW Florida
    Posts
    707
    Quote Originally Posted by MetroAir View Post
    I agree to check static pressure. If it's 80 degrees out with only 50/180 it seems to be undercharged. Once temps drop at night, that would cause it to freeze up. Everyone else gave good input too. Check sub and superheat...and split before replacing more parts. Good luck!
    What he said.

  13. #13
    Join Date
    Aug 2011
    Location
    Goose Creek ,SC
    Posts
    252
    Quote Originally Posted by ckartson View Post
    when you said you weighed in the charge, where are you getting the data?
    Just noticed you said weighed in . If the plate on the unit says 4.5 lbs, That is the factory charge the unit has when it is shipped to us if I am not mistaken. Add 25 to 30 ft of line set and indoor coil , a "weighed in charge" of what the outdoor units name plate says would result in an under charge . And probally save you a bunch of time checking all the duct stuff, Not saying it is a bad idea to check ,I have just seen really simple stuff cause guys a lot of heart ache , And spending three or so hours or how ever long checking this and that when it was a simple oversight .My thought is if you weighed in a charge according to that plate on the condenser ,you are defintely under charged espcially if it has a long line set or a larger size air handler to get the higher seer. Please excuse my spelling ,it has been one of those days
    "I just think things should work properly"
    Freon is a brand name-word


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