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  1. #27
    Join Date
    Sep 2009
    Location
    Arnold mo
    Posts
    3,967
    Quote Originally Posted by hvacrmedic View Post
    It helps to know what you're looking at. The SC that you select at the top of the chart is the "required subcooling" and that would be what is listed on the nameplate, and that differs fom one model to another, and even from one size to another of the same model. The chart is for use across the entire product line, it's generic, thus the reason for the range from 8 to 18. In this case the required subcooling is 15. You use the column below 15 and ignore the other columns. HTH. That chart is only for those who don't know how to subtract LLT from SCT. Ironically there is no 15 column, so you're expected to interpolate. I don't know how a tech who can't subtract is going to manage that, but there you go. That's how you use the chart.

    There are also a range of favorable conditions under which that SC value is valid. Instructions are to weigh in the charge when conditions aren't favorable and come back later when they ARE favorable to trim the charge.
    Thanks. Based on the chart, the subcooling seems correct, assuming the unit was in second stage when the pressure and temps were taken. I have no idea of what the pressure difference is between 1st & 2nd stage. I have no experience with 2 stage compressors.
    An answer without a question is meaningless.
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  2. #28
    Join Date
    Jul 2005
    Posts
    5,568
    Quote Originally Posted by tipsrfine View Post
    Thanks. Based on the chart, the subcooling seems correct, assuming the unit was in second stage when the pressure and temps were taken. I have no idea of what the pressure difference is between 1st & 2nd stage. I have no experience with 2 stage compressors.
    As a rule of thumb you can ballpark the expected SST in high speed by subtracting 35 from the indoor dry bulb temp. That also applies to single stage resi air conditioners. That would give us a SST of 44. I use my own rule of thumb, indoor wet bulb minus 22, which also gives us about 44 SST in this case. (Wet bulb is actually around 66-67 at the stated RH of 53% and 79 dry bulb.)

  3. #29
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    95
    He completed the "tune-up", but wants to come back Monday to take some airflow readings. He said the compressor is shifting to 2nd stage, but the suction pressure is increasing when it goes from 1st to 2nd stage. He doesn't think there are ventilation leaks because the return grille and supply register temps have appropriate temperature differences from the air handler.

    His numbers: (1st / 2nd stage)

    Compressor amp draw: 5.5A / 11.2A
    Liquid Line temp: 85F / 85F
    Liquid Line press: 308 / 312
    Suction Line temp: 64F / 68F
    Suction Line press: 140 / 152

    Suction Line Temp @ evap (only measured @ 2nd stage): 59-66
    He said normal TXV fluctuation

    Liquid Line Temp @ evap (only measured @ 2nd stage): 90

    Return air temp (@ evap): 79F / 78F
    Return air wet bulb: 65F
    Supply air temp (@ evap): 54.8 / 62.3

    Outdoor Ambient: 84F
    Outdoor Humidity: 66%

  4. #30
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    I'm thinking the compressor has something wrong with it that is causing a lack of capacity in 2nd stage.

    The suction pressure should have gone down, and the liquid line pressure up by more than 4 psig, when it changed to 2nd stage.
    There should also not have been a drastic rise in the supply air temperature in 2nd stage.
    If more government is the answer, then it's a really stupid question.

  5. #31
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,391
    How long was he there and did he do any other work as part of the tune-up?
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  6. #32
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    95
    Probably 2 hours. Did the other usual things, cleaned the condenser coil (was already clean), the cursory spraying of cleaner on the upper surface of the evap coil, looked at the bottom side as best he could (it was clean), checked amp draw on blower motor, condenser fan and compressor, condensate line cleanout, read the trouble codes off the controller (saw a 71 and 72, thermal cutout low stage and hi stage - from last summer, but he said that was a known issue with the original board at the condenser).

    I think the is the first time they've actually done comparisons from 1st to 2nd stage.



    Quote Originally Posted by comfortdoc View Post
    How long was he there and did he do any other work as part of the tune-up?

  7. #33
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    95
    The latest on this. My local HVAC company has conferred with Carrier, and they believe the TXV is bad. Carrier had them come back out and double check the amp draw on the compressor, since these Bristol TS compressors have had issues. Low stage: 5.6A, Hi Stage 9.2, so the thought is the compressor is ok.

    They will replace the TXV in a couple of weeks, (waiting for the part).

    I can't reason in my head how they get good subcooling and superheat values at hi stage, and the TXV not be working. I would have bet money that it was the compressor (suction pressure goes up low to hi stage). Carrier is replacing the TXV at their cost - so I can't argue with that. I'm just happy that they finally acknowledge something isn't right.

    Will post an update after it is replaced.

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