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  1. #1
    Join Date
    Feb 2007
    Location
    Napa
    Posts
    111

    Liquid Line temp question

    I have a Carrier 12 ton split system. Original issue was bubbles in the sight glass, SC and SH also indicated low charge.

    Charged system to clear sight glass. Ambient temp at troubleshooting time was 65-75 and minimal load in the zone.

    Before initial charge of system, head pressure was at approx 160 psi and suction was 75-78. Motor Master type c/fan control (Hoffman) was running c/fan at full speed even at lower ambient when initially started troubleshooting based on the LL temp (as per the manual, it goes to high speed at LL temp over 80 degrees).

    After charge, Head pressure increased slightly (but still had c/fan running at full based on LL temp), but suction pressures stayed about the same. This is odd, since everything indicated low charge except the suction pressure at initial beginning of troubleshooting

    What is also odd is that as my LL temp increases, my head pressure decreases and as the LL temp decreases, head pressure increases.

    There are 2 non adjustable TXV's for the stacked evap coils and they seem to be metering properly and do have temp/pressure drops through both TXV's (indicated by temp drop across TXV). I will do diagnostics on those today to confirm though.

    I doubt that there are any non condensables, as my head pressure is running more on the low side, even when ambient was closer to 75. I did have about 29 degrees air temp diff across condenser coil, but only about a 13 degree drop across the DX with both coils active.

    We do have re-heat coils which is more of what we use to maintain temps. Cooling is more to help maintain humidity levels since we have no other source for de-humidification, so there is an artificial load when temp is low enough in suite to activate RH coils.

    I did pump down the compressor using the service valves, and was able to get it into a slight vacuum, but pressure did slowly begin to climb every time (pumped down 5 times), however, I have had service valves leak by in the past around here, so I am not ready to condemn the valves just yet, unless it corrosponds to the questionable LL temp/pressure issues.

    I cleaned the snot out of the condenser coils, which are 2 coils sandwiched together. I have not yet pulled the top of the C/U to seperate the coils and do a better cleaning. This unit runs one of our clean rooms, and can't shut it down completely to do that for another month.

    I will update with more specific information relating to pressures, temps, SH & SC, but was more curious about the LL temp/pressure issues to start with as I can't really do much more until I figure out what is causing that as I can't get the c/fan control to control in low ambient with out switching to a pressure switch...

  2. #2
    Join Date
    Aug 2012
    Posts
    2,754
    What is your delta t, are you having problems maintaning temp in space?
    pressure temperature relationship would make it impossible for a temp drop with a pressure increase. You could have a restriction in a filter drier after your pressure gauge port. causing a temp. drop or rise.
    I do beleive this is in wrong forum you don't sound like a DIYer.

  3. #3
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,596
    Let's start with the easy things. Figure out your fan cycling and check the charge when it's a bit warmer. If you have to, block up the condenser. Is this a 2 stage unit? Also the sight glass is NOT for charging.

    Give us the rest of the info and maybe we can help a bit further.

  4. #4
    Join Date
    Feb 2007
    Location
    Napa
    Posts
    111
    I have reposted this in the correct forum, didn't realize I was in hte wrong one...

    But to address the 2 replies...

    "What is your delta t, are you having problems maintaning temp in space?
    pressure temperature relationship would make it impossible for a temp drop with a pressure increase. You could have a restriction in a filter drier after your pressure gauge port. causing a temp. drop or rise"

    Drop across DX coil is approx 12 degrees, maintaining temps OK, and I agree with your statement on temp pressure relationship, however since this unit is doing exactly that, I have to say that it is not impossible...ha ha, which is why I am asking for help here, it baffles the mind, not to mention the refrigeration logic. Filter driers were replaced a couple of years ago because I had to overtighten them to get them to stop leaking at the flares, and there is one for each TXV. There is no temp drop across either filter to indicate a restriction...

    Ryan, not too worried about c/fan, since it should be running at full speed when charging anyway...right? It will be a problem in low ambient, but shouldn't be part of the problem here. I also know that the sightglass does not indicate correct charge, just gets you in the ball park, which is why I was checking SH/SC to determine correct charge, however, both of those readings are fluctuation, so an actual reading is impossible. SH increases/decreases depending on where the head pressure is...higher SH at lower LL temp. Right now it is fluctuating between 20-30 degrees of SH depending on head pressure. SC has the same predicament...

    Ambient has increased by about 15 degrees since first post, but still seems to be reacting about the same, pressures are obviously higher, but LL pressures and line temps are still unstable...

  5. #5
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,793
    Sounds like a restriction before the LL service valve your connecting to.
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