Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Neg subcooling

5earlys
5earlys Member Posts: 8
edited September 26 in A-C, Heat Pumps & Refrigeration

I was doing a p.m. on my year and a half old unit. it's a Grand Air 3 and 1/2 ton. I have a normal suction pressure slightly high superheat and a negative subcooling. Head pressure is also low.
The system is cooling perfectly.

123.3 psig
17.2 SH
42.1 VSAT
59.3 SLT
74 ambient inside

227.0 PSIG
-13.3 SC
77.3 LSAT
90.6 LLT
87.7 Ambient outdoor

I did a pump down on the compressor. it would only go to 4 PSI. At 40 psi the compressor started to become noisy and rattled.

I am leaning towards a week compressor or a bad TXV. Your thoughts would be appreciated

Mike

Comments

  • pecmsg
    pecmsg Member Posts: 6,473
    edited 12:08AM

    Impossable!

    4 PSIG is unrealistic for an A/C compressor

  • 5earlys
    5earlys Member Posts: 8

    So isn't that how you store refrigerant in the condenser. I have pumped down a lot of units and always reached 0 psi with the liquid line service port closed.

    I am not understanding what you mean

  • HVACNUT
    HVACNUT Member Posts: 7,001

    The bulb for the TXV is in the correct position and insulated? Any static pressure readings? Fan on low speed, or a high MERV filter?

  • 5earlys
    5earlys Member Posts: 8

    The bulb is insulated and in the right position. I haven't taken a static reading at this point. The fan is on high speed. I opened all returns and removed filters.

    I am running a 18.5 degree split

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 18,555

    Your not supposed to pump scroll compressors down that low and according to some they shouldn't be pumped down at all which I don't agree with.

  • mattmia2
    mattmia2 Member Posts: 14,164
    edited 2:07PM

    are you using the right chart or scale on the gauges?

  • pecmsg
    pecmsg Member Posts: 6,473
    edited 11:13AM

    A/C compressors especially scroll's will only pump down to a determined deferential pressure at which point a safety opens.

  • 5earlys
    5earlys Member Posts: 8

    Im using sman 482 guages

    20250920_133056.jpg
  • JMWHVAC
    JMWHVAC Member Posts: 91

    Not sure how the readings you say you have are possible.

  • JMWHVAC
    JMWHVAC Member Posts: 91

    Did your gauge kick the bucket? Is this AC or HP?

    5earlys
  • pecmsg
    pecmsg Member Posts: 6,473

    You cannot have Negative superheat or subcooling.

    How do you cool the liquid line Below the ambient temp? The pressure will respond accordingly.

  • 5earlys
    5earlys Member Posts: 8

    Exactly the problem. I have never seen this before.

  • JMWHVAC
    JMWHVAC Member Posts: 91

    Yet, you are saying it is cooling fine, so all is good.🤷‍♂️

  • pecmsg
    pecmsg Member Posts: 6,473
    edited 12:33PM
    https://forum.heatinghelp.com/discussion/comment/1866147#Comment_1866147

    look for a restriction!

    Where are you taking the measurements? Pictures

  • JMWHVAC
    JMWHVAC Member Posts: 91

    Somehow the gauge has to be wrong. How can the evap side be good when the condensing side is so low?

    5earlys
  • ratio
    ratio Member Posts: 4,116

    If a scroll goes negative in the can—not necessarily at the suction port—the winding may start to arc over. You can hear it popping inside.

    I've seen negative subcooling and superheat on digitals before. Most of the time, it's because the unit isn't charged properly (especially on microchannel coils), or the digital was set to the wrong refrigerant.

  • JMWHVAC
    JMWHVAC Member Posts: 91

    The gauge is saying the saturation temperature is 10* below ambient, then it warms up to a few degrees above the ambient as it goes thru coil! ? ! ? Guess I am missing something…

  • JMWHVAC
    JMWHVAC Member Posts: 91

    Is that condensation on the high side of gauge set or just the way the surface looks on pic?

  • mattmia2
    mattmia2 Member Posts: 14,164
    https://forum.heatinghelp.com/discussion/comment/1866156#Comment_1866156

    Or the thermometer

    5earlys
  • pecmsg
    pecmsg Member Posts: 6,473

    or the placement

    mattmia2
  • 5earlys
    5earlys Member Posts: 8

    It's straight cool. Guages are a month old.

    Never pumped the compressor into vacuum.

    It's not condensation just a glare from the picture.

    I just hooked up my old sman 380 to verify that there's not an issue with the new guages. I'll let you know.

    At this point I'm leaning towards an internal issue with the compressor or a restriction at the txv. Checked the drier is not restricted

  • 5earlys
    5earlys Member Posts: 8

    Okay update. hooked up my old s-man 380s got normal subcooling 9. Which told me my new 482s had a problem. So so I did a factory reset on my new gauges and got correct readings.

    I would not haveI would not have guessed that a brand new set of gauges would be doing that. That explains why I never seen settings like that before.

    My thanks to everyone that contributed.

    JMWHVAC
  • pecmsg
    pecmsg Member Posts: 6,473
    edited 3:44PM

    Thanks for getting back with that


    the first red flag was - sub cooling

    5earlys
  • 5earlys
    5earlys Member Posts: 8

    I totally agree. I have been doing this for a while and have never seen it. I wouldn't think that a set of gauges out of the box would do that. My old s-man 380s were completely reliable never failed me. Only replaced them because the display is wonky. Thank you for your help

Categories