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Thread: a/c pressure

  1. #1
    Join Date
    Feb 2005
    Posts
    38

    Confused

    Haveing some problems with a/c pressure 85 degree outside air temp inside 80 pressure is 50 and 225 12seer unit when I first went to the job had 90 and 175 found compressor bad replaced condenser then had pressure 40 250 changed tx valve perfect pressure 70 190 then called back unit frozen found that I had leak by tx on equalizer my falt also I had soldered it closed so I replaced it now I have 50 225 tryed to adjust it but cant get good temp at right pressure

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,327
    First of all, what caused the original compressor to fail?

    Secondly, pressures only tell part of the story. Superheat and subcooling readings are what you need, in addition to pressures, to begin your diagnosis as to how the system is performing. Otherwise you're just wasting time.

    You should not even THINK about adjusting a TXV until you have eliminated every other possible cause for a coil freeze-up possible.

    Airflow is the Number One culprit when it comes to evap coils freezing over. Poor airflow due to dirty coils, restricted ductwork, dirty blower wheels, fan speed set too low, fan CFM improper for coil, etc.

    Next, but far down the list, is refrigerant charge. If airflow is good, a refrigerant shortage typically manifests itself by a partially frosted coil.

    Lastly, if charge is good and airflow is good, you can look at the TXV. But you darn better be sure all other factors are squared away first, or you'll NEVER get that TXV superheat right after fiddling with it.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    "you'll NEVER get that TXV superheat right after fiddling with it."

    Ol' Doc says, the only time you need to adjust a TXV is when the jackass before you did.

    Sounds like an expensive fix. In stead of replacing the compressor if it was bad, you replaced the condenser, then the TXV whithout noting the superheat.

    Contact your local distributor and see if he is offering any training in the near future.

  4. #4
    Did you follow the instructions when installing the TXV, if it was installed correctly you shouldn't have to adjust?

    You had a leak and fixed it. Did you adjust the charge after this?

    You said you had a bad compressor then replaced condensor. Did you mean that you replaced the condensing unit?

  5. #5
    Join Date
    Apr 2004
    Posts
    395
    Did you pull a Vac? How did you charge superheat,subcooling scale. Coils clean? Filter clean? Did you intsall new liquid line filter? Suction line filter? Pressure test the line? why start playing with a new txv. are you taking line temp? if so what are they?


    [Edited by fitter638nyc on 02-07-2005 at 03:28 PM]
    Tin Knockers BANG for a living

  6. #6
    Join Date
    Aug 2003
    Posts
    82
    what do you mean you have a perfect charge at 70 190????
    what is subcooling? 10 to 15 degrees subcooling would be a good charge 70 190 whats that? you need to learn how to charge first before you srvice the unit!

  7. #7
    Join Date
    Feb 2005
    Posts
    38
    First I replaced the condenser unit because the compressor was bad and then found that tx valve was bad if someone could tell me how to check tx with bad compressor that would be great my ? was if I had good pressor with 70 and 190 why dont I have it now

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,327
    Without superheat and subcooling readings, how did you know the original TXV was bad? Did you size the replacement TXV properly for the coil?

    You are spinning your wheels without superheat/subcooling readings. You're trying to draw a conclusion from incomplete data. It's like trying to determine why a car engine seized without noticing there's no oil in the crankcase. You know the engine locked up but you keep overlooking the culprit.

    There are no "perfect pressures". The x amount of degrees over ambient for head pressure went out years ago. If you haven't fully grasped the value of a pressure/temperature chart for refrigerants, available at any supply house, I encourage you to do so. That and a superheat chart. For subcooling -the value necessary to properly charge TXV systems- the subcooling temperature is often posted in the manufacturer's data right on the condensing unit.

    Your system pressures are going to vary with indoor heat load and outdoor ambient conditions, even with a TXV. If you're going by pressures alone you're about guaranteed to overcharge the system. Take advantage of the good resources on superheat/subcooling on this website in the "For Your Interest" section.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
    Join Date
    Dec 2004
    Location
    louisville
    Posts
    46
    Take a screw driver and scratch out the pressures on your guages. We deal with tempertures. Techs get too hung up on pressures. Superheat, subcooling, and approach all deal with temps not pressures. To determine the parameters of a system you may have to take 10 temperture readings. Then and only then will you be able to determine the source of the problem. Good luck

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