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Question on new condensing unit

Also,.the hi vol. units generally run 30 to 40 * S/H, but dont get me wrong. That is not how to check this charge. Sub cooling only.


  • Terry H.
    Terry H. Member Posts: 73

    I have a question about setting the charge on a hi-velocity air unit using a txv valve. Yesterday we had a contractor replace a condensing unit on our warehouse, this morning the coil and line set were frozen up.

    After turning the system off and allowing it to equalize and thaw out here's what I know (I hope I have more information than you need).

    This is a Hi-Velocity system running a txv valve.

    At the indoor unit I took these readings (while system was running):

    Return air-73.4 deg.
    supply air-45.7 deg.
    lineset before txv 87.5 deg. & 44.5 deg.
    lineset after txv on coil side 60.9 deg. & 33.8 deg.

    ambient air
    wet bulb temp 17
    dry bulb temp 24
    digital reading 74.8 deg. and 43%

    At the outdoor condensing unit
    suction air 81.8 deg.
    exhaust air 95.0 deg.
    line set 88.8 deg. & 36.3 deg.

    ambient air
    wet bulb 21
    dry bulb 28
    digital reading 81.2 deg. 43%

    I had the contractor check the gauges and they read:
    low 36
    hi 100

    I'm not an expert and when the contractor asked why I needed to take so many readings I told him just in case it helps figure what is wrong. Near the end of taking the readings I could tell the unit was starting to freeze again.

    Any insight would be appreciated.


    Terry H.
  • JeffD
    JeffD Member Posts: 41

    Hmmm, assuming this system is using R22 for the refrigerant your head pressure is way to low, suction pressure is way to low. Looking at a p/t chart for R22 shows that your refrigerant is boiling at around 14*F. You could have a restriction in the liquid line, but your head pressure is too low for this to be happening. My guesse would be that this unit is very low on refrigerant.
  • Terry H.
    Terry H. Member Posts: 73

    Yes Jeff it is an r-22 system. No refrigerant was added it only has the charge from the factory. The line set is about 14' long, it's a 7/8 x 3/8. The air handler coil is about 8' higher than condenser with a trap just below the txv valve. Since this was a compressor replacement I made the contractor add filters to both lines coming in to the compressor, even though compressor failure was not a burn out. Is it typical to need to add freon?


    Terry H.
  • JeffD
    JeffD Member Posts: 41

    After the unit is charged up initially, or recharged after a repair you should never have to add more refrigerant, unless there is a leak. It is possible for the filter driers to be plugged, but your head pressure is way down there, so I don't believe your filter drier(s) are plugged. If the compressor wasn't burned out you don't need a suction line drier, in fact it is good service practice to remove suction driers after they have done there job ie absorbed the acid and moisture from the system, liquid dryers are left in place however. Sounds like your system now has a leak and has lost most of it's charge. Have them come back and find the leak and recharge the unit to factory specs. If the leak is at one of the joints that they brazed when they put the compressor in they should fix it with no labor charge. At least I would as the leak would have been my fault, however different companies have different policies regarding this. Hope this helps and let us know how things turn out for ya.
  • Cosmo_3
    Cosmo_3 Member Posts: 845
    Thats right

    Only capillary, or orfice systems should be adjusted according to superheat. All TXV metered systems should be charged according to subcooling. The TXV's job is to control superheat, and thus will open/close as needed to try and maintain a specific superheat range.

    Cosmo Valavanis

    Dependable P.H.C. Inc.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Important Point

    Thanks Cosmo. Point well made and worth repeating again and again and again.

    Using evaporator superheat (only) to charge a TXV system will not yield desorable results.

    The TXV has a one line resume:


    Simply stated, the TXV will open or close (modulate) to maintain the prefetrmined superheat, no matter what the conditins are in other parts of the system.

    Thanks Cosmo.
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