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  1. #1
    Join Date
    Nov 2012
    Location
    Upstate New York
    Posts
    6

    Confused Vertical lengths ..... residential HVAC unit..

    went out to look at a AC unit this fall .... customer is complaining that the AH runs continuously during the summer and get minimal cooling. Have check the coils on the AH, pressures on the compressor .... they were a little low but not too bad.... it is a 2tn unit but the vertical rise on the line set is at least 25 ft with 2 horizontal runs of 10 ft each. 2 tns is adequate for the cubic footage but I am trying to find some data as to whether or not these vertical runs are too excessive and some sizing adjustments.

    The customer wants it fixed .... I am considering a Fujitsu split system I can mount on his back roof with two inside units.

    any input would be appreciated ..... thanks

  2. #2
    Join Date
    May 2006
    Location
    Austin, Texas
    Posts
    279
    Check with manufacture for installation guides. 25' verticals are fairly common without any issues

  3. #3
    Join Date
    Aug 2012
    Location
    NE wisconsin
    Posts
    401
    Is the suction line 3/4? Like the last guy said check with the manufacturer, you may need to put in a 7/8 suction line.

  4. #4
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,322
    Quote Originally Posted by philjafo View Post
    Is the suction line 3/4? Like the last guy said check with the manufacturer, you may need to put in a 7/8 suction line.
    You never increase diameter for vertical runs. If anything, you decrease it

  5. #5
    Join Date
    Aug 2012
    Location
    NE wisconsin
    Posts
    401
    The overall length of 45 ft is why I was thinking increase dia. But there maybe other issues like airflow or dirty coil that are not readily apparent on a msg board. Check with the manufacturers specifications for long linesets, that will tell you exactly what size lineset you need as well if you need an oil trap or something like that.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,704
    a little low but not too bad
    Doesn't tell us much.

    On a 25 foot rise to the air handler, your head pressure is 12.5 PSIG lower then at the condenser.
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  7. #7
    Join Date
    Sep 2008
    Posts
    577
    Quote Originally Posted by beenthere View Post
    On a 25 foot rise to the air handler, your head pressure is 12.5 PSIG lower then at the condenser.
    Wouldn't you need to know the diameter of the LL and subcooling changes to know this?

  8. #8
    Join Date
    Jul 2007
    Location
    Virginia
    Posts
    5,360
    Quote Originally Posted by CoolWine View Post
    Wouldn't you need to know the diameter of the LL and subcooling changes to know this?
    The greatest pressure drop due to a vertical column of liquid occurs when the temperature of the refrigerant is the lowest.

    The rule of thumb is assume a pressure drop of 0.5 psi for each foot of net liquid-line rise.

    It is actually slightly less than this, about 0.48 psi/ft, for a liquid temp of 115f.
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  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,704
    Quote Originally Posted by CoolWine View Post
    Wouldn't you need to know the diameter of the LL and subcooling changes to know this?
    No, R22 loses(evap above) or gains(condenser above) .5 PSIG for every vertical foot of liquid line.

    Just like a boiler. Even though you have 20 PSIG in the boiler. A radiator 40 foot above the boiler will only have 2.5 PSIG.
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