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  1. #14
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,964
    Quote Originally Posted by punklethal View Post
    Yeah the unit runs constantly never shuts down and can not cool the house down past 79 degrees with the windows all closed. I highly doubt a smaller unit would do the trick this almost makes me wish I would have stuck to my window ac units than going with central. What can I do now though the installer is saying I need to go bigger not on my ducts but system tonnage.
    For residential you can use a max of 1000-fpm velocity, but many recommend a max of 900-fpm on main, branch runs 600-fpm.

    For that 3-ton system, I would have sized the duct system for 1300-CFM; 16" metal; @ 932-fpm velocity.

    For a 2.5-ton; 15" rd. main if available; 1100-cfm; @ 897-fpm. Friction Loss Per 100' of duct, 0.09 IWC.

    For a 2-ton; 14" main; 900-cfm; 842-fpm.

    tinner73, my software for 2-ton, 800-cfm, I get a 13" duct; @ 868-fpm; FR is 0.10" | However, I would size for 900-cfm. Though, your figures are okay.

    With your figures I get; 12"; 800-cfm; 1019-fpm; FR 0.15" WC per 100'.

    Size the return equal or a little larger.
    Airflow is everything.

    The existing 3-ton with a poor duct system, may be delivering around 1-ton. (?)

    Reduce the heat-gain first; then do the manuals J, D,& S, & stop guessing. - Darrell

  2. #15
    Join Date
    Feb 2004
    Location
    chicago suburbs
    Posts
    4,422
    my Trane ductulator has me @ .2" of friction.
    FILL OUT YOUR PROFILE!!

  3. #16
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,964
    Quote Originally Posted by tinner73 View Post
    my Trane ductulator has me @ .2" of friction.
    Hi tinner, Your ductulator may be right, but that is very high for 100' duct run of friction loss. Most want 0.1" down to 0.06" or so, on the supply side, even lower on Return.

    All I have is software now, no ductulator. - Darrell

  4. #17
    Join Date
    Apr 2008
    Location
    san pedro,ca
    Posts
    10
    the ducts are flex ducting R 6.0 so maybe I am undersized a bit. I checked for leaks everything looks good and ok. The air is coming out of the vents and the furnance is set for 3.0 blower speed medium low setting on this unit. I guess maybe I am undersized thanks you guys.

  5. #18
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,619
    may your ac may not be charged right have it checked for proper operation could be way overcharged or under charged call a real tech to check it

  6. #19
    Join Date
    Apr 2008
    Location
    san pedro,ca
    Posts
    10
    Thats the first thing I did. He asked me if the original installer added any freon. I said no. He checked it with his gauges to see if charge was off and added some freon said the air was not cold enough to combat the heat. Since then the unit has been running better but still cant fight the heat. Maybe I will call him back and look at my duct work for me.

  7. #20
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Could be duct leaks,undersized ducts,etc..

    what R-value insulation do you have in the attic??

    Adding some could solve iy,if the ducts are good and the unit is operating properly.

  8. #21
    Join Date
    Apr 2008
    Location
    san pedro,ca
    Posts
    10
    they are r 6.0 ducting but like i said no air leaks so far.

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