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Thread: 3.5 ton evaporator

  1. #1
    DIY Junior Member mp6199's Avatar
    Join Date
    Jan 2011
    Seekonk Mass

    Default 3.5 ton evaporator

    Hi all,

    I wil be installing a 3.5 ton evaporator airhandler in a ranch home and am curious on how far I can run thr the main trunk. The house is rather long at 90 ft. I thought the best way would be to use a pair of pants off the supply and split the main trunk in two directions with the trunks instead of one long run. I am not sure if the blower at meduim speed(factory preset 1640 cfm) will be sufficient for such a long run or if I should change it to high. It would be roughly a 50 ft run if I ran it straight. I would reduce the trunk as it went naturally to accomadate the required cfm. Does anyone have an opinion on a run that long or is spliting it better.

    Thanks in advance.

  2. #2
    Retired prof. engr. gator37's Avatar
    Join Date
    Nov 2009


    You should have some papers with the AHU that tell you what the static capibility of your AHU is at 1640 CFM. Typically they would look like .2" SP @ 1700 CFM, or .4" SP @ 1500 CFM, etc. and it is usually in a chart form. The static pressure (SP) listed in these charts is telling you what CFM the AHU is capabile of delivering against the losses in reference to the duct system, duct fittings, filter, grills, etc. The return duct if any counts also.
    If there is no reason to split the duct trunk line (get around obsticles or what ever) I would recommend just one main trunkline. Simpler is cheaper. Minimum bends, fittings, etc. , your AHU should be able to handle that
    Typically you can size a duct system for .05"SP/100' of duct and be OK (especially for a furnace with cooling coils). Alot of contractor size for
    .1"SP/100'. You can probably find a "Ductulator" for sizing duct on the a web page or at your Local Trane dealer.

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