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ac system design help

hey guys,

I have a potential customer who wants zoned AC in a small office space using a multi-split heat pump. Did a load calculation & found that each office requires 5-600 BTUh of cooling, obviously mini splits are a bad idea (smallest indoor unit I can find cools to 9000 BTUh.). So I am looking at other ideas.

The entire office suite is only 1250 sq ft and could in theory be cooled by one mini split if it were open concept, but each office/room requires its own zone.

I've been thinking about presenting the customer with other options, such as zoned ducted AC with a variable-speed air handler and motorized zone-valve style dampers. However, I'm not up on AC duct sizing. Don't want to undersize.

Is there a sizing rule-of-thumb for AC ducts, something I can do to get a basic calculation of sizing for the purposes of working up an estimate?

I am doing a duct design course this winter but this job came up too early!

Thanks guys! If anyone has other ideas on how to configure this system, I'd love to hear these also


  • RJ_4
    RJ_4 Member Posts: 484

    I would find someone with a background in ductwork to help.    Get a Trane Ductulator, the Trane air conditioning manual ( hard back book ) and fan curves on your equip. to get started
  • Jack
    Jack Member Posts: 1,046
    The minisplit is still your best bet

    You could do a multi unit and use the small concealed air handlers. They have very low static so position to minimize ductwork. Your other option is a minisplit and add a few Tjernlund Aireshares.
  • hws
    hws Member Posts: 43
    looks like

    Looks like it's going to be mini-splits after all. Owner doesn't want to compromise celing height. Thanks!
  • hws
    hws Member Posts: 43
    hello RJ

    I'm doing an air system design course hopefully this winter.

    Since I wrote the original post, I have acquired a ductulator and learned some useful formulas, which has opened my eyes quite a bit.

    Regarding the Trane AC manual - is it available directly from Trane? I don't see it on their website.
  • RJ_4
    RJ_4 Member Posts: 484

    available online  Bookmark Inc   1-800-642-1288, or check your local Trane supply house  $40.00,  well worth it
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