We are installing a self-contained hvac unit that mounts to the roof. How do you determine how many vents and what sizes to run? Is there a calculation performed for this?
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Well, first of all, have you done a Manual J calculation to determine the design heating and cooling loads? For a homeowner, I would suggest using HVAC-Calc (http://www.hvac-calc.com). For $49 you can buy a one-time license to do the calculations for your home and the resulting output gives you the rom-by-room design heating and cooling loads. From there, you can use the program to set duct size to deliver the needed volumes to each room.
I'm not connected to HVAC-Calc in any way other than having used it on 2 of my own houses with very good results.