I have a new 3 ton goodman heating and air system in a horizontal installation. I had a really sloppy job done and I am trying to fix it myself. I have seven drops in an 1100 square foot house. How large should my ducts be on average and should they all run directly off of my plenum. The person that did the install came off the plenum with 12 inche ducts and used t's everywhere and my air flow is terrible. my duct size he used is 6 inche right now. Any help would be greatly appreciated before I tackle this. I have a good understanding of how to cut holes in my plenum and also of how to run the ducting, but do not know how to size it all.
The best way is to use a duct sizing calculator. Thats what the pros use (at least they did when assessing my duct size) You may be able find something online, not sure. Basically it takes the unit size, square foot of the house, duct supply length and number of vents and calculates the recommended size.