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Need help sizing a plenum here is some relevant info.

910 sq foot house with 2 ton system, heat pump outside and AHU centrally located in attic. The air handler has a supply air opening of 16.375" x 11.3" I was thinking just doing a 17 x 21 and making it long enough to the 3 runs on each side
There are 6 supply registers longest run is 15 feet. I'm planing on doing a radial duct system. Here is a diagram of my house and register locations flex duct sizing its going to be 8" and 4" for the bathroom.

A standard sized plenum is 16 x 20. It would be the most readily available without paying extra for a custom sized plenum.

I'd make the plenum long enough to keep any runs from terminating a good foot or two from the end cap and long enough that the first take offs don't take place within 4' to 6' of the air handler. I'd also try and keep each take off run 2' apart from each other. I'd drop the office room down in to a 7".

Most of us here are in Canada and don't typically have the pleasure of dealing with attic installs. Where abouts are you located?

Quote from: Porcupinepuffer on February 27, 2021, 08:53:06 AM
Most of us here are in Canada and don't typically have the pleasure of dealing with attic installs.