return air sizing?????

[ngerrald]

What is the best way to size a return air for a system with a single return

[xersw]

how many tons.

[ngerrald]

all tons, for example lets just say a 3 ton system

[dan sw fl]

Originally posted by ngerrald
all tons, for example lets just say a 3 ton system

3-tons = 1200 CFM typically

Return duct air velocity should be < 600 FPM
... Duct area ~ 2 square feet
20" X 12" works

IF

properly installed

[xersw]

a 3 ton a 14x30 could work as well,its easier to install because you can usually find a bay thats 14 and 1/4.14 on center is common.make the box taller if you think you need some more air,anyway a 16 inch flex is good enough to get on that box .youll have enough air for a 3 ton.For me,it all depends on the application.if you have a bang onstartup dont be cheap extend that flex to the return plenum.

[dash]

Use Manual D from , http://www.acca.org ,take a course on it ,or read and ask questions here.

[dajw02]

I would recommend that if you use a ductlulator, and while using that stay within a friction rate of .05 and make sure that your velocity does not exceed ACCA'a Manual D specifications.

For the actual grille open I would recommend taking total CFm out output and dividing that by 2.7 and then by .8 - which will give you a no less than number for the amount of needed sq inches -

David Wilson