I’m adding a wall in the second floor of my cape style house and want to build a chase for an AC return duct (from attic to 1st floor ceiling). The house is about 2300sq ft, and I’m guessing about 3ton of cooling load. Anybody have any ideas about how big the return should be??
Thanks.
Replies
David,
How much of the system is returned through this duct? A starting place would be about 100 square inches/ton of cooling (i.e. 12x8 or 10x10). Other factors would affect the size such as length of run, bends and branches, filters and registers/grilles sizes, etc.
Tim
Just my opinion but the return minimum should be no small than all the total square inch of the ducts that deliver air to the floor. Better yet add 20% to the return.
Fred, are you of the mind that most systems are way under designed with cold air returns? I am probably a bit odd in that I look for stuff like that. And it seems to me that far too often a house may have 12 or 15 heat registers and only 3 cold air returns. Yes some of those returns are larger than the heat registers, but not by a factor 4 or 5 to one. Secondly, I have seen many cold air registers which might measure 14" x 8" but they are placed in a 2 by 4 Stud wall. This makes the effective dimensions of the 14" x 8", 3 1/2" x 14 1/2". what's the point of the 8"? it buys you little except maybe reduced resistence through the grill.
I am not talking about measuring the grill sizes that cover the piping (ducts) into and out of the room but the size of duct piping or the chase that is built into the stud bay for air return.
lets say you have a single 4" supply size ducts to a room or floor. That will give you 12.56 sq. inch of area on one duct. Now if you have six ducts of 4" diameter that will be 75.39 sq. inch of air to let out of room.
Now if you have one 3.5 x 14.5 cold air return that is 50.75 sq. inch for the air coming out of 75.39 sq. inches from the supply side ducts.