11:31AM | 09/04/02
Member Since: 02/26/01
35 lifetime posts
I have two questions regarding a new central A/C system a contract recently installed.

For the return duct from the return intake to the plenum, the contractor used the attic joists with no liner, etc. He simply blocked each end with sheet metal plates. The house is very old and the attic is not conditioned/finished so I doubt the attic floor provides a very tight seal for this return vent. Is that method acceptable for my geographic area (Virginia)?

Next, even if this is acceptable, is there sufficient space for air flow on the return side given the following: Although the air return vent is about 18x24 inches (3 square feet), the joist bay openings (two joist bays were used) are somewhat less. The joists are about 12 inches on center and are 2x8s thus you only have an opening of about 1 square foot (12 inches times 2, less nominal width of center joist and outside joist, times 8 inch height yields about 152 square inches, about 1 square foot (144 squire inches)). The A/C size is a 3-ton unit.

Thanks in advance.

[This message has been edited by JamesPatrick (edited September 04, 2002).]


10:29AM | 09/07/02
Member Since: 09/06/02
2 lifetime posts
First in my area that is acceptable to use unconditioned spaces for return air ducts but it should be seal to avoid introducing dust and air leak into the system. You would also have to figure in the heat gain and lost associated with this area. Depending on the length and temperature difference of the space this may or may not present a problem.

Specification for your system are 3 Ton a/c requires about 1200 CFM (cubic feet per minute) with about .20 IWC (inches of water column pressure). The Manual D Recommends a maximum velocity of 700 FPM (feet per min) for return trunk ducts and a face velocity for filter grills of about 500 FPM. Most contractor will design the system with a pressure of .1 IWC that is half for return and half for supply of available pressure. So if you have a duct that is 8 X 24 you would have a velocity of about 1000 FPM which would be to high. But things to look at closely are the actual size of the duct most houses are built on 16 inch centers not 12 so that couple of inches would make the duct 32 X 8 which would get you real close to the 700 FPM for the duct.
Good luck.

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