Return size

[muncie]

I have a goodman heat pump system with two nice size return vents. One upstairs and one down. The filter size on the air handeler is 14X20. When I change the filter only approximatly 9X12 of the filter is dirty. Wouldn't this mean that the return size is only 9X12? I am having a problem with low airflow at the vents. A few vents do not have any flow at all just noise from the motor running.

[amickracing]

What are the dimentions of the duct itself? Also take the filter out and peek in there. Typically there's some form of channel in there for the filter to slide in. It's generally made of channel that's only 1" tall or less.. they might have made yours bigger for some reason. Another thing to check... Where the duct is cut into the furnace (also I'm assuming where the filter is), they might not have cut it out as big as the duct.

[johnl45]

Minimum return duct size for a heat pump system is 6 sq.in per 1000 BTU's.

[Irishmist]

john145: how does this method take into account your static? this 6 sa.in per 1000 btu sounds very much like a "rule of thumb." Dangerous at best.

[johnl45]

Originally posted by irishmist
john145: how does this method take into account your static? this 6 sa.in per 1000 btu sounds very much like a "rule of thumb." Dangerous at best.

It's not a sizing method just the absolute minimum size allowed by NFPA standard 90B and referenced in most codes.

[aircooled53]

I do not agree with that number,NFPA Standard is a little lite.144 square inches per ton was that old standard. Most new heat pumps require 200 square inches per 12,000 btu or 1) ton.

[johnl45]

Originally posted by aircooled53
I do not agree with that number,NFPA Standard is a little lite.144 square inches per ton was that old standard. Most new heat pumps require 200 square inches per 12,000 btu or 1) ton.

The manufacture may require a larger opening but if none is specified these minimums would apply.