Home Improvement → Re: [HVAC] A/C Duct Sizing

Without knowing the size of the room, it's hard to say whether or not the duct is sufficient for that room. But, without getting into friction loss, static pressure and other technical aspects of duct design, the general rule of thumb is 1 CFM per square foot, so if the room is 12' X 12', it would require 144 CFM of air, which would require a 7" duct (160 CFM at an air velocity of 600 ft/min). Two 5" ducts would supply 65 CFM/EA totaling 130 CFM (air velocity of 477 ft/min each), which would still not be enough for that room. In addition, if it's a master bedroom and/or has a lot of windows, it would be best to add 10%-20%. The air noise is because the duct work is undersized for the amount of CFM the blower puts out or the return air registers are too restrictive. You might consider different registers. But, my gut tells me the duct work is undersized because the duct work was probably originally designed for a heat only system.

I think the duct work is on the small side.

The room is 12 x 13. Two windows on 32x52, the other 32x40, both new, double pane, low e, argon, etc. One faces north, the other east, so don't really ever get sunlight. The walls have "some" insulation, probably 2-3" of fiberglass, and then maybe 3/4" foam under the siding, just based on what the previous owner tells me. Attic has maybe 16" of rock wool, but poor ventilation which hopefully will get fixed this year if I can come up with the cash for roof replacement. I was considering a higher efficiency a/c unit at that time too as our electric bills are very high in the summer. Also, location is northeast Iowa.

I already bought the stuff to add a 5" duct. Debating making the trip back to Menards to get 6" ductwork, but it's a 50 mile trip. I'll be doubling the amount of air to that room, so maybe it'll be okay.

I didn't think to ask what kind of duct work you have. I assumed it was sheet metal trunks and round sheet metal ducts. The numbers I gave were for round sheet metal duct work. Flex duct has a little more friction loss than sheet metal ducts, plus I didn't calculate any bends (45s and 90s), so the numbers (CFM and FPM) will be slightly lower. But, adding another duct will definitely help.

Sheet metal, not flex. Thanks for your help. I've added the second duct, it seems to move a lot more air than the original one did.