Home Improvement → [HVAC] A/C Duct Sizing

I have what I believe is a 2 ton central air conditioner (Lennox HS29-024-3P). I question whether the ductwork is adequately sized, as the house was built in 1962 and probably did not have central air at that time.

The air handler sits in the center of the basement. Coming off the air handler the supply plenum is 17x17 inches. It splits into two 8x10 trunks, each going in opposite directions. These then go down to 5" round ducts each feeding one of nine registers. The return is a 14x14 duct, that goes to two return air grills that are each 30x6, one in the hallway and one in the living room.

The problem is that one room is always hot in the summer, and for that matter, is cold in the winter time. There's two windows in that room, but only one register. The second bedroom has two registers, one under each window and maintains a good temp year round. Third room has one window, one register, and is smaller, so also maintains temp fine.

Another issue is that the return air registers whistle a little bit, like their trying to pull more air than they're able too.

When I bought the house 10 years ago the HVAC guy measured all the rooms and the ductwork, then went back to the shop to figure out what size furnace and air conditioner we needed. He claimed to run manual a D and a manual J calculation, but who knows?

I'm planning to add a second 5" duct to feed the bedroom that's too warm/cold to see if that helps, but since I need to remove the ceiling below, now would be the time to replace all the ductwork if it really needs it. The biggest challenge is that it passes through a concrete wall so upsizing will require cutting a larger opening in the wall, so I'm hoping I can get by with the current ducts.

Is there a "rule of thumb" to figure out if this ductwork is reasonably adequate?

just close off all the others a little so that one room gets more air flow?

I've tried that, but then people complain that other rooms aren't getting enough air

I'm going to replace the registers, most of them are nearly impossible to adjust so newer ones that actually work might help with that.

wait, normally, people need enough air to breath, if people complain they are getting enough air, this is the wrong forum.. but high velocity AC's use about a 2" duct, and do fine, so the existing duct is not the problem as far as heating/cooling if the room is insulated the same and generates the same heat/cooling as the others and has a return duct like the others..

I think your on the right path to add another duct to that one bedroom. I would also consider modifying the 2 return ducts so they are both located in an interior wall (which they probably are now) but moving the return register/grills up to within 9" - 12 inches of the ceiling which would suck the hot air back into the air handler/a/c coil. This setup is what all homes I've seen around here have been built since about the late '70's. Your whistle sound is probably caused by one of the registers vibrating due to the air flow through the fins. I had a return register in a bedroom do that, and you could gently place your finger tip on it and you could feel it. After bending a few of the fins in the return register, that cured it.

The return airs are on interior walls, but are at floor level. Moving them would require a lot of drywall work that I don't want to get into. I suck at drywall.

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.

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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.

Yesterday was hot and humid, so we ran the a/c. The bedroom is now staying as cool as the rest of the house. Looks like the extra duct was exactly what was needed.