Originally Posted by honola
Wasn't trying to bust chops on any competent techs. However, the 4 techs I had in my house said it's a big open house, the ac is going run longer and depending on the outside temp it will dictate the inside temp. I understand there is only a certain amount an ac can drop temp. But if you have a return on the left side of the house and t-stat on the right side of the house with a large stairwell in between that is wide open to the downstairs. It seems a smart solution would add additional return in the center and right side of the house. Considering the bedroom is 20x15, walkin closet is 15x15, bath is roughly 15x15 and the only way the air can get back to the return on the opposite side of the house is through a 32" wide door and across an open stairwell and through another 32" wide opening.
"if" 210 is the magic number of return I'm even shorter on sq in than I thought. 336 current and 210 * 3 = 630 or 294 sq in short.
upstairs configuration is 2 bedrooms and full bath on left side of house with the only return, hallway and 32" opening, center is a stairwell front to back of house with cathedral ceiling and 6" supply, right side of house has master bath/walk in closet and master bed with the thermostat and 32" opening to access the stairwell.
I wasn't planning on "maxing" out the return air, just adding adding additional return. The biggest I can add is 8" take offs and can possibly get 3, but would be safe with 2. The way I figure it an 8" flex would really be a 7" and measure out to about 40 sq in of return x 2 would be an additional 80 sq in or return or 336+80=416 total. Which is still shy of "rule of thumb" standards. Does this seem like a good idea or is there a better recommendation. Should I run the 8" into the attic with a box in the attic and put several 6" returns?
Thanks for the help
If you have 336 sqin then you are over you only need approx 210 sqin. But 336 is OK.
I understand what you are trying to do but if you run an 8" you are only going to be able to move approx 225 cfm so if you connect four 6" to a VAV and four 6" total approx 440 cfm you are only going to move about 225 because you are limited to the 8" pipe.
And when you start changing static pressures and changing ducting this will also change delta "T" and return/primary air difference.
THe problem I don't think you understand is it should be OK to add return but you may need to restrict existing returns.
By the way what is the static pressure of the return and supply?
And where can I get a rule of thumb book. I think yours needs some updating.
The only rule of thumb I have is that you need 350-450 CFM per ton of cooling but then I use a Ductulator for proper sizing