Hi,
I just started construction on a new home and am working with my HVAC contractor to spec out my equipment and design the system. I am trying to build a very tight home with icynene walls & roof deck keeping HVAC equipment in conditioned attic built on slab. it is a 2 story home with mechanical located in the attic (3rd story).

my question is about where to place cold air returns? my HVAC contractor is insisting that they need to be placed near the floor and an energy rater/consultant is telling me that it doesn't make that much difference in a tight house. It may be difficult and add expense to put them near the floor of the first floor.

also this house has all 9' ceilings, no vaults, etc. it is located in mixed-humid environment (memphis, tn).

can anyone help me to understand this a little better?

There's little to no rationale for having the cold returns at the floor level beyond some presumed stratification of the room air with cold at the bottom, a stratification that's rarely ever there. If the slab isn't insulated at the edges the slab might be cooling the air near the floor, but that won't be "fixed" by placing the return registers there. As long as the supply & return are place a reasonable distance apart, the entirety of the room air gets involved with the flow.

But DO insulate the slab edge.

Dana is correct in that there is no real advantage to low wall return placement.
Don't let your contractor do just a single large return opening. Several smaller returns placed throughout your home will give a much better balanced air flow. Since you will have a conditioned attic space, both the supply ducts and the return ducts can be located in this space. This should be easy to do, and a relatively inexpensive option.

I will agree with Dana and Wes above in regards to return air location. However, I don't see how it could be that difficult or expensive to have your return air near the floor. I personally would do it, just to make the house look better.

Oftentimes, placing an air return in an interior wall near the floor will result in giving up some closet space.  Ceiling returns and high wall returns are less likely to interfere with furniture placement.

I prefer returns in almost every room except kitchens and bathrooms.

And the kitchen, bathrooms, and utility areas are where you want your ERV returns located.
(In Memphis, ERV would be preferrable to HRV.)

Posted By Alton on 10 Mar 2010 10:49 PM
Oftentimes, placing an air return in an interior wall near the floor will result in giving up some closet space.  Ceiling returns and high wall returns are less likely to interfere with furniture placement.

I prefer returns in almost every room except kitchens and bathrooms.

Also with the returns higher on the wall, your house will cool better during the hot seasons when you are running your a/c. Warm air rises, so with the returns higher up, it will be pulling the warmer air through and cooling it.