I should have added one additional detail:

I live in Iowa and our heating season is about twice as long as our cooling season. Thanks.

Thanks for the great reply HKestenholz. What you explained makes a lot of sense, however, it is the opposite of what a lot of the new construction is in my area. All of the new construction that I have seen around here has the supplies under the windows on the exterior walls and the returns high on the interior walls. I thought the purpose of this was to cover the exterior wall with hot air and then carry it up and across the room to the high return grille.

Based on your response it sounds like the return should be below the window on the exterior wall and the supply should also be located on the exterior wall. Is that correct? It seems like having the supply and return on the same wall would prevent the warm air from covering the entire room. I think I'm missing something here. Can you help?

Thanks for the quick reply. So does this sound right:

Returns at the bottom of the exterior wall, floor level, under a window.

Supplies on the floor, also along the exterior wall. Perhaps one joist bay over from the return?

As always, thanks a million for all your help HK.

i was reading your questions and anwsers and i have one my system is setup with the returns high on the walls in each room and supplys by the windows how hard is it to reverse the system to the way you said "supply high on the walls and returns by the windows " can i just reverse the duct work at the air exchager i have a hot air system with a hot water coil in the duct does this mader plus acoil for the ac too and the filter is located at the bottom of duct do i need to relocat this to?the air comes in from the bottom and out the top if changed will this work?