I always leave the HVAC design to qualified experts. But just a couple of notes: you should have more than one return per level; HVAC does work best when it's centrally located and any ductwork in the attic is a big design mistake, unless your attic is non-vented.

Which hygro thermic region of the continent you're building in determines many of your choices. Check here for House Designs that Work in your climate.

First you need to locate the duct system.

Most of the systems I have seen in New England in the past 20 years have located an air-handler near the center of the basement so the ducts could run below the first floor framing and an air-handler in the attic to serve the second floor.

Of course, in such a system the attic would be included in the conditioned space of the house with insulation between the rafters.

It's difficult to advise you without knowing the climate or the nature of the system.

Thanks for your replies. I am in the western end of the Mixed Humid region. Around here, the HVAC for the second floor seems to always be located in a closet on the second floor, not in the attic.

I am not thinking that we novices design the whole system -- I will leave that to the experts. Rather, I am planning what parts of the basement can/should be finished and I'd like to know for aesthetic reasons where the large vents/returns will need to be located on the main level. We are a long way from getting an architect involved, but at this point, I would like to be aware of these issues so we can adjust our home-made floor plan accordingly.

Does this help? Thanks!

your HVAC contractor will design and lay out your HVAC system, including duct placement. i suggest you get them to look over your plans/house and see what they say. nothing worse than finding out you have to have a bulkhead for the ducts because you "planned" for it on the wrong end of the room!

worthy, around here ALL duct work is done in attics. very few homes even have a basement and even the ones on crawlspaces have ductwork in the attic. about the only thing you see with ducts below are mobile homes. all attics are well vented to prevent massive heat build up, mine used to get well over 160F before i put in power vents-even with 5 gable vents!

You should have a return-air duct in every room (except maybe laundry and pantry) in a new, custom home.

If I saw such a home, and they only had the one main big one in an upper area and a lower area, like you often see in low-end tract housing, I would run for the hills. It shouts "CHEAP" and it shouts "INEFFICIENT, POORLY DESIGNED HVAC."

As long as you have the room, a fresh air supply and venting for the system(s) they can be placed pretty well anywhere by an expert. They will design the duct runs and sizes accordingly. Just make sure where ever you place them that ducts for supply and return can be visualized going to each room. So, talk with someone now.

It does not "have" to be installed centrally. A lot depends on the house. You certainly have the ability to tell the contractor where you'd "like" it to be, but there may be design elements that make one location better than the other.

We located ours (for our 1 1/2 story home) pretty much in the corner of the basement, beneath the chase leading upstairs.

beware

there are so many variables that are needed to answer your question. however - i do think you are wise to think about hvac - especially the return air ducts - now because we got a big surprise when we started to have our ducts installed.

On the main level, i lost a niche and had another area where i was planning a built in cut in half to make room for the return air ducts - and I was not happy at all.

as far as second story areas, while it might be ideal to not put hvac ducts in the attic, sometimes that is the only way it can be done. we have a geothermal heat pump in a second floor closet (another space I lost) and the ducts were run through the attic space which is vented. they used insulated flex ducting. there just was no other way with our house design.

i think you need to create your preliminry plans and then have an hvac expert look them over to come up with an hvac plan. i bet you will find out that you will either loose space - or need to add space - to accomodate the ducting. make sure they lay in not only the duct dimensions, but any additional framing needed around the ducts.

I would also suggest that you plan for fireplace venting as well. We found out this can also be quite a challenge depending on the house design and location of the units. We lost more space in a main level powder room and a second story bathroom to accomodate a gas fireplace in our foyer which was a must have for the DW.

Good thinking on your part to consider this now.

Thanks sniffdog and others! I realize I may get a rude surprise or two even if I plan to the best of my ability now. I live in a spec house now and it has large return vents in places I HATE. The one on the main floor that I hate is not directly above where the HVAC unit is in my basement, so I am hopeful there's some flexibility that will be helpful to my design.