1 System/2 Zones or 2 Systems

[cinciguy]

Hello - new to the forum,

We are building in Cincinnati 4500 sq ft, 2 story with 1000 sq ft of finished in the basement. It's in Zone 5 and about 6 months of heating and 4 months of ac.

Getting very little help from our builder on the thermal envelope and hvac specs. Basically, we will be sealing the house up very well for air leakage (ZIP system, foam, well caulked). I would like to try to run the HVAC with one system and 2 zones to avoid having to put a second system on the second floor with ducts running through the attic. This would avoid having to condition the attic. (We lived in houses with conditioned ducts running through unconditioned attics and will never do it again). With the one system all the duct work will shoot up from the unit in the basement.

Most of the posts on the forum say - go with 2 systems, the zoning will never be right, but I notice a lot of the posts are for remodels or replacing old systems. Does anyone support the one system/2 zone model for a house this size. Or is it just a bad idea?

[garyed]

I wouldn't even consider trying to use one system if I was building a 4500 sq. ft. two story house. The only reason to zone is to save up front costs on the systems & you usually pay for it in the long run. Its harder on your compressor, more prone to failure & you don't have a spare system if one breaks down like you do with two separate systems. My own rule of thumb is to never zone an area that is big enough to have its own system. If you want each room to have its own separate control then zoning is what you need but if you're just looking to save money by using zoning then I would advise against it.

[cinciguy]

Thanks - that's what I expected.

What about the problem of running the ducts through the attic. I can insulate the rafters but that's not my first choice.

[CraziFuzzy]

If a single system could supply the entire house in the inter-floor space, then I see no reason why 2 systems couldn't do the same in the same space. Nothing is requiring the upstairs system to be in the attic. Personally, if I was building a 4500 sq ft house, it'd have a mechanical room with the water heater, air conditioning/furnace(s), electric panels, water filter, etc all grouped together neatly. No reason both systems can't be in that room, and duct in the space between floors - feeding up to the 2nd from one system, and down to the 1st from the other.

[cinciguy]

Thanks - I have considered that but the second floor furnace would have to be in the basement with ducts running up 2 floors walls. With the joists, I couldn't have the ducts travel between the floors, but this would be good. My HVAC contractor wants to put the second system in a closet on the second floor and shoot all of the ducts to the bedrooms through the attic with flexduct.

[BaldLoonie]

Foam insulation! Foam the roof deck and the attic will stay about 10° above the house temp. Otherwise you'll have 140° air penetrating ducts in the attic.

[cinciguy]

BaldLoonie - I notice you are close in Indy. Are you seeing a lot of foam going into roof decks in your area? My builder says he's never heard of it, but reading online indicates that it's happening more than fiber on the attic floor. Would you go with two systems then, foam the roof deck?