We are purchasing a new home (yet to be built) in Denver and our builder sort-of waffled when we asked about a dual HVAC systems. Our house is advertised as very efficient and has LowE windows and an external vapor wrap. It is a 2 story house with a full basement with 2900 sq ft in the upper 2 story's and 1150 sq ft in the basement. From everything I have read, the way to go is a 2 zone system but my builder insists on the single and will not allow a dual system. My question is this: Is the dual HVAC a convenience or does a house that will probably have 4000 sq ft of finished space require it?