For starters, I live in Atlanta, Ga and I am not an HVAC pro so this is a serious question that I would like answered for one of my clients. I understand the premise of a zoned system and on the surface, it sounds like a good idea. Then I considered the fact that I have always told my customers that it is a bad idea to close off vents in rooms they are not in because of duct leakage. Also, the second law of thermodynamics simply states the hot goes to cold, so now the unheated areas are doing their best to rob warm air from the heated areas. Finally, if your zoning a single system, aren't you creating on oversizing issue because the unit (that was probably oversized in the first place) is now servicing a smaller are than what it was designed for?
I look forward to your input.
Some of the benefit to 2 stage A/C units is the "oversized coils" in relation to compressor size when the unit is operating on low stage. If a 2 stager unit is 2ton/3ton you have a 2ton A/C system most of the time with 3 ton sized condenser and evaporator coils. More coil surface area = better heat transfer.
Thanks for the info Brad, it was very helpful. What actually got me to ask this question came from a recent client that spent a bunch of money adding zoning to an existing system. Oversizing is a common issue here because no one did manual J's because there were very few building codes (and even fewer enforced) during the 90's building boom. This guys actual reason for doing the zoning was due to serious comfort issues in a large bonus room over the garage. He was talked into zoning by a HVAC contractor. During my audit, I found ducts running to that area that were disconnected and the rest were not properly sealed so that contractor never even inspected the condition of the current system. The homeowner has used the zoning and weatherstripping on the door to completely shut off that room and they only use it when they have company (sad but true). This has now created other issues. Basically, the homeowner wasted a bunch of money and is now reluctant to spend more money to actually fix his problems.
I will definitely keep your info for future reference.
2 floors should have 2 separate systems in most cases. It just makes more sense because it reduces ductwork lengths and system complexity. If one system fails the occupants can resided in the other part of the house until they can get the broke system serviced.
Our house is a single story and I have "redneck zone control" installed. It amounts to a damper in the attic that I manually adjust for summer or winter since we like having the master bedroom cooler than the rest of the house. Simple, low cost, and effective.