Zoned system - layman
I've read that you shouldn't buy a HVAC system that is too large for your house.
But, if you have a zoned system, and you turn off the zone for one floor in a three story house, doesn't that result in the same problem--two large a system for the remaining floors?
The short answer is no...if designed properly.
Someone will be along shortly to go more in depth if you desire...but wifey says I gotta go to the grocery store right now
most zone systems incorporate a bypass damper, as well as a discharge air temp. sensor
when set-up properly the system will deliver proper CFM
as well as having overcooling or overheating protection