Unfortunately this one’s more complicated than it seems…20 yr HVAC professional here, system design and controls. I’ve zoned LOTS of homes…
The biggest hurdle to the theory of controlling each room individually with the supply vent/grills is- what do you do with the rest of the air? Here’s the conundrum-
Most of your HVAC systems are designed to deliver a specific amount of air when running. For round numbers we’ll say 2000 CFM (5 tons of cooling). Each duct in your home is sized to deliver a specific portion of that total air flow based upon heat load…
Let’s say you only want air to one room and the ducts to that room can only handle 200cfm. What do you do with the other 1800 CFM getting chucked down the pipe?
If you do nothing the pressure rises within the ductwork. They won’t explode but the increased static pressure/load on the blower motor will eventually kill it and possibly your compressor too as it slugs unvaporized/liquid refrigerant back to the compressor. Oh yeah, it’ll sound like a freight train at the supply grill and your evaporator coil will freeze too due to lack of air flow.
Current zone systems overcome this with appropriately sized zones, bypass loops that feed the extra air right back to the return or dump it in a common area, or have integrated system controls to reduce the systems capacity.
Think of it as a fire hose worth of water through a garden hose. It just ain’t gonna work. Not to mention all of the closed supply vents are going to sweat and drip water (zone dampers are upstream in the ductwork and insulated).
Trying to zone a house simply by opening and closing vents is a recipe for disaster.