Should I close AC vents in unused rooms to save on cooling, or does that backfire?

Short answer: on a DUCTED (central) system, closing a few vents in unused rooms saves far less than people think and can even backfire — the blower still pushes the same air, so closing registers RAISES duct pressure, which can force air out through leaks, make the system less efficient, unbalance the airflow, and in a bad case reduce airflow over the coil (risking the coil freezing). It doesn't neatly "cool less space" because the system isn't zoned to know you did it. Closing ONE or two isn't a disaster, but closing MANY is counterproductive. The right way to not cool unused rooms is proper ZONING (a system designed to shut off zones) or, on SPLIT units, simply turning OFF the unit in that room — a split serving one room can just be switched off, which genuinely saves.

Why closing ducted vents backfires: - The blower moves the same air regardless — close registers and the air has nowhere to go, so duct pressure rises. That pushes more air out through duct leaks (wasted), makes the fan work against resistance (less efficient), and unbalances the rest of the house. - Reduced airflow over the coil (if you close a lot) can drop the coil temperature too far and freeze it — weak cooling and a potential fault, the opposite of saving. - It's not zoning — the thermostat and system don't adapt; you've just choked the ducts.

What actually works: - Zoning (ducted): a properly zoned system with dampers and zone controls is designed to shut off areas you're not using — that saves correctly. Retrofitting zoning is a job, not a vent you shove closed. - Split units: if unused rooms are on their own split, just turn that unit off — one split per room means you genuinely stop cooling it, no downside. - Sensible setpoints + closing doors to unused rooms, and keeping the system serviced, save more reliably than choking vents. - Closing one or two registers slightly is usually fine; closing many is where it backfires.

Common mistake: shutting lots of ducted vents to "save," and instead getting an unbalanced, less efficient system (or a frozen coil) with little saving. Use zoning or switch off splits in unused rooms, and have the ducted system balanced and serviced by a proper AC service. Anyone measured a real saving from closing vents, or did it backfire?