Contrary to popular belief, closing off vents with today's high-efficiency heating and air conditioning systems results in little energy savings and can actually raise your energy bill. Increased suction in the return air ducts pulls in outdoor air from cracks around windows and exterior doors. Sealing off the room's return air duct can reduce this outdoor air filtration.
Closed vents cause increased system suction resulting in heated or cooled air seeping into basements, crawl spaces and floor cavities through improperly sealed seams on heating duct work; up to 15 percent of the heating or air conditioning is wasted instead of flowing into the rooms you use.
With a well-designed HVAC system, closing off too many rooms disturbs the balance of air flow throughout the home. Over time, the increased strain damages the system by causing it to work too hard to distribute the air.