Many homes have rooms that go unoccupied for long periods of time. Guest rooms, bedrooms and basements make up a large percentage of the air volume in the average home. So it seems like a good idea: if the room is unused, why not close the air registers and doors to save energy? The reality is that closing vents can actually waste energy, reduce the efficiency and lifespan your furnace and AC, while making your living space less comfortable.
When your central heating and cooling system was installed your HVAC technician carefully measured the volume of air in your entire home, then matched the size of your furnace and air conditioner to provide the most efficient performance. Reducing the air volume by closing vents will not make your AC cool less or the heating system heat less, it will simply send the same amount of air to the open air ducts and vents throughout your home.
In addition, during normal operation a home's ductwork will lose 20-30% of the air though leaks. Closing vents will simply cause more conditioned air to leak, wasting energy.
Closing air vents also increases the pressure inside the ductwork, which in turn will make the blower fan work harder to push the air through. The result is an increased risk of the AC evaporator coil freezing or a furnace's heat exchanger overheating.
Of course, closing one or two air vents is not going to cause major issues, but for each vent that is closed the performance of your central heating and cooling system will be diminished.