Your savings will probably be greatest if the rooms you want to close off are near the air handler/furnace. Because of their proximity to the source of the conditioned air, these rooms would normally be warmest in winter and coolest in summer. By closing off the air supply to them, you can redirect more conditioned air to other areas of your home. This will let  you improve the comfort in the spaces you use most often without the need to increase energy use.

If your duct joints aren't well sealed, pressurization caused by shutting supply vents at the end of a duct run (in the room) can cause conditioned air to leak through the duct joints. Wrapping the joints in aluminum foil duct tape will help reduce this problem. (Do NOT use regular duct tape to seal duct joints. It dries out too quickly and loses its effectiveness.)

If you have duct dampers, you should use them instead of shutting room vents. Located  inside the ducts, these devices can be closed to keep air from entering the ducts they control. This eliminates the potential the for duct pressurization problems mentioned above. If you have duct dampers, you will see a small metal tab on the duct about a foot from the main trunk. If the tab is lined up with the duct, the damper is open. To close the damper, turn the tab 90 degrees.

To avoid choking off too much air flow through your system, the rooms you close off should not represent over 20% of your home's total square footage. If you have a heat pump, we do not recommend closing off air supply to any part of your home.