Years ago, people would save energy use by closing off portions of their homes that were not being used. This occurred usually in the winter when people would huddle around a fireplace or wood stove and close off other non-heated rooms.
Some people still think that by closing off an unused room and its register saves energy. Some people close off all registers except for the one in the living room.
While this may have worked with older, non-insulated homes, it doesn't work with today's energy systems - forced air heating and cooling systems.
According to a 2003 study by the Lawrence Berkeley National Laboratory:
"Closing registers in forced-air heating systems and leaving some rooms in a house unconditioned has been suggested as a method of quickly saving energy for California consumers. This study combined laboratory measurements of the changes in duct leakage as registers are closed together with modeling techniques to estimate the changes in energy use attributed to closing registers.
"The results of this study showed that register closing led to increased energy use for a typical California house over a wide combination of climate, duct leakage, and number of closed registers. The reduction in building thermal loads due to conditioning only a part of the house was offset by increased duct system losses, mostly due to increased duct leakage. Therefore, the register closing technique is not recommended as a viable energy saving strategy for California houses with ducts located outside conditioned space.
"The energy penalty associated with the register closing technique was found to be minimized if registers farthest from the air handler are closed first because this tends to only affect the pressures and air leakage for the closed off branch. Closing registers nearer the air handler tends to increase the pressures and air leakage for the whole system.
"Closing too many registers (more than 60%) is not recommended because the added flow resistance severely restricts the air flow though the system leading to safety concerns. For example, furnaces may operate on the high-limit switch and cooling systems may suffer from frozen coils."