Vents serve primarily two purposes: to bring air into the heating and air conditioning system, and to push the temperature-controlled air into the building. This is most typically found in duct system air conditioners.
Typically vents that blow the temperature-controlled air back into the building have a small lever. This allows one to open and close the vent as needed. While this feature may allow for better room-specific control, closing the vent can increase the energy costs associated with the system. Most AC systems operate on a pressure-based system. Thus, when a vent is closed, the pressure ration is altered. This often leads to an increase in the amount of work the system must perform in order to output at the same rate. More work for the system means a higher energy bill.
In addition, closing vents can lead to greater leakage in the AC's ductwork, a compromised compressor, unwanted condensation and even a frozen air conditioner coil. As a result, it is best to leave any adjustable vents in the building open. If the temperature for a specific room is uncomfortable, it is best to adjust it with either blankets or more clothes, or with fans if it is too hot.