It makes sense to close HVAC registers when indoor temperature control becomes an issue. Keep conditioned air out of rooms where you don’t need it to keep the rest of the house comfortable and save money by not wasting unwanted heating and cooling. Unfortunately, it’s not that easy. HVAC units and ductwork systems are designed to deliver a specific volume of air to each room based on its square footage. One or more registers can affect that careful equilibrium. Unintended consequences can occur. HVAC systems don’t know whether all registers are open. As long as the thermostat is set, it produces the same amount of heated or cooled air and consumes the same amount of energy. Each room’s supply air volume is controlled by internal dampers inside the branch ducts. It ensures consistent temperatures regardless of whether a room is near or far from the system blower. When one or more supply registers are closed, airflow balance and temperatures are disrupted. Rooms near the blower may receive too much airflow, while rooms far away may not get enough. The supply registers may be closed to stop airflow, but the return registers remain open. Even without supply air, the return system draws air out of the room. This discrepancy depressurizes the room and/or part of the house. A depressurized room draws unfiltered cold or hot outdoor air into the room through small structural cracks and gaps. This infiltrating air destabilizes room temperature and can degrade indoor air quality. Closing supply vents increases static pressure inside the supply ductwork which in turn stresses the system blower, potentially leading to shorter service life of that component.