Air balancing is the process of adjusting an HVAC system so that each room receives the correct amount of conditioned air as designed by the Manual D duct design. Properly balanced systems deliver even temperatures throughout the building, prevent pressure imbalances, and operate at peak efficiency.
Signs of poor air balance include: some rooms consistently too hot or cold, doors slamming shut from pressure differentials, excessive noise in some registers, and high energy bills despite a functioning system. Causes include improperly sized ducts, leaky ducts, too many closed registers, or improperly adjusted dampers.
Air balancing is performed by a technician using air flow measurement devices (anemometers or flow hoods) and adjusting dampers, register sizes, and system settings. For new installations, air balancing should be documented. Building code in many jurisdictions now requires commissioning documentation including air balance reports for new HVAC systems.