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This addendum adds requirements to ensure that the air distribution system is capable of delivering outdoor air to the occupied spaces. In systems with AC units, heat pumps, or fan-coil units mounted in the ceiling or floor plenum, ventilation air is often supplied to the plenum to mix with recirculated air. Where outdoor air is poorly distributed in such systems, units that are far from the outdoor air supply to the plenum may not receive the intended levels of outdoor air. In any air distribution system, required airflow can only be assured by measuring actual airflow and adjusting dampers and orifices to account for the effect of as-installed duct pressure drops, i.e., the system must be balanced. Designers must consider and make provision for system balancing. Note that balancing is addressed in terms of designing the system so that it can be balanced; this addendum does not require balancing at any particular time, as that issue is covered in other addenda. Also, note that balancing is not a one-time event but must be reevaluated over time as indicated in other sections of the standard.