6.6.5 Zoning of HVAC Systems

Large buildings usually have multiple HVAC (heating, ventilation, airconditioning) zones, each zone with its own air-handling unit and duct system. In practice, these zones are not completely separated if they are on the same floor. Air circulates among zones through plenum returns, hallways, atria, and doorways that are normally left open. Depending upon the HVAC design and operation, airflow between zones on different floors can also occur through the intentional use of shared airreturn/supply systems and through air migrations via stairs and elevator shafts.

Isolating the separate HVAC zones minimizes the potential spread of an airborne hazard within a building, reducing the number of people potentially exposed if there is an internal release. Zone separation also provides limited benefit against an external release, as it increases internal resistance to air movement produced by wind forces and chimney effect, thus reducing the rate of infiltration. In essence, isolating zones divides the building into separate environments, limiting the effects of a single release to an isolated portion of the building. Isolation of zones requires full-height walls between each zone and the adjacent zones and hallway doors.

Another recommendation is to isolate the return system (i.e., no shared returns). Strategically locate return air grills in easily observable locations and preferably in areas with reduced public access.

Both centralized and decentralized shutdown capabilities are advantageous. To quickly shut down all HVAC systems at once in the event of an external threat, a single-switch control is recommended for all airexchange fans (includes bathroom, kitchen, and other exhaust sources). In the event of a localized internal release, redundant decentralized shutdown capability is also recommended. Controls should be placed in a location easily accessed by the facility manager, security, or emergency response personnel. Duplicative and separated control systems will add an increased degree of protection. Further protection may be achieved by placing low-leakage automatic dampers on air intakes and exhaust fans that do not already have back-draft dampers.