5.14 Pressurization and Ventilation

PressurizationPerimeter Zone. A dedicated 100 percent outside air unit shall be used to maintain positive pressure. The ventilation air for the perimeter air-handling unit shall be sized based on maximum occupancy with diversity and shall operate continuously during occupied hours and operate at 40 percent capacity during unoccupied hours. Industrial grade pressure sensors shall be located at several perimeter areas to communicate outside air pressure to maintain differential positive pressure (adjustable). The internal pressure need only be slightly higher than ambient on average to achieve the goal of excluding humid outdoor air from building cavities. In any case, internal pressure shall not be greater than 10 pascals. Maintain supply air discharge at the unit no more than 10°C (50°F) dew point when outside air dew point is above this temperature. Maintain neutral pressure when the outdoor ambient temperature falls below 3°C(37°F) dew point and neutral pressure. Differential pressure sensors and dew point sensors shall be connected to the building automation system. An alarm shall signal if positive or neutral pressures are not maintained, on average, based on multiple samples taken within a five minute period. Only industrial grade sensors are permitted.

Interior Zone. A dedicated outside air-handling unit shall be used to maintain positive pressure. The unit shall be sized based on the fresh air requirements for maximum occupancy with diversity. The unit shall have air monitoring devices and control the exhaust rate during occupied hours to be less than 10 percent of the supply air to ensure positive pressure in the space. The unit shall shut down during unoccupied hours. Maintain 10°C (50°F) dew point when outside air dew point is above that of the outside air. Use humidification equipment, if necessary, to maintain 3°C (37°F) dew point whenever outside dew point is below 3°C (37°F). Also maintain neutral pressure by setting the exhaust air quantity to equal the supply air rate. Air monitoring devices shall be connected to the building automation system to indicate positive and neutral pressure.

Special Ventilation RequirementsVentilation requirements for all building spaces shall comply with ASHRAE 62.

Entrance Vestibules and Lobbies. Sufficient heating and cooling should be provided to offset the base load plus the infiltration load of the space. The entrance vestibule should be positively pressurized relative to atmospheric pressure to minimize infiltration. A separate variable air volume (VAV) system shall serve entrance vestibules and lobby spaces. The VAV system shall operate to vary the flow of air for the space through a differential pressure control system designed to maintain positive pressure relative to the outdoors and neutral or negative pressure relative to adjacent spaces. Also provide air monitoring devices in the unit. The air-handling unit and the variable volume dampers at the VAV boxes shall have self-contained microprocessor controls capable of connecting to and interoperating with a BACnet or LONWORKS direct digital control (DDC) Building Automation System.

Atriums. A dedicated air-handling system shall be provided to control heat gain/loss in the occupiable areas of the atrium. The atrium area should maintain negative pressure relative to adjacent interior and perimeter spaces or zones and positive or neutral pressure relative to adjacent vestibules and lobbies, and positive pressure relative to the outdoors. The design of the HVAC system must be fully coordinated with the smoke control system.

Toilets. Toilet areas must have segregated exhausts and should be negative in pressure relative to surrounding spaces.

Janitor/Housekeeping Closets. Janitor/housekeeping closets must have segregated exhausts and should be negative in pressure to surrounding spaces.

Food Service Areas. Kitchen areas shall be negative in pressure relative to adjacent dining rooms, serving areas and corridors. Tempered make-up air shall be introduced at the kitchen hood and/or the area adjacent to the kitchen hood for at least 80 percent of exhaust air. Duct air velocity in the grease hood exhaust shall be no less than 7.5 to 9 m/s (1,500 to 1,800 FPM) to hold particulate in suspension. Dishwashing areas must be under negative pressure relative to the kitchen, and dishwashers shall be provided with their own exhaust hoods and duct systems, constructed of corrosion-resistant material.

High Occupancy Areas. High occupancy areas, which also have largely variable occupancies, such as conference rooms, lecture theatres, etc., and are served by dedicated ventilation and air-handling systems, shall incorporate a CO2 demand controlled ventilation (DCV) system to minimize energy consumption, while maintaining appropriate levels of ventilation and pressure relationships between spaces and the outdoors. The DCV system devices shall be located for ease of maintenance and shall provide appropriate operation of the ventilation system it is controlling. Enthalpy heat recovery system shall be utilized if economically feasible.

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