ASHRAE Journal - October 2011 - 22

Optimal start (Section 6.4.3.3.3) and zonelevel demand-controlled ventilation (Section 6.4.3.9) are mandatory requirements of ASHRAE/IES Standard 90.1-2010.6 Fan-pressure optimization (Section 6.5.3.2.3), supplyair-temperature reset (Section 6.5.3.4), and system-level ventilation reset (Section 6.5.3.3) are prescriptive requirements of the standard.

VAV Air-Handling Unit With Flow-Measuring OA Damper OA
• Reset Outdoor Airﬂow

RA SA

zone, the controls can determine that this amount CO2 OCC TOD of reset is too much, and either change the SAT setpoint back to 55°F (13°C), or lower the SAT DDC/VAV Terminals Communicating BAS setpoint a degree or so and see if that eliminates • Required Outdoor Airﬂow (TOD, OCC, CO2) • New OA Setpoint (Vot ) • Actual Primary Airﬂow (Flow Ring) the overheating problem. Per ASHRAE Standard 62.1 Ventilation optimization. In a typical VAV system, the VAV air-handling (or rooftop) unit Figure 5: Ventilation optimization (DCV at zone level + ventilation reset at delivers fresh outdoor air to several, individually system level). controlled zones. Demand-controlled ventilation (DCV) involves resetting intake airﬂow in response to varia- lers and solves the ventilation reset equations (prescribed by tions in zone population. Although commonly implemented ASHRAE Standard 62.15) to determine how much outdoor air using carbon dioxide (CO2) sensors, occupancy sensors or must be brought in through the system-level OA intake to sattime-of-day (TOD) schedules can also be used. isfy all zones served. Finally, the BAS sends this outdoor airOne approach to optimizing ventilation in a multiple-zone ﬂow setpoint to the VAV air-handling (or rooftop) unit, which VAV system is to combine these various DCV strategies at the modulates a ﬂow-measuring outdoor-air damper to maintain zone level (using each where it best ﬁts) with ventilation reset this new ventilation setpoint. at the system level. “Occupied standby” mode. When an occupancy sensor is In the example system depicted in Figure 5, CO2 sensors are used in combination with a time-of-day schedule, the sensor installed only in those zones that are densely occupied and ex- can be used to indicate if the zone is unoccupied although the perience widely varying patterns of occupancy (such as con- BAS has scheduled it as occupied. This combination can be ference rooms, auditoriums, or a lounge area). The VAV con- used to switch the zone to an “occupied standby” mode. troller resets the ventilation requirement for that zone based In this mode, all or some of the lights in that zone can be on the measured CO2 concentration. shut off, the temperature setpoints can be raised or lowered Zones that are less densely occupied or have a population by 1°F to 2°F (0.5°C to 1°C), and the ventilation requirement that varies only a little (such as private ofﬁces, open plan of- for that zone can be reduced, typically to the building-related ﬁce spaces, or many classrooms) are probably better suited ventilation rate, Ra, required by Standard 62.1. for occupancy sensors. When a zone is unoccupied, the VAV In addition, for a VAV system the minimum airﬂow setting controller lowers the ventilation requirement for that zone, of the VAV terminal can be lowered to avoid or reduce the typically to the building-related ventilation rate, Ra, required need for reheat. This minimum airﬂow setting is typically set by ASHRAE Standard 62.1. to ensure proper ventilation. However, when nobody is in the Finally, zones that are sparsely occupied (such as open of- room and with the ventilation requirement reduced, the miniﬁce areas) or have predictable occupancy patterns (such as mum airﬂow setting can be lowered signiﬁcantly during this cafeterias or auditoriums) may be best controlled using a time- occupied standby mode. This reduces both reheat and fan enof-day schedule. This schedule can either indicate when the ergy use. zone will normally be occupied versus unoccupied, or can be When the occupancy sensor indicates that the zone is again used to vary the ventilation requirement based on anticipated occupied, these settings are switched back to normal occupied population of that zone. mode. These various zone-level DCV strategies can be used to reset Variable airﬂow during heating. The conventional way the ventilation requirement for their respective zones. This zone- to control a VAV reheat terminal has been to reduce primary level control is then tied together using ventilation reset at the airﬂow as the zone cooling load decreases. When primary airsystem level (Figure 5). In addition to resetting the zone ventila- ﬂow reaches the minimum setting, and the cooling load contion requirement, the controller on each VAV terminal continu- tinues to decrease, the reheat coil is activated to warm the air ously monitors primary airﬂow being delivered to the zone. and avoid overcooling the zone. When this occurs, the airﬂowThe BAS periodically gathers the current ventilation re- modulation damper typically maintains a constant heating airquirement and primary airﬂow from all the VAV control- ﬂow.
22 ASHRAE Journal ashrae.org October 2011

ASHRAE Journal - October 2011

Table of Contents for the Digital Edition of ASHRAE Journal - October 2011