Approach Description of Logic Possible Advantages Possible Disadvantages No Reset Maintain design intake airflow at all conditions. Simple and never underventilates. Overventilation wastes energy. Compliance with airflow requirements may be difficult to validate without airflow measurement. May need CO2 sensing in many zones. Inaccurate CO2 sensing may introduce significant errors. Large population changes can result in small changes in CO2 concentration, especially in high-density zones. Requires base outdoor airflow level for building (Ra). Compliance with airflow requirements may be difficult to validate without airflow measurement. Requires base outdoor airflow level for building (Ra). Zone-Level CO2Based Reset Sense zone(s) CO2 level; increment the outdoor air damper open or closed in proportion to apparent outdoor air needs of the most critical zone. No outdoor airflow calculation required during operation. Supply Air CO2-Based Sense supply air CO2 level with respect to outdoor air; increment outdoor air damper open or closed to maintain supply CO2 concentration at the design level. No outdoor airflow calculation required during operation. Very few CO2 sensing locations needed, and no outdoor airflow calculation needed. Responds to changes in efficiency based solely on airflow values, which are usually available in DDC VAV systems. Solves the MZS equations dynamically to reset intake airflow setpoint. Allows validation of compliance with outdoor airflow requirement. Lowest possible Vot at all conditions since it accounts for both actual population and actual airflow variations. BAS can solve MZS equations. Ensures base outdoor airflow level for building (Ra). TOD scheduling capability is standard for most building automation systems (BAS). Schedules can be entered without adding sensors. Primary airflow values are usually available in DDC VAV systems. BAS can solve MZS equations. Ensures base outdoor airflow level for building (Ra). Only zones with significant unoccupied intervals require occupancy sensors. Primary airflow values are usually available in DDC VAV systems. BAS can solve MZS equations. Ensures base outdoor airflow level for building (Ra). CO2 sensors only needed in zones with highly variable population. Primary airflow values are usually available in DDC VAV systems. BAS can solve MZS equations. Ensures base outdoor airflow level for building (Ra). Airflow-Based Ventilation Reset Control (VRC)1 Sense current zone airflow; using design population and sensed airflows, solve the MZS equations to find current outdoor air intake flow setpoint. No direct adjustment for changes in zone population. VRC With Population Counting2 Count people and sense airflow in each zone; use these actual values to solve the MZS equations to find current outdoor air intake flow setpoint.1 Accurate people-counting systems may be prohibitively expensive, impractical, or not readily available. VRC and Population Estimating Based on Schedules Estimate zone population based on time-of-day schedule and sense airflow in each zone; use estimated (rather than actual) population and actual airflow to solve the MZS equations to find current outdoor air intake flow setpoint. Population estimates based on TOD can be very inaccurate, leading to under- or overventilation at many conditions. VRC and Occupancy Sensing Estimate zone population based on binary occupancy sensors (design population or zero population) and sense airflow in each zone; use estimated population and actual airflow to solve MZS equations to find current outdoor air intake flow setpoint. Use CO2 sensors to estimate breathing zone outdoor airflow currently required; sense airflow in each zone; use estimated breathing zone outdoor airflow and actual discharge airflow to solve the MZS equations to find current outdoor air intake flow setpoint. Population never goes to zero in many zones so savings beyond VRC-only may be small. VRC and CO2-Based Breathing Zone Outdoor Air Estimating Inaccurate CO2 sensing may introduce significant errors. Large population changes can result in small changes in CO2 concentration, especially in highdensity zones. 1 Tables 3 and 4 show the results of using this approach 2 Tables 5 and 6 show the results of using this approach, assuming each zone includes accurate people-counting capability Table 7: Some possible multiple-zone systems (MZS) dynamic reset approaches. 32 ASHRAE Journal ashrae.org March 2010