Describe a basic zone-level control loop for a VAV system in a BAS.

Describe a basic zone-level control loop for a VAV system in a BAS.

• A. The BAS directly sets the global building temperature regardless of zone loads.
• B. The AHU supply temperature is controlled by the zone thermostat.
• C. A zone thermostat sets a cooling/heating demand, and the BAS modulates the VAV damper to meet the zone load while the AHU supply temperature is controlled.
• D. The VAV damper is fixed and does not respond to zone load.

Answer: (C)

The key idea is that comfort and energy balance begin at the zone, with the zone thermostat setting the desired condition and the BAS translating that into airflow through the VAV box, while the air temperature coming from the AHU is kept under its own plant-level control. The zone thermostat compares the actual zone temperature to its setpoint and generates a cooling or heating demand. The BAS then modulates the VAV damper to change how much supply air reaches that zone, so the zone load is met by adjusting airflow rather than trying to force a specific temperature everywhere. The supply air temperature from the AHU is handled by a separate control loop that maintains a target supply temperature or follows economizer logic, independent of the zone’s immediate demand. This separation lets different zones with different loads be treated individually while the overall plant maintains its temperature characteristics. Why the other ideas don’t fit: having the BAS directly set the global building temperature ignores the localized comfort needs of each zone; letting the zone thermostat control the AHU supply temperature would couple zone and plant controls inappropriately and can cause instability and conflict between zones; and a fixed VAV damper would not respond to changing zone loads, defeating the purpose of zone-level control.

The key idea is that comfort and energy balance begin at the zone, with the zone thermostat setting the desired condition and the BAS translating that into airflow through the VAV box, while the air temperature coming from the AHU is kept under its own plant-level control. The zone thermostat compares the actual zone temperature to its setpoint and generates a cooling or heating demand. The BAS then modulates the VAV damper to change how much supply air reaches that zone, so the zone load is met by adjusting airflow rather than trying to force a specific temperature everywhere. The supply air temperature from the AHU is handled by a separate control loop that maintains a target supply temperature or follows economizer logic, independent of the zone’s immediate demand. This separation lets different zones with different loads be treated individually while the overall plant maintains its temperature characteristics.

Why the other ideas don’t fit: having the BAS directly set the global building temperature ignores the localized comfort needs of each zone; letting the zone thermostat control the AHU supply temperature would couple zone and plant controls inappropriately and can cause instability and conflict between zones; and a fixed VAV damper would not respond to changing zone loads, defeating the purpose of zone-level control.