Overview
This lecture covers Variable Air Volume (VAV) systems, focusing on their components, control strategies, zoning, and methods for optimizing energy efficiency in commercial HVAC applications.
VAV System Fundamentals
- VAV systems adjust the volume of supply air (CFM) to each zone based on temperature demand.
- Air handlers provide constant 55°F (13°C) supply air, varying the airflow according to the combined needs of all zones.
- Two airflow control levels: zone-level VAV boxes and system-level air handler.
VAV Box Operation & Modes
- VAV boxes contain a damper controlled by an actuator and a temperature sensor in the space.
- Three basic modes: Cooling (damper modulates 30–100% open, heat valve closed), Dead Band (damper at minimum for ventilation), Heating (damper at minimum, heat valve modulates).
- Airflow sensors measure velocity pressure (velocity pressure = total pressure - static pressure).
Air Handler & Fan Control
- Fan speed is managed by a Variable Frequency Drive (VFD) based on static pressure feedback from the supply duct.
- VFD slows when pressure increases (VAV dampers close) and speeds up when pressure drops (VAV dampers open).
- Static pressure sensor maintains minimum pressure (e.g., 1.25" static pressure) at setpoint.
Zoning Concepts
- Zoning divides a building into areas with similar load profiles, each served by its own VAV box.
- Zones may be grouped by window exposure, interior vs. exterior, or similar thermal demands.
- Proper zoning reduces system cost and improves comfort and efficiency.
- Air handler is sized for the building’s maximum block load (peak combined load).
Reheat & Ventilation
- Reheat coils (hot water, steam, or electric) are used mainly in exterior zones for heating.
- Minimum CFM settings ensure required ventilation is met (per codes such as ASHRAE 62, Title 24).
- During heating, VAV box maintains minimum airflow and reheat activates as needed.
Control Strategies for Energy Efficiency
- Constant Static Pressure Control: Maintains set static pressure in supply duct by adjusting VFD.
- Static Pressure Reset: Lowers static pressure setpoint so that the most demanding VAV box damper is nearly open, reducing fan energy (required by codes in some regions).
Key Terms & Definitions
- VAV (Variable Air Volume) — HVAC system that varies air supply to each zone according to temperature demand.
- CFM (Cubic Feet per Minute) — Unit of airflow measurement.
- VFD (Variable Frequency Drive) — Device that adjusts fan speed to control airflow and pressure.
- ASHRAE 62 — Code specifying minimum ventilation requirements.
- Block Load — Combined peak heating/cooling load for all zones served by an air handler.
- Dead Band — Mode where neither heating nor cooling is needed; damper set to minimum for ventilation.
- Static Pressure — Air pressure in the supply duct, used to control fan speed.
Action Items / Next Steps
- Review your local building codes for VAV design and minimum ventilation requirements.
- Study control strategies for VAV systems, focusing on static pressure reset and energy optimization.
- Prepare for questions on VAV zoning logic and component functions for upcoming assessments.