Constant Volume Air Handling

In the ever-evolving landscape of HVAC engineering, constant volume air handling systems remain a cornerstone of reliable climate control for commercial and institutional buildings. Their ability to maintain consistent airflow while adapting to varying thermal demands makes them particularly valuable for applications requiring stable ventilation rates.

System Architecture and Components

The foundation of an effective constant volume air handling system lies in its thoughtfully sequenced control arrangement. Figure 1 illustrates a comprehensive four-channel control system serving a single zone, with channels controlling a logical progression:

1. Heating (via hot water valve)
2. Heat recovery (via face and bypass dampers)
3. Fresh air introduction (“free cooling”)
4. Mechanical cooling (via chilled water valve)

This arrangement optimizes energy efficiency by prioritizing passive or lower-energy strategies before engaging mechanical cooling. Heat recovery components typically appear in applications with high minimum fresh air requirements, such as densely occupied spaces where ventilation demands are substantial.

It’s worth noting that in some configurations, the primary controlled parameter shifts from space temperature to supply air temperature, particularly when the air handler serves multiple zones with local temperature control.

Operational Mechanics

The heart of the system lies in space temperature control (θr), typically measured in the return air duct to ensure accurate readings from well-mixed air at higher velocities. This approach delivers more responsive control compared to room-based sensors in relatively still air.

The system manipulates four key variables to maintain desired conditions:

• Heating water flow via diverting valve (VH)
• Air distribution at the heat recovery exchanger through damper positioning (DHR)
• Fresh/recirculated air ratio via mixing dampers (DFA)
• Chilled water flow through a diverting valve (VC)

During warm weather, the system maximizes free cooling by fully opening fresh air dampers and bypassing heat recovery. However, this strategy reaches its limit when fresh air enthalpy exceeds return air enthalpy-a condition that triggers the system to return to minimum fresh air position while engaging heat recovery for pre-cooling. This sophisticated enthalpy-based control requires additional sensor modules (he and hf) but delivers significant efficiency benefits.

Frost Protection Strategies

Two critical frost protection mechanisms safeguard system integrity during cold weather:

1. Heat Exchanger Protection: A dedicated upstream frost coil with thermostatic control (FTH) prevents ice formation on the heat recovery exchanger.
2. System-Wide Protection: A secondary frost thermostat (FTF) provides fail-safe protection by shutting down fans if heating plant failure occurs during freezing conditions. Many installations add automatic intake damper closure to prevent natural drafts through idle equipment.

Typical setpoints position the heat exchanger protection (FTH) at 5°C and the system-wide protection (FTF) at a slightly lower 3°C threshold.

The Intelligence of Sequencing

Perhaps the most elegant aspect of constant volume systems is their control sequencing logic (Figure 2). This approach ensures all control actions remain mutually exclusive while prioritizing energy-efficient strategies:

1. During low-temperature conditions, heating modulates while heat recovery remains maximized, fresh air stays minimal, and cooling remains off.
2. As temperature rises, heating gradually reduces until it deactivates completely.
3. Heat recovery then begins modulating while maintaining minimum fresh air and keeping cooling off.
4. When heat recovery reaches its minimum position, free cooling engages through increased fresh air introduction.
5. Only after maximizing free cooling does the system activate mechanical chilled water cooling.

This sequential approach guarantees minimum energy consumption while maintaining comfort. Engineers often incorporate strategic dead zones between heating and cooling operations, creating a buffer that prevents simultaneous operation and establishes separate winter heating and summer cooling setpoints-further enhancing energy efficiency across seasonal transitions.

Conclusion

Constant volume air handling systems demonstrate how thoughtful engineering can balance consistent ventilation with energy-efficient temperature control. Through strategic sequencing of heating, recovery, ventilation, and cooling operations, these systems deliver reliable performance while minimizing energy consumption-a particularly valuable attribute in today’s sustainability-focused building environment.

While variable air volume (VAV) systems have gained popularity for their additional energy-saving potential, constant volume systems remain the preferred choice for applications where consistent air delivery takes precedence, such as laboratories, healthcare facilities, and specialized industrial environments.