VAV vs CAV — What’s the Difference?

 VAV vs CAV — What’s the Difference?

VAV (Variable Air Volume) and CAV (Constant Air Volume) are two HVAC air distribution strategies: CAV delivers a fixed airflow with varying temperature, while VAV varies airflow to match load, saving energy and offering better zone control.

🔹 Constant Air Volume (CAV)

• Operation: Supplies a fixed airflow rate at all times; adjusts supply air temperature to meet load.

• Advantages:

  • Simple design and controls

  • Lower first cost (approx. $5–10 per CFM)

  • Reliable for spaces needing constant ventilation (labs, hospitals, clean rooms)

• Limitations:

  • Energy inefficient (requires reheating cooled air)

  • Limited zone control (system-wide only)

  • Higher operating costs over time

🔹 Variable Air Volume (VAV)

• Operation: Maintains nearly constant supply air temperature; airflow varies by zone demand using VAV boxes.

• Advantages:

  • Significant energy savings (30–50% compared to CAV)

  • Individual zone control

  • Lower operating costs; payback in 3–7 years

• Limitations:

  • Higher first cost (approx. $8–15 per CFM)

  • More complex controls (requires BMS and VFD fans)

📊 Quick Comparison Table

Feature	CAV	VAV	Winner
Energy Efficiency	Baseline, reheating wastes energy	30–50% savings	VAV
First Cost	$5–10/CFM	$8–15/CFM	CAV
Operating Cost	Higher	Lower	VAV
Zone Control	System-wide only	Individual zones	VAV
Complexity	Simple	Advanced (BMS required)	Depends on project
Best Use	Labs, hospitals, clean rooms	Offices, schools, hotels, retail	Depends on need

Variable airflow per zone in VAV

In a Variable Air Volume (VAV) system, variable airflow per zone means each space receives only the amount of conditioned air it needs — no more, no less — based on its real‑time thermal load.

🔹 How It Works

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• VAV box installed in each zone modulates a damper to adjust airflow.

• Thermostat feedback signals the damper to open or close depending on temperature.

• Supply air temperature remains nearly constant, while airflow varies.

• Fan speed control via VFD reduces energy use when overall demand drops.

🔹 Benefits

• Energy efficiency: Less fan power and reduced reheat energy.

• Comfort control: Each zone maintains its own temperature setpoint.

• System flexibility: Ideal for offices, hotels, and multi‑zone buildings.

• Reduced noise: Lower airflow in lightly loaded zones minimizes sound levels.

🔹 Example

If Room A faces the sun and Room B is shaded:

• Room A’s VAV box opens wider → higher airflow.

• Room B’s VAV box closes partially → lower airflow. Both rooms stay at their desired temperature without wasting energy

• Choose CAV for critical environments requiring constant ventilation (e.g., hospitals, labs).

• Choose VAV for multi-zone commercial buildings (offices, schools, hotels) where energy efficiency and zone comfort are priorities.