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VAV vs CAV — What’s the Difference?

Gul Bahar Shah
  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: High...
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Air Changer Per Hour (ACH)

Gul Bahar Shah

 Air Changer Per Hour (ACH)

In a lot of projects, I still see ACH being selected very early, sometimes based on old projects, rules of thumb, or simply “this is what we usually use”, but ACH is not a universal value you can apply everywhere. When it’s used without really understanding the space, it can create more problems than it solves: Too high → unnecessary fresh air, higher energy use, and humidity issues Too low → poor air quality and uncomfortable spaces What often gets missed is that ACH is just a way to express airflow, it doesn’t tell you why that airflow is needed, but in reality, ventilation should come from: 1- how many people are in the space 2- what kind of contaminants are present 3- requirements from standards like ASHRAE 62.1 ACH works well for certain applications like toilets, parking areas, or some industrial spaces where you’re dealing with dilution. But for offices, data centers, and comfort cooling spaces, airflow should be driven by occupancy, heat loads, and indoor air quality needs, not a predefined ACH assumption Remember, a good engineer underdtands what the space actually needs, then sizing the system accordingly.

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