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What Is Constant Air Volume?

Ray Hawk
Ray Hawk

Constant air volume (CAV) is a method for climate control systems in buildings that supplies a steady level of air flow at two different temperatures, and maintains a standard air pressure throughout the system. Since it is one of the ways that a standard heating, ventilation, and air-conditioning (HVAC) system is built, the actual function of the machinery can vary based on the needs of the building. The alternative to a constant air volume system is a variable air volume (VAV) system. While both are used in building design, each has a few unique advantages over the other.

The main challenge in using a constant air volume system is that it usually requires two sets of duct-work throughout a building — one for heated air and one for cooled air. These air supplies are provided by either individual air handling units, or one central unit known as a CAV box, that can switch back and forth between heating and cooling of the air that it pumps into the duct-work. Individual rooms or businesses in an office building then control the room temperature by regulating the mix of hot and cold air that is allowed into the room. This gives a constant air volume system the advantage over its alternative in that room temperatures can be changed more rapidly than they can with a VAV system.

A digital thermostat.
A digital thermostat.

Variable air volume systems tend to be much more complex than their CAV counterparts because they must employ temperature controls and manipulate the actual volume of air that is pumped into each room. Dampers and fans with variable speeds are built into the duct-work to facilitate this, and system controls automatically cycle them open and closed, or on and off, as needed. While a VAV system is more difficult to design and implement, it is more energy efficient than a CAV system, since components of the variable air design only run as needed. The air supply from a constant air volume system is continuously supplied at quantities that meet the thermostat settings of the building.

Incorporating a CAV system into a building can be physically more extensive to construct from the start, but it is often used in smaller buildings with simple HVAC needs or large buildings with open floor plans, such as those with auditoriums, gyms, or lecture halls. Publicly-utilized spaces often have CAV systems for climate control, such as shopping malls, government centers, and libraries. The constant air volume approach is built upon machinery that is easy to maintain and relatively maintenance-free as compared to a VAV system. It works best for spaces such as large, open sports arenas with little restriction on air flow. If a building layout is changed over time to provide increasingly smaller rooms, a CAV system is modified to provide multi-zone capabilities that make it more like a VAV system, with each room given more individual treatment for environmental controls.

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Do CAV boxes require disconnect switches per California code?

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    • A digital thermostat.
      By: Dave Friedel
      A digital thermostat.