A Constant Air Volume (CAV) system is a type of HVAC air distribution system that delivers a fixed, unchanging airflow rate to one or more conditioned zones. Rather than modulating the volume of air supplied, a CAV system achieves temperature control by varying the temperature of the supply air through heating or cooling coils. This approach represents one of the most established methods of mechanical climate control and remains widely used where simplicity, reliability, and consistent ventilation rates are primary design objectives.
Technical Details
In a CAV system, the supply fan operates at a constant speed, delivering a set airflow rate measured in cubic feet per minute (CFM). A typical single-zone CAV unit might supply anywhere from 400 to 2,000 CFM for a small commercial space, while larger systems serving multiple zones can deliver 10,000 CFM or more. Because the airflow volume does not change, temperature adjustments are made at the air handling unit (AHU) by modulating hot water valves, chilled water valves, or staged electric or gas heating elements.
Key technical characteristics include:
- Fan speed remains constant during normal operation, though some modern installations incorporate Variable Frequency Drives (VFDs) to optimize energy use during startup or to fine-tune static pressure without altering delivered volume.
- Zone-level dampers are not typically modulated for temperature control in basic CAV configurations. When multiple zones with differing load profiles are served, reheat coils at individual zone terminals are often employed to prevent overcooling.
- Static pressure within the ductwork remains relatively stable compared to Variable Air Volume (VAV) systems, simplifying duct design and reducing the risk of pressure-related noise issues.
- Minimum outdoor air ventilation rates, often governed by ASHRAE Standard 62.1, are easier to maintain because supply airflow does not fluctuate.
Common Applications
CAV systems are well suited to spaces with uniform and predictable thermal loads. Typical applications include small office buildings, retail stores, classrooms, and manufacturing or industrial facilities where process requirements demand consistent airflow. They are also preferred in environments where strict humidity control is necessary, such as laboratories, hospital isolation rooms, and data centers, because the constant airflow rate supports more stable moisture removal across the cooling coil.
Relevant Standards and Codes
Design and installation of CAV systems are guided by several industry standards. ASHRAE Standard 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) addresses energy efficiency requirements, including limitations on the use of reheat in CAV systems to reduce energy waste. ASHRAE Standard 62.1 governs ventilation rates for acceptable indoor air quality, which CAV systems inherently support through their fixed airflow delivery. Local mechanical codes, often based on the International Mechanical Code (IMC), further regulate ductwork sizing, fan capacity, and system controls.
Practical Significance
The primary advantage of a CAV system is its straightforward design, which translates to lower initial equipment and installation costs, simpler controls, and reduced maintenance complexity compared to VAV alternatives. However, CAV systems can consume more energy in buildings with highly variable or diverse zone loads, since the fan runs at full volume regardless of demand and reheat may be required to avoid overcooling lightly loaded zones. For this reason, engineers often reserve CAV designs for single-zone applications or spaces with stable occupancy and load patterns, while selecting VAV systems for larger, multi-zone buildings where energy efficiency is a higher priority.