When designing a ventilation system, terms like "VAV" (Variable Air Volume), "CAV" (Constant Air Volume), and "Constant Pressure Control" often come up. In this article, I will explain these concepts, when to use each option, and what to consider when designing and adjusting an automated ventilation system.

 

What are VAV and CAV?

 

Modern ventilation systems in homes and commercial buildings often use a central ventilation group or air handling unit (AHU), which may include a heat recovery system. In homes, this system is usually located in the attic, while in commercial buildings, it is typically installed outside on the roof or next to the building. The central ventilation system connects to a network of air ducts with ventilation grids at the ends. Traditionally, these grids were manually adjusted per room to achieve the desired flow rate. Nowadays, there are electrically operated grids, and even better, fully automatic VAV or CAV control valves placed at the end of the duct before the grid.

 

 Constant Pressure Control

 

Regardless of whether we use a manually adjustable grid or an automated VAV/CAV damper, we must ensure that the central ventilation system's fans adjust their speed according to the total demand for fresh air within the network. Otherwise, controlling the dampers alone would result in increased draft and noise in the rooms. To address this, a differential pressure controller is installed in the main duct before the central ventilation unit. This controller ensures that fan speed adjusts according to total demand, enabling the dampers to achieve the desired air volume for each room. Some central ventilation systems have built-in differential pressure controllers, while others accept control signals from external differential pressure controllers. Sentera offers a wide range of differential pressure controllers, some of which can measure air velocity using a Pitot tube, providing convenience and accuracy.

 

VAV Dampers for Variable Air Volume

 

For energy efficiency, we prefer demand-based ventilation in homes or buildings. This means using sensors to measure room air quality and adjusting ventilation to maintain good air quality, avoiding unnecessary heat loss. Room sensors, duct sensors, or sensors built into control dampers adjust the VAV-damper position based on air quality. Poor air quality opens the valve further, creating a variable air volume (VAV) that adjusts to the measured air quality.

 

CAV Dampers for Constant Air Volume

 

In contrast to VAV dampers, which provide variable air volume based on air quality, CAV dampers ensure a predetermined volume of fresh air for a room or zone. Constant air volume (CAV) is often used when mandatory or when it's challenging to measure air quality for demand-based ventilation. CAV dampers adjust their position to maintain a set air volume when changes occur in other rooms connected to the same central ventilation group.