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What is the difference between VAV and CAV? What does Constant Pressure Control involve?

14/03/2023 Benny Van Moeseke

When desgning a ventilation system, we often come across the terms "VAV", "CAV" and "Constant Pressure Control". In this article I would like to explain what these terms mean exactly, when which option is used and what you need to pay attention to when designing and adjusting an automated ventilation system.

What is the difference between VAV and CAV? What does Constant Pressure Control involve?

Modern ventilation systems for homes or (commercial) buildings often use a central ventilation group or air handling unit (AHU) that may or may not be equipped with a heat recovery system. In a home, this central ventilation system is usually located in the attic, while in a (commercial) building it is usually installed outside on the roof or somewhere next to the building. Attached to this central ventilation system is a network of air ducts, each of which has a ventilation grid at the end. In the past, these grids were often manually adjusted per room to obtain the desired flow rate. Nowadays, there are electrically operated grids. Even better is to provide a fully automatic VAV or CAV control valve at the end of the duct, right before the grid.

Constant pressure control
Whether we control with a manually adjustable grid or with an automated VAV / CAV damper, by controlling the dampers per room or per zone, we are obviously not there yet. If the central ventilation system, equipped with one or more fans, does not adjust its speed according to the total demand of fresh air within the network, then controlling the dampers would simply result in more draft and noise entering the room through the grids. It is therefore necessary to install a differential pressure controller in the main duct, just before the central ventilation unit. This controller guarantees that the speed of the fans gets adjusted in relation to the total demand. Only thanks to this constant pressure control will the position of the dampers actually result in the desired air volume for each individual room. 

Some central ventilation systems have a built-in differential pressure controller, while others have an analog or digital input to which a control signal from an external differential pressure controller can be sent. Sentera offers a wide range of differential pressure controllers. Most of them can be equipped with a Pitot tube, which allows air velocity measurement. This is even more convenient and often more accurate than differential pressure measurement.

VAV dampers for variable air volume
For reasons of energy efficiency, we prefer demand based ventilation of the rooms in our home or in our building. This means that we will no longer work with manually adjustable grids, but will use sensors to measure the air quality in a room and ventilate just enough to maintain good air quality. After all, ventilating more than necessary would often involve loss of heat (in winter, that is), and that is obviously irresponsible, both for environmental and economic reasons. The room sensor, duct sensor or sensor built into the control damper will open or close the VAV-damper position proportionally to the measured air quality. The worse the air quality, the further the valve opens and vice versa. This creates a variable air volume (VAV), related to the measured air quality.

CAV dampers for constant air volume
Unlike a VAV damper that provides a variable air volume according to the air quality, a CAV damper ensures that a room, or a zone receives a predetermined volume of fresh air. A constant air volume (CAV) is often used because it is mandatory, or because it is difficult to measure the air quality in a particular space and ventilate demand-based as is the case with a VAV damper. Every time changes happen in other rooms connected to the same central ventilation group, the CAV damper is going to notice a pressure difference and adjust its valve position to maintain the desired constant air volume setpoint for this particular room.

Can we combine VAV and CAV within one ventilation system?
Yes, it is possible to combine both systems. For example, a demand-based VAV control could be used for landscape offices and meeting rooms and a CAV control for production rooms or workshops. After all, constant pressure control takes into account the sum, the total demand of fresh air for the entire building. Regardless of whether it is VAV or CAV.

What do we need to watch out for?
Minimum flow rate
If duct sensors or sensors built directly into the control damper are used, it is necessary to ensure minimum air circulation to allow the sensors to measure the room air quality. Because if the air from the room does not reach the sensors, obviously no correct measurement is possible.

VAV and CAV consecutively
As mentioned above, it is ok to combine VAV and CAV in parallel.  I.e. that some rooms are controlled by CAV and others by VAV. In some cases however, systems are designed where upstreams in the duct system CAV dampers are used and further down the ducts VAV dampers are used to provide demand based air volume to the different rooms. As you might expect, this creates additional challenges in the balancing process. Partly because CAV dampers need a certain minimum volume (pre-pressure) to keep their air volume constant. If the VAV dampers that are located behind a CAV damper no longer demand enough flow, the CAV damper will have difficulty maintaining its set flow rate.

SenteraWeb cloud services make balancing easier
Needless to say, designing and controlling a central ventilation system can be quite complicated. The fact that Sentera VAV or CAV control valves are equipped with Modbus communication and can therefore be remotely monitored and adjusted can result in enormous time savings. Savings are also made in travel costs and working hours. Even if the customer subsequently wants to make parameter changes or expand the system, large costs can be saved thanks to SenteraWeb cloud services.

Sentera CAV valves
With the ACDPH-125, Sentera has a 125 mm round CAV control valve in its range. Modbus RTU allows it to be remotely controlled and read out or it can be included in your HVAC network. 160 and 200 mm CAV valves are currently under development.

Sentera VAV valves
With the ACT-H-125 and ACT-H-160, Sentera currently has 2 motorised circular VAV valves in the range while a 200 mm version is under development. These ACT-H valves can be controlled via Modbus RTU or via a 0-10 VDC control signal. This makes them compatible not only with Sentera sensors, but also with third-party sensors.  Sentera VAV valves with built-in CO2, TVOC, CO, temperature and relative humidity sensors are currently under development.
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