How VAV Boxes Work? A Variable Air Volume box is typically installed in HVAC systems in commercial buildings and provides heating and cooling for occupants.

There are different types of Variable Air Volume boxes but how a Variable Air Volume box works is fundamentally the same from one Variable Air Volume box type to another.

Essentially a Variable Air Volume system allows an HVAC systems design engineer a way to zone large areas in workspaces such as office buildings and other places where there are large spaces that need heating and cooling or simply just cooling in rare cases.

Furthermore, the concept is simple but making it energy efficient and keeping everyone comfortable has taken some time from the inception of the Variable Air Volume box.

How VAV Boxes Work

Thermostat and Controls

A Variable Air Volume (VAV) box, is a part of an HVAC system in commercial buildings. A VAV box is integral to the ductwork connecting primary ductwork to secondary ductwork.

In many cases, a Variable Air Volume box is nothing but an assembly of a sheet metal box with a damper and controls inside a control panel. A thermostat connects to the controller while the controller positions the damper based on programmed inputs and variables that include airflow and temperature.

The VAV box regulates the volume of the air to the zone by opening or closing a damper, therefore controlling the volume of air in a specific zone. The primary air comes from a central VAV air handler through the primary duct system and the VAV Box distributes the air into a secondary duct system with the zone it controls. In a VAV system, it is all about controlling air flow and room temperature.

Heat Source | How VAV Boxes Work | HVAC Zoning

Some Variable Air Volume boxes also have a heat source inside the box. The heat source in many Variable Air Volume boxes is typically electric but can also be a hot water coil. The thermostat and the controller will also turn on the heat whenever the temperature in the space falls below the set point, as determined by the thermostat and pre-programmed temperature limitations in the programming. How VAV Boxes Work | HVAC Zoning

Usually, the building engineer or maintenance staff controls the temperature limitations in the program. Consequently, this prevents occupants from running the thermostat above certain settings that are unreasonable and possibly overload the HVAC system. Lastly, these limited setpoints are usually within reasonable ranges for most peoples’ comfort factors.

Typical Sequence of Operation for a VAV Box - Non-Fan Powered Heating and Cooling Mode

Heating Mode Sequence of Operation (Non-Fan Powered)

1. On a call for heat from the thermostat, a signal is sent to the DDC controller.
2. The DDC controller (attached to the VAV box) sends a response to the damper actuator. The VAV damper rotates to a minimum position reducing airflow.
3. The DDC controller also sends a signal to an actuator that controls hot water flow to the hot water coil. The actuator opens the valve the coil receives hot water.
4. Based on all the variables such as temperature and airflow, the DDC controller, sending an analog signal to the damper actuator, will modulate the damper until a desirable position is determined by inputs into the DDC controller. Inputs are CFM’s, thermostat temperature and setpoint, and supply air temperature (SAT). An SAT temperature sensor is located in the secondary duct after the flow ring. A controller that is not tuned (PID) will modulate erratically.
5. When the thermostat is satisfied, the box resumes normal operations based on the best possible conditions to satisfy any demand. The hot water valve actuator closes the valve.

In DDC, you have different types of control. See the link above to understand various types of control. e.g., a hot water valve actuator can be a two-position actuator, or it can be a modulating actuator. The two-position actuator responds with a simple static voltage of 24 volts.

However, the modulating actuator has different types of response control. It can be milliamps or a zero to ten DC volt signal. As one may surmise, the modulating type of actuator offers better performance and more options but is the more expensive type of actuator.

Cooling Mode Sequence of Operation (Non-Fan Powered)

1. On a call for cooling the thermostat sends a signal to the DDC control on the VAV box.
2. The VAV damper will modulate to a maximum position increasing conditioned airflow from the VAV air handler.
3. As with heating, all variables are taken into consideration. If the temperature is deficient for the load, the DDC controller for the VAV box will send a signal to the DDC controller for the VAV air handler. Essentially, it is a nag signal. It nags the DDC controller for the air handler to reduce the temperature setpoint for the supply air being delivered in the primary duct. If some basic parameters are met, the DDC Controller for the VAV air handler will reduce the setpoint giving the boxes colder air. That helps satisfy demand while also minimizing mechanical cooling. That saves energy and lowers utility bills.
4. When the setpoint of the thermostat is satisfied the box resumes normal operation and stops nagging the VAV air handler controller.

The whole control sequence is set up to maximize efficiency, satisfy comfort parameters for occupants, and reduce energy use. Welcome to the age of computer control and algorithms.

VAV Air Handlers | How VAV Boxes Work

Flow Ring inside a VAV Box - How Do VAV Boxes Work?

The air that is supplied to a Variable Air Volume box in a VAV system comes from a large air handler and central ductwork. Therefore, the air handler must have the ability to supply the Variable Air Volume boxes with a variable amount of air. The dampers in the Variable Air Volume system will modulate to different positions.

The positions fluctuate on pressure setpoint requirements and demand. When the box is in cooling mode the damper will modulate to a nearly fully open position as air conditioning or cooling requires more airflow. Furthermore, heating requires less airflow, so the damper will modulate to a minimum position.

The air handler blower fan needs to modulate to vary the airflow depending on demand. Typical Variable Air Volume boxes have a flow sensor inside them to measure airflow. The airflow and temperature variables control damper position based on temperature demands and C.F.M. demands.

Another air pressure sensor inside the main duct that supplies air to the VAV system measures the pressure inside the duct. There is a pressure set point inside the control program for the air handler. The air handler controls will ramp the blower up and down depending on the duct pressure. Furthermore, this is to maintain the duct pressure setpoint requirement in the program.

Energy Efficiency | How VAV Boxes Work

Big gains have been made in the last few decades because of Direct Digital Controls or DDC. These are computer controls retrofitted into the HVAC system. Controllers with microprocessors are linked together over a network.

The controllers can work stand-alone, but most are networked to each other, a master controller, and possibly to a server. The server allows monitoring and control of all the HVAC equipment. Furthermore, DDC can also be integrated into the building security and lighting systems for added benefits.

A VAV air handler supplying multiple VAV boxes with air and cooling can be precisely controlled with DDC to save energy. From controlling the cooling setpoint to controlling the airflow, DDC systems have proven themselves to be very efficient over the older methods, which were typically pneumatic controls. Schedules can be introduced to minimize use when demand is very low.

e.g., An office building where the workers will be in the building from 8 a.m. to 7 p.m. Furthermore, the controls can be set up to schedule the system to shut down during the off-hours. Additionally, also programming for a morning warm-up can occur, so all the spaces are at the desired temperature when the workers arrive. How VAV Boxes Work | HVAC Zoning

Conclusion | How VAV Boxes Work

Finally, in conclusion, Variable air volume systems provide comfort for many people working in commercial buildings. The VAV system is different than the air conditioning and heating systems in our homes. However, these systems work well and provide good comfort for commercial buildings.

Lastly, the systems provide zoning for large work areas such as office spaces and other large work areas and office buildings. More information on Variable Air Volume Systems.

How VAV Boxes Work

Resource: The Fundamentals of HVAC Direct Digital Control