How VAV Boxes Work? A Variable Air Volume box is typically installed in commercial buildings and provide 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 a 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. The concept is simple but making it energy efficient and keeping everyone comfortable has taken some time from the inception of Variable Air Volume box.
How VAV Boxes Work
Thermostat and Controls
In many cases, a Variable Air Volume box is nothing but 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 air flow and temperature.
Some Variable Air Volume boxes also have a heat source inside the box. The heat source in many Variable Air Volume box 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.
Usually, the building engineer or maintenance staff controls the temperature limitations in the program. This prevents occupants from running the thermostat above certain settings that are unreasonable and possibly overload the HVAC system. These limited set points are usually within reasonable ranges for most peoples’ comfort factor.
How VAV Boxes Work | VAV Air Handlers
The air that is supplied to a Variable Air Volume box comes from a large air handler and central ductwork. 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 are based 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 air flow. Heating requires less air flow so the damper will modulate to a minimum position.
The air handler blower fan needs to modulate to vary the air flow depending on demand. Typical Variable Air Volume boxes have a flow sensor inside them to measure airflow. The air flow 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 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. This is to maintain the duct pressure set point requirement in the program.
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 that are usually retrofitted into the HVAC system. Controllers with microprocessors are linked together over a network. The controllers can work stand-alone but most are linked to each other, a master controller, and possibly to a server. The server allows monitoring and control of all the HVAC equipment. 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 set point to controlling the air flow 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. The controls can be set up to schedule the system to shut down during the off hours. It can also be programmed for a morning warm-up so all the spaces are at the desired temperature when the workers arrive.
How VAV Boxes Work | Conclusion
Finally, in conclusion, Variable air volume systems provide comfort for many people working in commercial buildings. The 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. The systems provide zoning for large work areas such as office spaces and other large work areas and office buildings.
How VAV Boxes Work