Variable Air Volume Systems


What distinguishes a variable air volume system from other types of air delivery systems is the use of a variable air volume box, as shown above, in the ductwork. The variable air volume box is interlocked with a thermostat, and can then control the temperature in a space. As a space requires cooling because of a large cooling load, the damper in the VAV box will be placed in the full open position, to allow the maximum volume of air to enter the room, as shown in the left picture. As the space comes closer to its setpoint temperature, the damper inside the VAV box begins to close, thus decreasing the volume of air delivered to the room, as shown in the second illustration. The damper may close to provide only 15-30% of the original design cfm for a space. By regulating the volume of air delivered, a variable air volume system can control the temperature of a space without changing the temperature of the supply air.

There are several variations on the traditional VAV system, including VAV boxes with fans and heating coils. These variations were created to counteract some of the negative effects of a traditional VAV system, which will be discussed below.

VAV System Schematic

The graphic above outlines the major components included in a variable air volume system. The above diagram shows the air handling unit, which includes the mixing box, filter, heating and cooling coils, fan, and the ductwork. In addition to the air handling unit, the illustration shows the VAV box and the diffuser. All of these elements will be discussed below.

In addition to the components shown above, the VAV system also used chilled water and hot water in the coils to accomplish heating and cooling. Chillers, boilers, and heat exchangers provide the mediums required for heating and cooling. A description of these components can be found on the HVAC components page of this website.

Air Handling Unit: The air handling unit is the location where air is conditioned to an acceptable level for distribution within the building. An air handling unit is composed of several components and subsystems, defined as follows:

1. Mixing Box: The mixing box is the location where return air and outdoor air are mixed to satisfy outdoor air requirements set forth in the ASHRAE standards.

2. Filter: The filter removes particulates from the air. The amount and size of particulates removed depends on the rating of the filter.

3. Cooling Coil: The cooling coil cools the air to the desired temperature. In addition to cooling the air, moisture is removed from the air in a dehumidification process based on the properties of air.

4. Heating Coil: The heating coil can have a hot water or steam medium. The primary purpose of this coil is to heat the air to a required temperature.

5. Humidifier: A humidifier adds moisture to the air in the heating mode, if required. Humidifiers are commonly served by a steam manifold.

6. Fan: The fan pressurizes the air so it can travel through the ductwork to the space requiring the air.

7. Ductwork: The ductwork serves as a pathway for the transportation of the air from the air handling unit to the space.

Diffuser: The diffuser is the component of the system that ultimately delivers the air to the space. A diffuser regulates velocity of the entering air, throw pattern, and volume of entering air through the use of an integral volume damper. In VAV systems, linear slot diffusers are usually preferred to traditional square diffusers. Since linear slot diffusers have a plenum attached to the register, they can more evenly distribute air at times when the delivered air volume is low. If a traditional square diffuser is used on a VAV system, a puddling effect often occurs where the cold air just drops to the floor, instead of being circulated around the room.

VAV Box Components:

VAV Box with Inlet and Actuator

Casing: See diagram above for description.

Inlet: The inlet is a round connection to the VAV box from the often square supply ductwork.

Damper: The damper is a component which can open or close to vary the volume of air passed through the VAV box.

Damper Shaft: The damper shaft is the operational tool that allows the damper to open or close. The shaft is connected to a motor (actuator), which is interlocked with the room thermostat.

Fan: A fan is a component of a fan powered VAV box, which is not shown above. The fan helps to improve air velocity during times of low volumetric flow rates.

Reheat Coil: A heating coil can be added to a VAV box to improve air volume and velocity when a room nears its design temperature.

Thermostat: The thermostat is placed within the space and allows the occupant to control the temperature. The thermostat is interlocked with the damper shaft to control the position of the damper.

Types of VAV Systems:

Traditional: A traditional VAV system consists of a VAV box with a damper to control the volume of air delivered to a space. When the space approaches design conditions, the damper may close to provide only 15% of the design cfm.

Fan Powered VAV: The addition of a fan to a VAV box improves air movement at times when a space is near its design temperature and supply air volumes are low. The fan will usually be set to begin running when the damper is 50% closed. The addition of a fan, however, reduces the efficiency of the system because the fan motor requires electric input for power.

VAV with Reheat: A reheat coil (hot water or electric) can be added to a VAV box to improve air volume and movement when a space approaches design temperature. When the damper closes to a determined position (usually 50% of the design cfm), the reheat coil becomes operational and heats the air. Since the supply air is now warmer, a larger amount will have to be delivered to the space, which improves the volume of air and velocity.

Advantages of a VAV System:

A variable air volume system is highly efficient because the minimal amount of air required is used to keep a space at its design temperature. When the damper closes in a VAV box, the fan in the air handling unit can run at a slower speed (through the use of a variable speed drive) and the amount heating and cooling mediums running through the coil can be reduced (through electric or pneumatic control valves). In essence, a VAV system allows the air handling unit, chillers, and boilers to run a part load, which is more efficient.

2. Individual Temperature Control

Since a VAV box is linked to a thermostat, the ability to control temperature in a space is independent of other spaces. Rooms with similar loading patterns are often placed on the same VAV box, and through the use of a VAV system, areas with very different loading patterns can be placed on the same air handling unit.

VAV systems are very cost effective in operation, but also offer a fairly low first cost. Since traditional VAV systems do not require piping or coils, they are cheaper to install and require simple electrical installation.

4. Flexibility

VAV systems are highly flexible because the boxes can easily be removed from one ductwork branch and placed into another, assuming that the design cfm for the spaces are similar.

Disadvantages of a VAV System:

As a VAV system reaches its design setpoint, the volume of air delivered to a room is decreased. This decrease in air volume is a problem because the outdoor air requirements of 20 cfm/person are not met, the velocity of air is decreased resulting in discomfort, and if a space requires positive pressure the needs cannot be met with a VAV system. To combat this trend, the addition of a fan or heating coil to the VAV box can alleviate many of these problems, however there is a greater expense involved.

The installation of a VAV box requires considerable space, both in the vertical and horizontal directions. As a rule of thumb, the linear duct length before a VAV box should be three times the diameter of the inlet. This length is required for the air profile in the duct to even out before entering the box. In the vertical direction, the VAV box can require up to 18, which can be a problem if above ceiling heights are relatively small.

3. Inability to Simultaneously Heat and Cool

In many building situations, it is common to require heating in some parts (usually at the perimeter) and cooling in others (usually interior). Since a traditional VAV system does not have a heating coil in the box, heating in one space and cooling in another cannot take place. This problem can be overcome by adding a coil to the VAV box.

4. VAV Box Location

Since VAV boxes must be located in the branch ductwork, they are often placed above the ceiling. In situations where an acoustical ceiling is used, there is easy access to the box. However, in situations where a plaster ceiling is present, an access panel must be provided to allow access to the box.

Typical Uses For a VAV System:

VAV systems are commonly used in offices because of their efficiency and ability to grant independent temperature control. A VAV system is also rather flexible, as the boxes can easily be moved into new ductwork branches to accommodate office renovations. Offices are usually not as pressure sensitive as other scenarios (such as hospitals or laboratories), so the deficiencies in pressure associated with a VAV system are not a large concern.

2. Auditoriums

VAV systems work well in auditoriums because these spaces are occupied at intermittent intervals. When the space is occupied, the damper in the VAV box is completely open to combat the cooling load. However, when the auditorium is not in use, the dampers will be mostly closed, allowing the rest of the mechanical system to run at part load.

VAV systems are commonly used in stores because of their efficiency. Since a store might have occupancy only 8 hours of the day, having the mechanical system running at part load for the unoccupied hours can provide quite a savings. In addition, pressurizing issues are not as important in stores as in other cases.

Inappropriate Uses for a VAV System:

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