Question

Discuss the two main types of VAV systems ?

”Not very long answers, and don’t write for hand Plz”

Answer 1

Variable air volume (VAV) systems :were developed to be more energy efficient and to meet the varying heating and cooling needs of different building zones. A zone can be a single room or cluster of rooms with the same heat gain and heat loss characteristics.

VAV systems can save as much as 30 percent in energy costs as compared to conventional dual duct systems. In addition, they are economical to install and to operate. Duct sizes and central air handling units are smaller and the design and installation is generally much simpler.

The main duct for a typical VAV system provides cooling only (at approximately 55°F). This is called primary air. Room thermostats control the amount of primary air delivered to each zone through modulating dampers for each zone. These dampers vary the volume of air to each zone according to the cooling needs.

Early VAV systems varied the fan cfm output according to the total need of the zones. The fan was sized for the maximum probable load. As the air volume for the zones varied, the static pressure (SP) in the main duct tended to vary. An SP sensor in the main duct controlled the fan output to maintain a constant supply duct static pressure. The fan output was varied either by fan inlet vanes or by a damper at the fan outlet. These systems were variable volume-constant temperature (VV-CT)

Early VAV systems were cooling only, so a separate source of heat was needed for the outer rooms. This was usually supplied by perimeter heating in the rooms.

These early VAV systems were low-cost to install. However, depending upon the position of the zone dampers, the zones were subject to delivering too much cold supply air, which sometimes created drafts and air noise. These systems were very difficult to balance.

Two main types of vav systems are:

SINGLE ZONE SYSTEM

The first HVAC systems were single zone systems . These systems treat the building as a single zone. They deliver air from a central air handling unit at a constant volume and a varying temperature (CV-VT). The central air handling unit in any HVAC system is the equipment in the mechanical room or on the roof that conditions the primary supply air and delivers it to the spaces.

The single zone system is still used in residences and in small commercial buildings. The single zone system is effective in a small building because the heat gain and heat loss for different areas may not vary a great deal.

Fig: single Zone System

The multizone system : was an early system that was designed to meet the varying needs of different zones. It has a separate supply air duct to each zone in a building. There is a heating coil and a cooling coil in the central air handling unit. Both coils are in operation at the same time. Dampers after the coils mix the hot and the cold supply air to the temperature needed to satisfy each zone.

When heating and cooling occur at the same time it is called bucking because the heating and cooling coils are working against each other. The supply air to each zone is mixed to a temperature somewhere in between the hot and the cold supply air. The multizone system uses too much energy to heat and cool the air at the same time.

Because they waste energy, multizone systems are no longer being installed. They are generally banned by local building codes throughout the country.

Fig: multizone system