Ventilation (outside air) must be provided to every occupied building. Supplying a certain amount of outside air prevents the indoor air from becoming stale and unhealthful. Building codes specify the amount of ventilation air that must be introduced by the HVAC system.
An HVAC system conditions the supply air to provide the spaces with an acceptable combination of humidity and temperature within the comfort zone. It also provides a sufficient amount of outside air for ventilation. The effectiveness of the system depends upon two factors:
To heat or cool a space, these two factors are combined in different ways depending upon the type and design of the particular HVAC system. The combinations are:
As the indoor air temperature varies, the humidity also increases or decreases. The indoor humidity generally remains within the comfort zone. However in dry climates, many HVAC systems have a humidifier unit in the central air handler to increase the moisture level in the conditioned air when it is needed. In humid climates, systems may have a means of removing moisture from the supply air.
In the past, large commercial buildings were usually designed with a central light well. This admitted light and air to the inner rooms. As building and land costs increased and HVAC systems developed, this inner light well was eliminated. Now buildings are designed with a core of inner rooms where the light well used to be (Figure 2). The development of this inner core created a new air conditioning problem. The core spaces do not require heating, but do require cooling and ventilation.
The rooms along the outer walls require either heating or cooling as well as ventilation. These outer rooms are also called perimeter spaces.
Consider the needs of different zones in a typical office building which have different heat loss and heat gain characteristics. The offices are arranged around the outer walls of the building, with an inner core of rooms that have no outside exposure (Figure 2).
The outer rooms have at least one wall exposed to the outside temperatures. This means that these offices have more heat loss and heat gain than the interior rooms:
The outer rooms gain or lose heat at a varying rate. For example, when the sun strikes one side of a building, that side has more heat gain than the sides that are shaded. The position of the sun, color of the wall, insulation, amount of glass, and shading all affect this solar heat gain.
The core of the building gains heat from the interior load such as people, lights, and equipment. Therefore, these spaces generally do not require heating except for a top floor where heat is lost through the roof. The normal condition is that they gain too much heat and therefore require cooling when occupied.
The cooling load for both core spaces and outer spaces depends on many factors such as these:
Since different spaces have different rates of heat gain and heat loss, it is impossible for an HVAC system that delivers the same temperature of air at a fixed volume to every space
to provide comfort conditions for all of them. Therefore, heating and cooling must be supplied at varying rates to different zones of the building. A zone is a space or group of spaces in a building with similar requirements for heating and cooling. All rooms in a zone can be supplied with the same temperature supply air at the same flow rate.
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.
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.
This system also uses too much energy because the hot air and cold air are bucking each other. Therefore the dual duct system that mixes hot and cold air is now generally banned.
The dual duct system also has other problems. The cold duct usually requires most of the supply air. This results in less flow in the hot duct at times and therefore a higher hot duct static pressure. When a zone called for heating, the high static pressure in the hot duct resulted in a high cfm that created drafts and noise in the conditioned spaces.
HIGH PRESSURE MIXING BOXES
To solve the problems of non-constant airflow rate in a dual duct system, a high pressure mixing box was developed. This replaced the mixing dampers. The mixing boxes control the cfm to a constant flow rate. The boxes change the system into a true constant volume-variable temperature (CV-VT).
The dual duct mixing boxes require a high static pressure to operate - usually a minimum of 1.5 inches wg. The system itself generally requires about 3.0 inches wg duct pressure to operate properly. The high fan horsepower required to maintain the high static pressure, plus the bucking condition, means a high energy usage. The cost of operating the dual duct system was too high.
Another problem was that, because of the higher pressure often present in the hot duct, the hot air might flow back through the mixing box and into the cold duct. This could raise the air temperature in the cold duct so that the supply air could not cool the spaces adequately.
LOW PRESSURE REHEAT BOXES
Later, low pressure reheat units for the zones were developed. The supply air had to be cold enough to meet the needs of the zone with the greatest cooling load. The supply air to all other zones had to be reheated. There was no temperature control unless the boiler was operating. In the summer when the boiler was normally turned off, the system could only deliver cold air that was produced by the central cooling system. Often the conditioned spaces were too cold.
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 55Â°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.
Each terminal unit (Figure 8) receives primary air from the central air handling unit at the same temperature (about 55Â°F). The terminal unit contains a primary-air damper (a butterfly damper) which modulates (changes position) according to signals from the automatic control system. (A modulating damper is not just open or closed. It can be at any position in between.) The primary-air damper regulates the volume of cold primary air delivered to the terminal unit according to the needs of the spaces. This is also the volume of secondary air delivered to that space. Figure 8 shows only the key features of a terminal unit.
The big advantage of VAV systems with terminal units is that they are able to meet the comfort requirements of different zones in a building without heating and cooling at the same time.
PRESSURE DEPENDENT OR PRESSURE INDEPENDENT
VAV systems are either pressure dependent or pressure independent.
The first VAV terminal units were pressure dependent. They had no means for limiting the quantity of supply air.
In pressure dependent systems, the volume of air supplied by the terminal unit varies depending upon the static pressure (SP) in the primary air duct. The primary-air damper in the terminal unit is controlled by a thermostat in the space. However, the airflow through the damper varies according to the SP in the main duct. Terminal units that are close to the supply fan are likely to supply too much primary air. Terminal units that are farthest from the supply fan are not likely to supply enough primary air.
Pressure independent terminal units have flow-sensing devices that limit the flow rate through the box. They can control the maximum and minimum cfm that can be supplied and are therefore independent of the SP in the primary air duct.
Almost all HVAC systems installed or retrofitted at present have pressure independent VAV terminals. Pressure independent systems can be balanced and will allow the correct airflow from each terminal.
VARYING FAN SPEED
Because each terminal unit regulates its primary air volume independently, the volume (cfm) of primary air delivered by the central air handling unit varies according to the demands of the terminal units in the system. This means that the supply fan in the central air handling unit must vary its output in order to meet the needs of all the terminal units. If the primary-air dampers of most terminal units are full open, the cfm required for the entire system is high. If most terminal unit dampers are closed, the cfm required for the system is much less.
In many current systems, the rpm (speed) of the central supply fan is regulated by the control system to meet the changing demands of the system. A static pressure (SP) sensor in the primary air duct sends a signal to a controller that regulates the fan speed to maintain a constant SP in the primary air duct.
The location of the SP sensor in the primary duct is critical to the performance of the system. It is best placed near the terminal unit that is most difficult to supply. This is the location that has the greatest pressure drop from the fan. If the sensor is placed too close to the supply fan, the SP in the supply duct will be too high during periods of low cfm demand.
Many VAV systems and terminal units have been developed to provide for the particular needs of a building. The following are the commonly used types:
These seven types of VAV systems and their characteristics are summarized in the chart above.
Digital control systems have greatly improved VAV system performance. Digital controls are more accurate, and they can manage more complex functions. In addition, digital controls input information into a central processing unit (CPU). The CPU is a computer that generates reports analyzing system performance. The CPU can also be used to change the parameters of the system by remote control.
Better sensors are used with the control system to measure such factors as temperature, static pressure, and total pressure more accurately and reliably. This allows for faster and more consistent control of the terminal units.
Better fan speed control allows a VAV system to meet varying demands.
Better design of terminal units has simplified and improved them. Design of outlet diffusers has improved to allow more complete penetration of the conditioned air into the space.
Installation and balancing methods have been improved.
At present, most VAV systems are controlled by digital stand-alone control systems. Digital control systems have many advantages:
Excerpted and reprinted from Variable Air Volume Systems by Leo A. Meyer, one of the books in the Indoor Environment Technician's Library series published by LAMA Books. For over 30 years, Meyer has been writing and publishing training materials for the HVAC industry. His books cover a wide range of topics, including heating and cooling, indoor air quality, sheet metal work, electricity basics, safety, and others. For more information, visit www.lamabooks.com.
Publication date: 05/01/2006