HVAC air systems can be either centralized or decentralized. Centralized air systems receive their cooling and heating energy from a remote central plant. Decentralized HVAC air systems contain the central heating and cooling plant equipment within the air system itself. HVAC air systems can also be constant air volume (CAV) or variable air volume (VAV). CAV systems deliver constant supply airflow at a variable temperature. VAV systems deliver variable supply airflow at a constant temperature. CAV and VAV air systems can be further subdivided into systems that condition a single temperature zone and systems that condition multiple temperature zones.
Whether the HVAC air systems are centralized or decentralized, CAV or VAV, serve a single zone or multiple zones, the choices for heating and cooling coils within the units are the same. Generally, the heating coil in centralized HVAC air systems will be hot water, steam, or electric, and the cooling coil will be chilled water. The heating coil in decentralized HVAC systems will be electric, direct or indirect gas-fired or (reverse cycle) direct expansion (DX) refrigerant (for heat pumps only); the cooling coil will be DX refrigerant.
Single-Zone
A single-zone CAV system consists of an air handling unit that delivers constant supply airflow (Fig. 1). The heating and/or cooling capacity of the unit is modulated by varying the temperature of the supply airflow to meet the needs of a single thermostat mounted in one of the spaces served by the unit.
Single-zone CAV systems may serve multiple spaces; therefore, it is important to ensure that all of the spaces served by a single-zone CAV system have similar HVAC load characteristics.
Heating and Ventilating
Heating and ventilating (H&V) systems are used to serve spaces that require heating only and also require outdoor air ventilation for occupant ventilation or exhaust air makeup. H&V units are often configured to deliver 100% outdoor air. However, they may have return air capabilities if the outdoor airflow is less than the supply airflow that is required to provide effective heating of the spaces served during the winter. If the units heat the spaces they serve during unoccupied periods when the exhaust systems are off and the outdoor air dampers are shut, H&V units may also be equipped with return air capabilities. H&V units normally recirculate 100% of the air during the unoccupied mode of operation.
During the summer it is common for H&:V units to position their outdoor and return air dampers to deliver 100% outdoor air to the spaces served in order to maintain the space temperature at 5 to 10°F higher than the outdoor temperature. If the H&:V units have return air capabilities, provisions must be made within the spaces served to either relieve or exhaust the excess outdoor air that is introduced during the summertime mode of operation.
Temperature control for H&:V systems is the same as for the single-zone CAV system described earlier, except the space thermostat will control the heating capacity of the H&V unit only. Some common examples of spaces requiring H&V systems are gymnasiums, locker rooms, and kitchens:
Multiple-Zone
CAV systems are not well suited to provide multiple zones of temperature control because serving multiple zones with CAV systems incurs a higher energy cost and, in some cases, a higher first cost than the VAV options that are normally available. However, through the use of reheat, dual-duct, and multizone configurations, CAV systems can serve multiple temperature .zones for certain applications, particularly renovations of existing systems.
Reheat
A CAV reheat system consists of a CAV unit whose supply air ductwork branches out to serve multiple zones within the area served by the unit (Fig. 2). A reheat coil is mounted within the branch duct to each zone. Typically, the discharge air temperature from the CAV unit is maintained at approximately 55°F so that it can meet the cooling needs of the temperature zones, if required. A heating-only thermostat located in each zone controls the heating output of its associated zone reheat coil. This coil reheats the supply air from the CAV unit as required to maintain the setpoint of the zone thermostat. If no heating is required in the zone, the heating coil will be shut off. The zone reheat coil needs to be sized not only to meet the building envelope heat losses for the zone but also to raise the supply airflow from the discharge air temperature (typically 55°F) to the zone cooling setpoint (typically 75°F).
CAV reheat systems are not energy efficient because simultaneous cooling (at the CAV unit) and heating (at the reheat coil) of the supply airflow occu.rs. Care must be taken when designing CAV reheat systems because the 2018 International Energy Conservation Code, Section 503.4.5, requires the supply airflow to be reduced to at least 30% of the maximum supply airflow to each zone before reheating can occur. One exception to this rule is for .zones that have special humidity control requirements.
Dual-Duct
A dual-duct CAV system includes two separate supply air ducts, one hot duct and one cold duct, connected to a specially configured CAV air handling unit (Fig. 3). The supply airflow is divided within the unit downstream of the supply fan into what are called the hot deck and cold deck. A heating coil is installed within the unit in the hot deck and a cooling coil is installed within the unit in the cold deck. A portion of the supply airflow is blown through the hot deck and disclw:ged through the main hot duct connected to the unit. The remaining supply airflow is blown through the cold deck. and discharged through the main cold duct connected to the unit. Both the hot and cold supply ducts are routed parallel to each other through the building and branch out to serve multiple zones within the area served by the unit.
Each zone is equipped with a dual-duct mixing box, which has both hot and cold duct inlet connections, each of which is equipped with a motor-operated damper and an inlet airflow sensor. The dual-duct mixing box has a single duct outlet through which air is supplied to the zone. The temperature of the air supplied to the zone is controlled by modulating the hot and cold airflows at the dual-duct mixing box as required to maintain the setpoint of the zone thermostat. For CAV dual-duct mixing boxes, the total supply airflow to the zone remains constant.
Typically, the cold deck air temperature from the CAV air handling unit is maintained at approximately 85°F so that cooling is available if required by the temperature zones. The hot deck temperature from the CAV unit is maintained at approximately 55°F so that heating is available if required by the temperature zones. The heating coil in a dual-duct air handling unit can be hot water, steam, or electric. The cooling coil will normally be chilled water. Dual-duct air handling units rarely, if ever, utilize a DX refrigerant cooling coil.
Dual-duct CAV systems generally do not have an application for commercial buildings because of the high first cost. Thus, we will not discuss dual-duct systems in any more detail Dual-duct CAV systems were designed years ago mainly to serve laboratory areas within buildings. Therefore, it is necessary for the HV AC system designer to understand th.is type of system should a project involve the renovation of a building with a dual-duct CAV system. However, today there are more modem HVAC systems available to serve laboratory areas that have a lower first cost and lower operating cost than dual-duct CAV systems. Therefore, it is unlikely for a new building to require a dual-duct CAV system.
Multizone
Multizone CAV systems are similar to dual-duct CAV systems in that there is a hot deck and a cold deck within the air handling unit (Fig. 4). The difference is that the hot and cold airstreams for each zone are mixed at the air handling unit. There is a hot and cold air motor-operated damper mounted on the discharge of the air handling unit for each zone; that is, if the unit is a five-zone unit, there will be five hot deck motor-operated dampers and five cold deck motor-operated dampers mounted on the discharge of the unit. There is a single duct connection on the combined outlet of each hot and cold damper serving each zone through which air is supplied to the zone. The temperature of the air supplied to each zone is controlled by modulating the hot and cold airflows at the air handling unit as required to maintain the setpoint of the zone thermostat. For CAV multizone systems, the total supply airflow to each zone remains constant. The hot and cold deck temperatures for multizone CAV systems are similar to the deck temperatures for dual-duct CAV systems.
Multizone CAV systems are not commonly used in commercial buildings because the VAV options that are available typically have a lower first cost and lower operating cost. Also, the number of zones that can be accommodated by a multizone unit is limited by the physical space available to install the hot and cold motor-operated dampers for each zone on the discharge of the air handling unit. Therefore, multizone units can serve no more than about eight zones. The number of zones that can be served by a VAV system is not limited by the equipment but is determined by the needs of the areas served.
HVAC Design Sourcebook - W. Larsen Angel, P.E., LEED AP, is a principal in the MEP consulting engineering firm Green Building Energy Engineers. He has worked in the MEP consulting engineering industry for more than 30 years. Mr. Angel has contributed to the development of design standards and continues to find new ways to streamline the HVAC system design process.
