**6. Central HVAC systems**

A central HVAC system may serve one or more thermal zones, and its major equipment is located outside of the served zone(s) in a suitable central location whether inside, on top, or adjacent to the building [4, 5]. Central systems must condition zones with their equivalent thermal load. Central HVAC systems will have as several control points such as thermostats for each zone. The medium used in the control system to provide the thermal energy subclassifies the central HVAC system, as shown in **Figure 2**.

The thermal energy transfer medium can be air or water or both, which represent as all-air systems, air-water systems, all-water systems. Also, central systems include water-source heat pumps and heating and cooling panels. All of these subsystems are discussed below. Central HVAC system has combined devices in an air handling unit, as shown in **Figure 3**, which contains supply and return air fans, humidifier, reheat coil, cooling coil, preheat coil, mixing box, filter, and outdoor air.

*6.1.1. Single zone*

mal zone when improperly applied.

**Figure 4.** All-air HVAC system for single zone.

**Figure 3.** Equipment arrangement for central HVAC system.

A single zone system consists of an air handling unit, a heat source and cooling source, distribution ductwork, and appropriate delivery devices. The air handling units can be wholly integrated where heat and cooling sources are available or separate where heat and cooling source are detached. The integrated package is most-commonly a rooftop unit and connected to ductwork to deliver the conditioned air into several spaces with the same thermal zone. The main advantage of single zone systems is simplicity in design and maintenance and low first cost compared to other systems. However, its main disadvantage is serving a single ther-

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#### **6.1. All-air systems**

The thermal energy transfer medium through the building delivery systems is air. All-air systems can be sub-classified based on the zone as single zone and multizone, airflow rate for each zone as constant air volume and variable air volume, terminal reheat, and dual duct [5].

**Figure 2.** Horizontal hierarchy representation of the main types of central HVAC systems.

**Figure 3.** Equipment arrangement for central HVAC system.

#### *6.1.1. Single zone*

The piping system is used to deliver refrigerant, hot water, cooled water, steam, gas, and condensate to and from HVAC equipment in a direct, quiet and affordable way. Piping systems can be divided into two parts: the piping in the central plant equipment room and the delivery piping. HVAC piping may or may not be insulated based on existing code criteria.

A central HVAC system may serve one or more thermal zones, and its major equipment is located outside of the served zone(s) in a suitable central location whether inside, on top, or adjacent to the building [4, 5]. Central systems must condition zones with their equivalent thermal load. Central HVAC systems will have as several control points such as thermostats for each zone. The medium used in the control system to provide the thermal energy sub-

The thermal energy transfer medium can be air or water or both, which represent as all-air systems, air-water systems, all-water systems. Also, central systems include water-source heat pumps and heating and cooling panels. All of these subsystems are discussed below. Central HVAC system has combined devices in an air handling unit, as shown in **Figure 3**, which contains supply and return air fans, humidifier, reheat coil, cooling coil, preheat coil,

The thermal energy transfer medium through the building delivery systems is air. All-air systems can be sub-classified based on the zone as single zone and multizone, airflow rate for each zone as constant air volume and variable air volume, terminal reheat, and dual duct [5].

**Figure 2.** Horizontal hierarchy representation of the main types of central HVAC systems.

**6. Central HVAC systems**

54 HVAC System

mixing box, filter, and outdoor air.

**6.1. All-air systems**

classifies the central HVAC system, as shown in **Figure 2**.

A single zone system consists of an air handling unit, a heat source and cooling source, distribution ductwork, and appropriate delivery devices. The air handling units can be wholly integrated where heat and cooling sources are available or separate where heat and cooling source are detached. The integrated package is most-commonly a rooftop unit and connected to ductwork to deliver the conditioned air into several spaces with the same thermal zone. The main advantage of single zone systems is simplicity in design and maintenance and low first cost compared to other systems. However, its main disadvantage is serving a single thermal zone when improperly applied.

**Figure 4.** All-air HVAC system for single zone.

In a single zone all-air HVAC system, one control device such as thermostat located in the zone controls the operation of the system, as shown in **Figure 4**. Control may be either modulating or on–off to meet the required thermal load of the single zone. This can be achieved by adjusting the output of heating and cooling source within the packaged unit.

internal mixing dampers. It is recommended that one multi-zone serve a maximum of 12 zones because of physical restrictions on duct connections and damper size. If more zones are required, additional air handlers may be used. The advantage of the multi-zone system is to adequately condition several zones without energy waste associated with a terminal reheat system. However, leakage between the decks of air handler may reduce energy efficiency. The

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A terminal reheat all-air system is a multiple zone, which considers an adaptation of single zone system, as shown in **Figure 6**. This can be performed by adding heating equipment, such as hot water coil or electric coil, to the downstream of the supply air from air handling units near each zone. Each zone is controlled by a thermostat to adjust the heat output of heating equipment to meet the thermal condition. The supply air from air handling units is cooled to the lowest cooling point, and the terminal reheat adds the required heating load. The advantage of terminal reheat is flexible and can be installed or removed to accommodate changes in zones, which provides better control of the thermal conditions in multiple zones. However, the design of terminal reheat is not energy-efficient system because a significant amount of extremely cooling air is not regularly needed in zones, which can be considered as waste energy. Therefore, energy codes and standards regulate the use of reheat systems.

The dual duct all-air system is a terminal-controlled modification of the multi-zone concept. A central air handling unit provides two conditioned air streams such as a cold deck and a hot deck, as shown in **Figure 7**. These air streams are distributed throughout the area served

**Figure 6.** Single duct system with reheat terminal devices and bypass units.

main disadvantage is the need for multiple supply air ducts to serve multiple zones.

*6.1.3. Terminal reheat*

*6.1.4. Dual duct*

Although few buildings can be a single thermal zone, a single zone can be found in several applications. One family residential buildings can be treated as single zone systems, while other types of residential buildings can include different thermal energy based on the occupation and building structure. Movements of occupants affect the thermal load of the building, which results in dividing the building into several single zones to provide the required environmental condition. This can be observed in larger residences, where two (or more) single zone systems may be used to provide thermal zoning. In low-rise apartments, each apartment unit may be conditioned by a separate single zone system. Many sizeable single story buildings such as supermarkets, discount stores, can be effectively conditioned by a series of single zone systems. Large office buildings are sometimes conditioned by a series of separate single zone systems.

#### *6.1.2. Multi-zone*

In a multi-zone all-air system, individual supply air ducts are provided for each zone in a building. Cold air and hot (or return) air are mixed at the air handling unit to achieve the thermal requirement of each zone. A particular zone has its conditioned air that cannot be mixed with that of other zones, and all multiple zones with different thermal requirement demand separate supply ducts, as shown in **Figure 5**. Multi-zone all-air system consists of an air handling unit with parallel flow paths through cooling coils and heating coils and

**Figure 5.** All-air HVAC system for multiple zones.

internal mixing dampers. It is recommended that one multi-zone serve a maximum of 12 zones because of physical restrictions on duct connections and damper size. If more zones are required, additional air handlers may be used. The advantage of the multi-zone system is to adequately condition several zones without energy waste associated with a terminal reheat system. However, leakage between the decks of air handler may reduce energy efficiency. The main disadvantage is the need for multiple supply air ducts to serve multiple zones.

#### *6.1.3. Terminal reheat*

In a single zone all-air HVAC system, one control device such as thermostat located in the zone controls the operation of the system, as shown in **Figure 4**. Control may be either modulating or on–off to meet the required thermal load of the single zone. This can be achieved by

Although few buildings can be a single thermal zone, a single zone can be found in several applications. One family residential buildings can be treated as single zone systems, while other types of residential buildings can include different thermal energy based on the occupation and building structure. Movements of occupants affect the thermal load of the building, which results in dividing the building into several single zones to provide the required environmental condition. This can be observed in larger residences, where two (or more) single zone systems may be used to provide thermal zoning. In low-rise apartments, each apartment unit may be conditioned by a separate single zone system. Many sizeable single story buildings such as supermarkets, discount stores, can be effectively conditioned by a series of single zone systems. Large office buildings are sometimes conditioned by a series of separate single zone systems.

In a multi-zone all-air system, individual supply air ducts are provided for each zone in a building. Cold air and hot (or return) air are mixed at the air handling unit to achieve the thermal requirement of each zone. A particular zone has its conditioned air that cannot be mixed with that of other zones, and all multiple zones with different thermal requirement demand separate supply ducts, as shown in **Figure 5**. Multi-zone all-air system consists of an air handling unit with parallel flow paths through cooling coils and heating coils and

adjusting the output of heating and cooling source within the packaged unit.

*6.1.2. Multi-zone*

56 HVAC System

**Figure 5.** All-air HVAC system for multiple zones.

A terminal reheat all-air system is a multiple zone, which considers an adaptation of single zone system, as shown in **Figure 6**. This can be performed by adding heating equipment, such as hot water coil or electric coil, to the downstream of the supply air from air handling units near each zone. Each zone is controlled by a thermostat to adjust the heat output of heating equipment to meet the thermal condition. The supply air from air handling units is cooled to the lowest cooling point, and the terminal reheat adds the required heating load. The advantage of terminal reheat is flexible and can be installed or removed to accommodate changes in zones, which provides better control of the thermal conditions in multiple zones. However, the design of terminal reheat is not energy-efficient system because a significant amount of extremely cooling air is not regularly needed in zones, which can be considered as waste energy. Therefore, energy codes and standards regulate the use of reheat systems.

#### *6.1.4. Dual duct*

The dual duct all-air system is a terminal-controlled modification of the multi-zone concept. A central air handling unit provides two conditioned air streams such as a cold deck and a hot deck, as shown in **Figure 7**. These air streams are distributed throughout the area served

**Figure 6.** Single duct system with reheat terminal devices and bypass units.

**Figure 7.** All-air HVAC dual-duct system.

by the air handling unit in separate and parallel ducts. Each zone has a terminal mixing box controlled by zone thermostat to adjust the supply air temperature by mix the supply cold and hot air. This type of system will minimize the disadvantages of previous systems and become more flexible by using terminal control.

and convectors. However, all-water cooling-only systems are unusual such as valance units mounted in the ceiling. The primary type that is used in buildings to condition the entire

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Fan-coil unit is considerably small unit used for heating and cooling coils, circulation fan, and proper control system, as shown in **Figure 9**. The unit can be vertically or horizontally installed. The fan-coil unit can be placed in the room or exposed to occupants, so it is essential to have appropriate finishes and styling. For central systems, the fan-coil units are connected to boilers to produce heating and to water chillers to produce cooling to the conditioned space. The desired temperature of a zone is detected by a thermostat which controls the water

space is a fan-coil unit.

**Figure 9.** All-water system: fan-coil units.

**Figure 8.** All-air HVAC systems with VAV terminal units.

*6.2.1. Fan-coil units*

#### *6.1.5. Variable air volume*

Some spaces require different airflow of supply air due to the changes in thermal loads. Therefore, a variable-air-volume (VAV) all-air system is the suitable solution for achieving thermal comfort. The previous four types of all-air systems are constant volume systems. The VAV system consists of a central air handling unit which provides supply air to the VAV terminal control box that located in each zone to adjust the supply air volume, as shown in **Figure 8**. The temperature of supply air of each zone is controlled by manipulating the supply air flow rate. The main disadvantage is that the controlled airflow rate can negatively impact other adjacent zones with different or similar airflow rate and temperature. Also, part-load conditions in buildings may require low air-flow rate which reduces the fan power resulting in energy savings. It may also reduce the ventilation flow rate, which can be problematic to the HVAC system and affecting the indoor air quality of the building.

#### **6.2. All-water systems**

In an all-water system, heated and cooled water is distributed from a central system to conditioned spaces [4, 5]. This type of system is relatively small compared to other types because the use of pipes as distribution containers and the water has higher heat capacity and density than air, which requires the lower volume to transfer heat. All-water heating-only systems include several delivery devices such as floor radiators, baseboard radiators, unit heaters,

**Figure 8.** All-air HVAC systems with VAV terminal units.

and convectors. However, all-water cooling-only systems are unusual such as valance units mounted in the ceiling. The primary type that is used in buildings to condition the entire space is a fan-coil unit.

#### *6.2.1. Fan-coil units*

by the air handling unit in separate and parallel ducts. Each zone has a terminal mixing box controlled by zone thermostat to adjust the supply air temperature by mix the supply cold and hot air. This type of system will minimize the disadvantages of previous systems and

Some spaces require different airflow of supply air due to the changes in thermal loads. Therefore, a variable-air-volume (VAV) all-air system is the suitable solution for achieving thermal comfort. The previous four types of all-air systems are constant volume systems. The VAV system consists of a central air handling unit which provides supply air to the VAV terminal control box that located in each zone to adjust the supply air volume, as shown in **Figure 8**. The temperature of supply air of each zone is controlled by manipulating the supply air flow rate. The main disadvantage is that the controlled airflow rate can negatively impact other adjacent zones with different or similar airflow rate and temperature. Also, part-load conditions in buildings may require low air-flow rate which reduces the fan power resulting in energy savings. It may also reduce the ventilation flow rate, which can be problematic to

In an all-water system, heated and cooled water is distributed from a central system to conditioned spaces [4, 5]. This type of system is relatively small compared to other types because the use of pipes as distribution containers and the water has higher heat capacity and density than air, which requires the lower volume to transfer heat. All-water heating-only systems include several delivery devices such as floor radiators, baseboard radiators, unit heaters,

the HVAC system and affecting the indoor air quality of the building.

become more flexible by using terminal control.

*6.1.5. Variable air volume*

58 HVAC System

**Figure 7.** All-air HVAC dual-duct system.

**6.2. All-water systems**

Fan-coil unit is considerably small unit used for heating and cooling coils, circulation fan, and proper control system, as shown in **Figure 9**. The unit can be vertically or horizontally installed. The fan-coil unit can be placed in the room or exposed to occupants, so it is essential to have appropriate finishes and styling. For central systems, the fan-coil units are connected to boilers to produce heating and to water chillers to produce cooling to the conditioned space. The desired temperature of a zone is detected by a thermostat which controls the water

**Figure 9.** All-water system: fan-coil units.

flow to the fan-coil units. In addition, occupants can adjust fan coil units by adjusting supply air louvers to achieve the desired temperature. The main disadvantage of fan-coils is ventilation air and only can be solved if the fan-coil units are connected to outdoor air. Another disadvantage is the noise level, especially in critical places.

#### **6.3. Air-water systems**

Air-water systems are introduced as a hybrid system to combine both advantages of all-air and all-water systems [5]. The volume of the combined is reduced, and the outdoor ventilation is produced to properly condition the desired zone. The water medium is responsible for carrying the thermal load in a building by 80–90% through heating and cooling water, while air medium conditions the remainder. There are two main types: fan-coil units and induction units.

#### *6.3.1. Fan-coil units*

Fan-coil units for air-water systems are similar to that of all-water systems except that the supply air and the conditioned water are provided to the desired zone from a central air handling unit and central water systems (e.g., boilers or chillers). The ventilation air can be separately delivered into space or connected to the fan-coil units. The major types of fan-coil systems, are 2 pipes or 4-pipes systems, as shown in **Figure 10**.

#### *6.3.2. Induction units*

Induction units are externally similar to fan-coil units but internally different. An induction unit induces the air flow in a room through cabinet by using high-velocity airflow from a

**Figure 11.** Air-water HVAC system using induction units.

central air handling unit, which replaces the forced convection of the fan in the fan-coil by the induction or buoyancy effect of the induction unit, as shown in **Figure 11**. This can be performed as mixing the primary air from the central unit and the secondary air from the

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Water-source heat pumps are used to provide considerable energy savings for large building under the extreme cold weather [6]. A building of various zones can be conditioned by several individual heat pumps since each heat pump can be controlled according to the zone control. A centralized water circulation loop can be used as a heat source and heat sink for heat pumps. Therefore, heat pumps can act as the primary source of heating and cooling. The main disadvantage is the lack of air ventilation similar to the all-water systems as in fan-coil units. For a heating process, the boiler or solar collectors will be used to supply heat to the water circulation, while a cooling tower is used to reject heat collected from the heat pumps to the atmosphere. This system does not use chillers or any refrigeration systems. If a building requires a heating process for zones and cooling process for other zones at the same time, the heat pump will redistribute heat from one part to another with no need for a boiler or cooling

Heating and cooling panels are placed on floors or walls or ceilings where can be a source of heating and cooling [7]. It also can be called as radiant panels. This type of system can be constructed as tubes or pipes impeded inside the surface where the cooling or heating media is circulated into the tubes to cool or heat the surface. The tubes are contacted to the adjacent large surface area to achieve the desired surface temperature for cooling and heating process. The heat transfer process is mainly by the radiation mode between the

room to produce a suitable and conditioned air into the room/zone.

**6.4. Water-source heat pumps**

tower operation,

**6.5. Heating and cooling panels**

**Figure 10.** Air-water HVAC system using fan coil units with 4-pipes configuration.

central air handling unit, which replaces the forced convection of the fan in the fan-coil by the induction or buoyancy effect of the induction unit, as shown in **Figure 11**. This can be performed as mixing the primary air from the central unit and the secondary air from the room to produce a suitable and conditioned air into the room/zone.

#### **6.4. Water-source heat pumps**

flow to the fan-coil units. In addition, occupants can adjust fan coil units by adjusting supply air louvers to achieve the desired temperature. The main disadvantage of fan-coils is ventilation air and only can be solved if the fan-coil units are connected to outdoor air. Another

Air-water systems are introduced as a hybrid system to combine both advantages of all-air and all-water systems [5]. The volume of the combined is reduced, and the outdoor ventilation is produced to properly condition the desired zone. The water medium is responsible for carrying the thermal load in a building by 80–90% through heating and cooling water, while air medium

Fan-coil units for air-water systems are similar to that of all-water systems except that the supply air and the conditioned water are provided to the desired zone from a central air handling unit and central water systems (e.g., boilers or chillers). The ventilation air can be separately delivered into space or connected to the fan-coil units. The major types of fan-coil systems, are

Induction units are externally similar to fan-coil units but internally different. An induction unit induces the air flow in a room through cabinet by using high-velocity airflow from a

conditions the remainder. There are two main types: fan-coil units and induction units.

disadvantage is the noise level, especially in critical places.

2 pipes or 4-pipes systems, as shown in **Figure 10**.

**Figure 10.** Air-water HVAC system using fan coil units with 4-pipes configuration.

**6.3. Air-water systems**

60 HVAC System

*6.3.1. Fan-coil units*

*6.3.2. Induction units*

Water-source heat pumps are used to provide considerable energy savings for large building under the extreme cold weather [6]. A building of various zones can be conditioned by several individual heat pumps since each heat pump can be controlled according to the zone control. A centralized water circulation loop can be used as a heat source and heat sink for heat pumps. Therefore, heat pumps can act as the primary source of heating and cooling. The main disadvantage is the lack of air ventilation similar to the all-water systems as in fan-coil units. For a heating process, the boiler or solar collectors will be used to supply heat to the water circulation, while a cooling tower is used to reject heat collected from the heat pumps to the atmosphere. This system does not use chillers or any refrigeration systems. If a building requires a heating process for zones and cooling process for other zones at the same time, the heat pump will redistribute heat from one part to another with no need for a boiler or cooling tower operation,

#### **6.5. Heating and cooling panels**

Heating and cooling panels are placed on floors or walls or ceilings where can be a source of heating and cooling [7]. It also can be called as radiant panels. This type of system can be constructed as tubes or pipes impeded inside the surface where the cooling or heating media is circulated into the tubes to cool or heat the surface. The tubes are contacted to the adjacent large surface area to achieve the desired surface temperature for cooling and heating process. The heat transfer process is mainly by the radiation mode between the

**Figure 11.** Air-water HVAC system using induction units.

occupants and the radiant panels, and the natural convection mode between the air and panels. Temperature restriction is recommended for radiant floor panels, a range of 66–84°F, to achieve thermal comfort for occupants (ASHRAE Standard 55). Radiant ceiling or wall panels can be used for cooling and heating process. The surface temperature should be higher than the air dew point temperature to avoid condensation on the surface during the cooling process. Also, the maximum surface temperature is 140°F for ceiling levels at 10 ft. and 180°F for ceiling levels at 18 ft. This temperature is recommended to avoid too much heating above occupants' heads.

The installation of such systems is often expensive compared to other types as mentioned above, but they can be useful and has a lower running cost mainly because of the surface temperature restriction. A control signal is connected to the thermostat of each zone to manipulate the medium temperature to condition the space. The used medium can be refrigerant or water mixing with inhibited glycol (anti-freeze) instead of plain water to prevent icing inside the tubes for the cooling process. The main advantage is no space required, only a few inches for the panels to be installed and no more collected dirt in the standard ceiling or the ductwork. Many designs are available to produce attractive panels.

**7.3. Local ventilation systems**

by conventional mode [5, 6].

*7.4.1. Window air-conditioner*

*7.4.2. Unitary air-conditioner*

**7.4. Local air-conditioning systems**

Local ventilation systems can be forced systems by using devices such as window fan to allow air movement between outdoor and a single zone without changing in the thermal environment of the zone. Other systems used for ventilation are air circulation devices such as desk or paddle fans to improve thermal comfort of the space by allowing the heat to be transferred

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**Figure 12.** Horizontal hierarchy representation of the main types of local HVAC systems.

A local air conditioning system is a complete package that can contain cooling and heating source, a circulation fan, a filter, and control devices. There are three main types listed below [5, 6].

This system is a packaged device consisting of a vapor compression refrigeration cycle that contains a compressor, a condenser, an expansion valve, and an evaporator, in addition to a fan, a filter, control system and housing. Window air-conditioners can be installed in a framed or unframed opening in building walls and in window openings without any ductwork and distribution the cooling or heating air effectively inside the conditioned space. The air conditioning contains both evaporator and condenser where the condenser is located outside the space while the evaporate is inside the space, however, it serves the entire single zone with the thermal requirements. The heating process can be achieved by adding electric resistance coil in the air conditioning or reversing the refrigeration cycle to act as a heat pump. Many feature designs are produced to provide aesthetical values and improve the quality and response.

It is similar to window air conditioners from the equipment perspective, but it is designed for commercial buildings. It is installed on the exterior wall of the building and generally located
