When a system is designed to provide independent control in different spaces, each space is called a
“zone.” A zone may be a separate room. A zone may also be part of a large space. For example, a
theater stage may be a zone, while the audience seating area is a second zone in the same big space
this need for zoning leads us to the four broad categories of air-conditioning systems, and
consideration of how each can provide zoned cooling and heating.
The four systems are:
1. All-air systems.
2. Air-and-water systems.
3. All-water systems.
4. Unitary, refrigeration-based systems.
System 1: All-air Systems :-
All-air systems provide air conditioning by using a tempered flow of air to the spaces. These all-air
systems need substantial space for ducting the air to each zone.
Q (Btu) =Constant * mass flow* temperature difference
Q (Btu) =Constant * cfm * (°Fzone - °FSupply air)
To change the heating or cooling capacity of the air supply to one zone, the system must either alter
the supply temperature, °F, or alter the flow, cfm, to that zone.
Reheat system: The simplest, and least energy efficient system, is the constant volume reheat
system. (not used anymore).
Variable Air Volume (VAV) System: It is an another type zoned system, called a Variable Air ,Volume system, VAV system, because it varies the volume of air supplied to each zone. Variable Air Volume systems are more energy efficient than the reheat systems. (VAV is widely used).
System 2: Air-and-water Systems :-
Another group of systems, air-and-water systems, provide all the primary ventilation air from a
central system, but local units provide additional conditioning. The primary ventilation system also provides most, or all, of the humidity control by conditioning the ventilation air. The local units are usually supplied with hot or chilled water. These systems are particularly effective in perimeter spaces.
System 3: All-water Systems :-
When the ventilation is provided through natural ventilation, by opening windows, or other means,
there is no need to duct ventilation air to the zones from a central plant. This allows all processes
other than ventilation to be provided by local equipment supplied with hot and chilled water from a
central plant. These systems are grouped under the name “all-water systems.”
System 4: Unitary, Refrigerant-based Systems :-
The final type of system uses local refrigeration equipment and heaters to provide air conditioning.
They are called “unitary refrigerant–based systems” The window air-conditioner is the simplest
example of this type of system.
other than ventilation to be provided by local equipment supplied with hot and chilled water from a
central plant. These systems are grouped under the name “all-water systems.”
System 4: Unitary, Refrigerant-based Systems :-
The final type of system uses local refrigeration equipment and heaters to provide air conditioning.
They are called “unitary refrigerant–based systems” The window air-conditioner is the simplest
example of this type of system.
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