104 Hvac System Types

The energy efficiency of systems used to heat and cool buildings varies widely but is generally a function of the details of the system organization. On the most simplistic level the amount of energy consumed is a function of the source of heating or cooling energy, the amount of energy consumed in distribution, and whether the working fluid is simultaneously heated and cooled. System efficiency is also highly dependent upon the directness of control, which can sometimes overcome system inefficiency.

HVAC system types can be typically classified according to their energy efficiency as highly efficient, moderately efficient or generally inefficient. This terminology indicates only the comparative energy consumption of typical systems when compared to each other. Using these terms, those system types classified as generally inefficient will result in high energy bills for the building in which they are installed, while an equivalent building with a system classified as highly efficient will usually have lower energy bills. However, it is important to recognize that there is a wide range of efficiencies within each category, and that a specific energy-efficient example of a typically inefficient system might have lower energy bills than the least efficient example of a moderately, or even highly efficient type of system.

Figure 10.2 shows the relative efficiency of the more commonly used types of HVAC systems discussed below. The range of actual energy consumption for each system type is a function of other design variables including how the system is configured and installed in a particular building as well as how it is controlled and operated.

To maximize the efficiency of any type of HVAC system, it is important to select efficient equipment, minimize the energy consumed in distribution and avoid simultaneous heating and cooling of the working fluid. It is equally important that the control system directly control the variable parameters of the system.

Most HVAC systems include zones, which are areas within the building which may have different climatic and/or internal thermal loads and for which heat can be supplied or extracted independent of other zones.

The four-volume ASHRAE Handbook, published sequentially in a four-year cycle by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., provides the most comprehensive and authoritative reference on HVAC systems for buildings. The reader is specifically referred to the ASHRAE

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