The heating, ventilating, and air conditioning (HVAC) systems maintain and control temperature and humidity levels to provide an adequate indoor environment for people activity or for processing goods. The cost of operating an HVAC system can be significant in commercial buildings and in some industrial facilities. In the U.S., it is estimated that the energy used to operate HVAC systems can represent about 50% of the total electrical energy use in a typical commercial building ( Krarti, 2000). It is therefore important that buildings designers recognize some of the characteristics of the HVAC systems and determine if any available design and operating options can be considered to improve the energy of these systems.
A basic HVAC air distribution system consists of an air-handling unit with the following components as shown in Figure 1:
Dampers to control the amount of air to be distributed by the HVAC system including: outside air (OA) damper, return air (RA) damper, exhaust air (EA) damper, and supply air (SA) damper.
Preheat coil in case the outside air is too cold to avoid any freezing problems.
Filter to clear the air from any dirt.
Cooling coils to condition the supply air to meet the cooling load of the conditioned spaces.
Humidifiers to add moisture to the supply air in case a humidity control is provided to the conditioned spaces.
A distribution system (i.e., ducts) where the air is channeled to various locations and spaces.
Each of the above listed components is available in several types and styles. The integration of all the components constitutes the secondary HVAC system for the sole purpose of conditioned air distribution.
The main objectives of the HVAC systems installed within buildings include:
maintaining thermal comfort for all building occupants. In most climates, HVAC systems have to meet both heating and cooling requirements.
providing fresh air intake (i.e., ventilation) to maintain acceptable indoor air quality.
In the following sections, energy efficient alternatives for HVAC systems and operating and control strategies are presented. These alternatives should be considered in the early stages of designing sustainable buildings.