## 2 Load Classification in Commercial Buildings
### HVAC Loads  
Heating, ventilation, and air-conditioning (HVAC) systems are utilized to regulate the temperature, moisture content, circulation, and purity of the air within a place to achieve the intended effects on the occupants of the space or the manufactured items and equipment stored there. They are used in commercial buildings all year. A typical HVAC system is shown in Figure 3. An HVAC system also refers to central air-conditioning [23]. High-capacity central systems are primarily used in large buildings. The main A/C unit is usually placed in a mechanical room, generally at a distance from the conditioned spaces. The air is then conditioned, according to the operating mode (cooling or heating) of the system. For cooling purposes, the air is cooled and, if necessary, it is also dehumidified. Similarly, for heating purposes, the air is pre-heated by passing it through a system supplied with steam, hot water, or an electric heating element. If necessary, water vapor is also added by passing it through a humidifier, and finally, the air is heated again by using steam, hot water, or an electric heating element. The central unit is connected to the air handling unit (AHU) through a duct/piping system. Then, the air is transported with fans through a duct system to individual units which are installed usually at ceiling height, floor level, or near windows, in order to ensure the best possible air circulation in each air-conditioned space. The outdoor air is introduced into the main unit and is mixed with a certain amount of recirculated air. The mixture then passes through air filters to remove any dust or other foreign particles [24; 25]. In addition, a fan coil unit (FCU) is used in commercial buildings to heat or cool a room without connecting the ductwork. To condition a specific space of a room, an FCU circulates hot or cold water through a coil. The FCU draws hot or cold water from a central plant to make a human-occupied zone comfortable, with all the air-indexed properties under consideration, using a mechanical HVAC system. That results in a significant amount of energy consumption, which is associated with three factors that lead to excess electricity consumption, such as an HVAC sizing capacity that does not meet consumer needs accordingly, unnecessary usage, and lack of best practices in installation. In European countries, space cooling constitutes 40% to 60% of total building energy use. In the US, HVAC systems contribute to 50% of the energy use in buildings which is about 20% of their total energy consumption [26]. Cooling systems in the Middle East utilize more than 70% of all building energy [27]. Fan and supply air cooling account for 60% of HVAC energy use in Singapore and it is predicted to reach 70% [28]. Inefficiencies such as unneeded HVAC activity and exaggerated temperature settings waste a total of 10-40% of this electrical energy [29]. Without a doubt, due to economic growth and rising occupant  
Figure 2: Energy usage data of commercial buildings in the US [22].  
comfort demands, the global energy demand for buildings will continue to climb in the foreseeable future.