Performance Ratings The performance rating table offers readers a high-level system evaluation to facilitate cross comparison according to eight metrics. Each metric is scored on a five-point scale from "++" (best) to "--" (worst) performance along with a brief explanatory note. The eight metrics are described in the following. Energy performance Energy performance indicates of how much electricity and (if applicable) natural gas an HVAC system requires to condition a building relative to the same building outfitted with a different system. Systems with higher efficiency use less energy to operate than those with a lower efficiency. Energy performance is often expressed in terms of energy use intensity (EUI) which is defined as the sum of fuel uses divided by the conditioned floor area (CDDI-2). For example, the variable air volume (VAV) system in Fig 1.3 has a lower energy performance than room fan coil units with a dedicated outdoor air system, due to the larger volume of air that the VAV needs to centrally heat, cool and distribute through the building. Thermal comfort Thermal comfort describes the ability of a system to reliably provide individualized temperature and humidity control. HVAC systems that provide high thermal comfort allow for independent control of zone-level equipment and deliver conditioning and supply air directly to occupants. In Fig 1.3, the fan coil units provide more thermal comfort flexibility than the VAV system since individual fan coil units can be locally adjusted depending on zone conditions. To make up for this lack of control, some VAV systems include additional heating and cooling registers right at the room outlet at the expense of higher system costs and energy use. Acoustic comfort Mechanically ventilated or hydronic HVAC systems generate noise from to moving air and water which may create acoustic discomfort for building occupants. HVAC systems with good acoustic comfort make less noise since they have fewer pieces of equipment with less moving parts or slower moving air. In Fig 1.3, the fan coil units generate more noise than the VAV system since there is a fan located in each zone. The VAV system is not perfect, either as it generates noise from moving potentially large volumes of air at high speeds through the ducts. First cost First cost corresponds to the installation cost of the system. A higher first cost is associated with more pieces of equipment or the use of unconventional systems. In Fig 1.3, the fan coil unit system costs more than the VAV since it effectively requires two HVAC systems, the dedicated
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