ASHRAE Journal - April 2010 - 38

point (drier than the space), to remove the entire indoor latent load (Figure 3).
Safety Factor for Wca (Humidity Ratio of the Conditioned Air Leaving the Unit)

Target Space Humidity Level (Wsp). The first influence of safety factors in determining the required humidity ratio of the conditioned air (Wca) is in the selection of the target humidity level for the occupied space (Wsp). For most comfort-cooling applications, engineers will typically design HVAC systems using a target relative humidity (RH) of 50%. However, the allowable upper limit (worst case) for indoor humidity may be higher. Some engineers consider 60% RH an acceptable upper limit, while others use 55%, 65%, or some other limit. The new ASHRAE Guide for Buildings in Hot and Humid Climates10 recommends limiting indoor humidity to no greater than 55°F (13°C) dew point, which equates to about 51% RH at 74°F (23°C) dry bulb. This decision has a huge impact on the sizing of the dedicated outdoor air unit. Using the same four-classroom example, the humidity ratio of the conditioned air (Wca) and the resulting dehumidification capacity of the dedicated OA unit were determined for various RH limits. As Table 4 (see Page 35) demonstrates, to maintain a lower RH in the zones, Wca must be lower, resulting in greater dehumidification capacity required. For this example, designing the system to limit indoor humidity to 50% RH increases the required dehumidification capacity of the dedicated OA unit by 15% compared to allowing indoor humidity to rise to 60% RH at worst-case conditions. However, be sure to analyze the impact on the entire system. When a chilled-water or DX dedicated OA unit delivers the conditioned air cold (that is, not reheated to “neutral”), a dedicated OA unit that is selected to deliver drier air (lower Wca) delivers the air at a colder dry-bulb temperature. This colder air offsets more of the space sensible cooling load, allowing for the local HVAC units to be smaller and use less energy.11 Design Indoor Latent Loads (qLspace). The second influence of safety factors in determining the required conditioned-air humidity ratio (Wca) is in the calculation of the design latent loads for each zone. As discussed earlier, the estimated zone population (Pz) not only impacts ventilation calculations, it also affects indoor latent load calculations. Another source of latent load in a zone is infiltration of humid air from outside. Calculating loads due to infiltration is often more of an art than a science because it is highly dependent on the quality of the building envelope construction.9 Since precise calculations are often difficult, many engineers choose to select the dedicated OA unit with reserve (or extra) dehumidification capacity, rather than attempt to apply safety factors to the calculation of the latent load due to infiltration.
Summary and Discussion
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Excessive use of safety factors in the design of a dedicated outdoor air system (DOAS) can result in significant over April 2010

ASHRAE Journal

ASHRAE Journal - April 2010

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