While the principle of diversity as it applies to the mechanical systems of occupied buildings is an often-discussed topic among engineers during the design phase of a project, its benefits and applications often go unquantified in the process. As such, neither the building owner nor the design professional truly appreciates the energy and cost savings realized by the application of this simple principle.

Simply put, diversity is nothing more than the variation of occupant location throughout the day. The primary elements of an HVAC system affected by diversity are internal space load (cooling/heating demand) and air load (ventilation and outdoor-air [OA] tempering). While HVAC loads and code-based ventilation must be met for each individual occupied space, how this can be accomplished depends on the distribution of occupants throughout a building and the types of HVAC systems employed.

Saving Money and Energy
When the principles of diversity are applied to total building-ventilation requirements, substantial cost and energy savings can be realized. To illustrate this principle, consider a simple single-zone office building with 30 occupants. If the ventilation requirement is 20 cfm per person, the total requirement is 600 cfm OA—the maximum OA setting for the entire building regardless of the location and distribution of the occupants within the structure.

Conversely, the structure's ventilation requirement would be much higher if it was based on the OA required for each space when fully occupied. Not only would the higher OA value require a larger fan motor (which would increase electrical demand), the equipment capacity required to condition the OA would increase as well.

Carbon-dioxide- (CO2-) based demand-control led ventilation (DCV) or HVAC-system-integrated occupancy sensors can be used to apply diversity ventilation principles. Because multiple spaces or zones served by a common OA unit may be partially or completely occupied, CO2 limits must remain at appropriate levels at all times. In addition to the mitigation of bioeffluents, the regulation of OA introduction helps reduce or prevent adverse effects of elevated CO2 levels, such as headaches and drowsiness, as well as assist in the maintenance of overall comfort.

Many lighting applications use occupancy sensors to conserve energy; these sensors also can be used to open OA dampers for units serving occupied spaces. The same sensors can be used to prevent OA from being introduced to unoccupied spaces. While this application is practical when utilized in conjunction with two-position dampers, occupancy sensors otherwise are ineffective when careful OA modulation is required in a single space. Therefore, this ventilation-control method must be used with caution and only when single-space OA modulation is not required.

To illustrate how energy and cost savings can be quantified through the application of diversity principles, let’s utilize the previous example's values for a small building with Figure 1's layout. While the building's total maximum occupancy is 30 people, adding the maximum occupancies of each space in Figure 1 nets 49 occupants. If the required ventilation for the break room and both restrooms is included, the total apparent OA requirement is a whopping 1,310 cfm.

Obviously, the break room’s 200 cfm and the restrooms' total 150 cfm can be omitted because they can be handled via the building's standard ventilation. However, this still leaves a disparity of 19 people. Based on the ventilation requirements for each occupant, the total apparent OA requirement still is 960 cfm—a volumetric airflow 60-percent greater than that required to ventilate the entire building for the actual number of occupants at the standard office-space rate of 20 cfm per person.

Table 1 illustrates the difference in the energy required to cool or heat ventilation air based on the lack or application of diversity principles. Table 1 reveals the OA conditioning requirement of a 3½-ton cooling load without diversity vs. that of a 2¼-ton cooling load with diversity. In a 3,000-sq-ft occupied building, the savings associated with diversity can add up to 15,000 Btuh during warm-weather months. Further, the size of the unitary system for this type of application could be reduced by at least 1 ton, requiring less energy to start and operate.