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.
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Likewise, heating energy can be reduced significantly. In this example, the diversity-related savings of 41 mbh could be applied to electrical energy or natural-gas usage in a gas-fired unit. In larger buildings, a proportional amount of savings that would reduce the size of fuel-oil tanks and piping, boilers, and hydronic piping could be realized. The potential areas in which savings can be achieved by applying simple diversity principles include:
- Cooling systems—unitary-equipment sizes and capacities, central-plant chiller/cooling-tower sizes, and geothermal-well quantity.
- Heating systems—boiler sizes and quantity, fuel-oil/gas consumption, heating-coil/element sizes, and fuel-/gas-line and regulator sizes.
- Hydronic systems—hot- and cold-water pumps, hydronic piping sizes and quantities, and valving and hydronic specialties.
- Non-HVAC disciplines—size and quantity of structural members required to support equipment, amount and size of architectural screening, electricity demand, wire sizes, and amount of conduit.
Additionally, if ventilation systems utilize energy-recovery equipment, heat-exchanger size may be reduced, resulting in secondary energy savings.
Diversity Through Thermal Control
Diversity principles can be applied to a variety of buildings and systems, such as offices, schools/universities, public halls, libraries, sports arenas, and churches. Almost any building that has a relatively constant occupancy with a potential for occupant migration can be a good candidate for diversity.
Diversity principles also can be applied through thermal control. This type of diversity allows systems to be regulated in various spaces within a building based on occupant comfort and can be achieved via operational scheduling and controls, such as unoccupied- and night-setback control modes.
While reducing cooling or heating demand in unoccupied spaces is a practical method of saving energy, engineers must employ design safeguards to prevent repeated short-term cycling of HVAC equipment in areas in which occupant migration might be frequent. Trying to save energy by continuously starting and stopping equipment can have the opposite effect while also shortening equipment life and potentially creating excessive noise. Often, using a simple programmed lead/lag time or delayed start/stop during periods of varying occupancy is more than sufficient to prevent excessive equipment cycling.
When employed correctly, thermal diversity principles can yield additional paybacks when coupled with ventilation diversity methods. Consider the previous small-office-building example. If the building's HVAC unit were sized for the peak cooling loads of each space based on full occupancy, then another 19 people would need to be included in the maximum occupancy count. With an average total heat output of 450 Btuh per person, the apparent cooling load would need to be increased by 8,550 Btuh, nearly three-fourths of a ton. Although this value is not large for a 3,000-sq-ft office building, consider the disparity that would occur in a 30,000-sq-ft office building with 300 occupants. Such a scenario would call for an additional 7½ tons of cooling capacity, if diversity were not taken into account.
Ventilation and thermal diversity principles also may be applied to centrally ventilated systems with zoned terminal units, such as fan coils or water-source heat pumps. While individual terminal units need to be sized and selected based on the peak loads of the spaces they serve, the central ventilation equipment (supply/exhaust fans, ventilation-only or packaged energy-recovery units, etc.) used to deliver and temper the OA can be sized significantly smaller because of the reduction in overall air load.
Table 2 illustrates how the application of diversity principles in this example produces myriad savings in multiple areas of construction, operation, and maintenance.
This article discussed only one of many scenarios in which the application of diversity principles can yield energy and cost benefits for a building owner and occupants. The larger and more complex a building becomes and the greater the occupant activity level, the greater the potential for cost and energy savings.
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ANSI/ASHRAE. (2004). Ventilation for acceptable indoor-air quality. ANSI/ASHRAE Standard 62-2004. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
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A mechanical-systems designer and energy-modeling specialist for Gatter & Diehl Inc., Jack L. Burton, EIT, LEED AP, attended the U.S. Naval Academy, is a graduate of Baltimore Polytechnic Institute, and has a bachelor’s degree in mechanical engineering from Warren National University. He has been employed in the engineering and construction fields since 1984, working within the civil-, electrical-, structural-, and mechanical-engineering disciplines.