![]() ![]() ![]() In the example, that’s 100/250-a potential reduction of 60% in the required people outdoor airflow. This means that the system population is 100, not the sum of all zones’ populations.ĪSHRAE 62.1-2016 Section 6.2.3.5.1 allows the designer to determine the diversity (D) by dividing the number of total building occupants by the sum of all possible occupants of all spaces. In a case where a building actually has a total of 100 employees (full-time-equivalent occupants of the building), those 100 people are either at a desk, in a private office, in the cafeteria, or sitting in a conference room, but not occupying two spaces at once. The same scenario noted above can be designed for outdoor ventilation by applying occupant diversity within the calculation. This strategy calls for adjusting the system needs based on the actual number of people who will be working in the building, or the building’s system population (Ps.) Keep in mind that, with most building codes, it is not necessary to design a building for every person that every room is designed to accommodate, but the design must include a reasonable approximation of the expected use of the building. Yet the real occupancy rate may be much lower, and that’s where occupant diversity may be applied, which is permitted under Section 6.2.5.3.1. The sum of the zone populations results in a maximum potential occupancy of 250 total, and the formulas help to determine the ventilation needs based on that number. To demonstrate the strategies, a sample building with a large, open office space that will hold 100 people in addition to five conference rooms accommodating up to 20 occupants each and a cafeteria accommodating 50 people will be used throughout. Such corrections to the outdoor airflow can be examined independently of the additional factors that affect an HVAC system’s outdoor airflow and supply airflow requirements. These corrections to the outdoor airflow are omitted from the examples in order to focus attention on the calculation that results from use of the specific strategies mentioned within the article. This article does not examine the prescriptive formulas accounting for multiple-zone systems, zone air-distribution effectiveness, or zone primary outdoor airflow fractions. Note that this article focuses on the calculation of uncorrected outdoor airflow values as generally defined in ASHRAE 62.1-2016 Section 6.2.5.3. In simple terms, find the maximum occupants in each space, find the areas of each space, calculate the breathing-zone outdoor airflows, and then add them up. For a dedicated outdoor-air single-zone system, summing up the ventilation requirements for each zone results in the total possible uncorrected outdoor-air intake value. The breathing-zone outdoor airflow (Vbz) is calculated by summing the people ventilation requirements and the area ventilation requirements for the space (ASHRAE 62.1-2016 Section 6.2.2.1). ![]() Refer to the ASHRAE 62.1-2016 Table 6.2.2.1, Minimum Ventilation Rates in Breathing Zone for the specific per-person airflow rates (R p) and per-square-foot airflow rates (R a) based upon the use of the space. Those requirements that are due to the zone area/square footage of the spaces (Az) are calculated at a different rate, the "area rate" (R a), in cubic feet per minute per square foot. Outdoor-air requirement rates of airflow ("R") due to occupants ("p" for population or people) are calculated at one rate, the "per person rate" (R p) in cubic feet per minute per person based upon the design-zone population (P z). That’s on top of air contamination that comes from paint, carpet, upholstery, and other fixtures that emit minute particles and vapors, all of which are already figured into the mathematic formulas found in ASHRAE 62.1-2016, the standard that regulates outdoor air. People that occupy a room will contaminate indoor air by exhaling carbon dioxide, sweating, coughing, etc. ![]()
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