Four strategies for implementing ASHRAE 62.1 in HVAC systems

There’s room in ASHRAE 62.1 to improve energy efficiency in a commercial ventilation system.



Learning objectives

  • Understand how to design ventilation systems using ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality.
  • Learn four strategies to realize cost and energy savings.




When designing HVAC systems to meet local codes and ASHRAE 62.1- 2016: Ventilation for Acceptable Indoor Air Quality, reducing the amount of necessary outside air that needs to be conditioned for acceptable indoor use is allowed, and there are several means by which the designer can approach such reductions, all of which are described within this ASHRAE Standard.

Four strategies will be examined to save energy and realize cost savings. Approaching HVAC outdoor-air calculations in this manner may not be the easiest way to design ventilation systems, but the payoff could make it worthwhile. The potential reduction in required outdoor airflow could exceed 50% depending on what combination of strategies are implemented for a given HVAC system or a combination of systems. 

Strategy 1: Occupant diversity

People that occupy a room will contaminate indoor air by exhaling carbon dioxide, sweating, coughing, etc. 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. Outdoor-air requirement rates of airflow ("R") due to occupants ("p" for population or people) are calculated at one rate, the "per person rate" (Rp) in cubic feet per minute per person based upon the design-zone population (Pz). Those requirements that are due to the zone area/square footage of the spaces (Az) are calculated at a different rate, the "area rate" (Ra), in cubic feet per minute per square foot. Refer to the ASHRAE 62.1-2016 Table, Minimum Ventilation Rates in Breathing Zone for the specific per-person airflow rates (Rp) and per-square-foot airflow rates (Ra) based upon the use of the space.

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 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. 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.

Figure 1: A conference room that is often sparsely occupied, or even empty, provides an opportunity for use of time-averaging. Courtesy: WD PartnersNote that this article focuses on the calculation of uncorrected outdoor airflow values as generally defined in ASHRAE 62.1-2016 Section This article does not examine the prescriptive formulas accounting for multiple-zone systems, zone air-distribution effectiveness, or zone primary outdoor airflow fractions. 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. 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.

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. 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.

Yet the real occupancy rate may be much lower, and that's where occupant diversity may be applied, which is permitted under Section 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. The same scenario noted above can be designed for outdoor ventilation by applying occupant diversity within the calculation. 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. This means that the system population is 100, not the sum of all zones' populations.

ASHRAE 62.1-2016 Section 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 the example, that's 100/250-a potential reduction of 60% in the required people outdoor airflow. That can mean a significant savings when heating and cooling the building. One thing that engineers can do when creating such systems is to ask specific questions about the building's use and the realistic intended occupancy before deciding on the final ventilation plan.

Example 1: Calculation of outdoor airflow requirements in the example building:

Open office space: 20,000 sq ft (Az1), 100 occupants (Pz1)

Conference rooms: 5 at 400 sq ft (Az2), 20 occupants (Pz2)

Cafeteria/break room: 2,000 sq ft (Az3), 50 occupants (Pz3)

From Table, office, conference, and break room: Ra = .06 cfm/ sq ft; Rp = 5 cfm/person

System population provided by building owner: Ps= 100

Occupant diversity, D = Ps/(Pz1 + 5 x Pz2 + Pz3)

D = 100/(100 + 5 x 20+50) = 0.40

Uncorrected outdoor-air intake, Vou = D(Rp1 x Pz1 + 5 x Rp2 x Pz2 + Rp3 x Pz3) + (Ra1 x Az1 + 5 x Ra2 x Az2 + Rp3 x Az3)

Vou = 0.40 x (5 cfm/person x 100 people + 5 rooms x 5 cfm/person x 20 people + 5 cfm/person x 50 people) + (0.06 cfm/sq ft x 20,000 sq ft + 5 rooms x 0.06 cfm/sq ft x 400 sq ft + 0.06 cfm/sq ft x 2,000 sq ft)

Result: Vou using diversity = 1,940 cfm

Vou without using diversity: 2,690 cfm

The use of occupant diversity in Example 1 reduces the required outdoor airflow by 28%. 

Figure 2: This example shows an opportunity for use of time-averaging in a conference room that is rarely fully used. Courtesy: WD PartnersStrategy 2: Time averaging

ASHRAE 62.1-2016 Section allows the designer to account for situations where the occupancy will peak for only short durations of time. Depending on the building and the business it holds, some people may occupy a space for only a short period of time. In such situations, an appropriate strategy is to apply time averaging. For example, in a transient-occupancy situation in which a conference room often sits completely empty but is occasionally full for a short period of time. The typical meeting will last for only 45 minutes, requiring occupied outdoor-air ventilation for that short period of time. However, the prescriptive formulas would indicate the need for full ventilation for that space all day during occupied hours, every day, representing a potentially overventilated room.

By focusing on the amount of time people are staying in a space, the required outdoor-air requirements can be reduced anywhere from 30% to 50%. Similar to this example would be retail spaces and restaurants where the occupancy varies depending on the day or time. The ultimate idea is that meaningful energy savings can result from using these different deductions to design the right system for the space-for exactly the way it's meant to be used. The key to the calculation is to determine the allowable averaging time period, T. Equation is the calculation for the averaging time period:

Equation T (min) = 3 ν/Vbz

The variable (ν) is the volume of the space in cubic feet.

Example 2: A 400-sq-ft conference room has a 10-ft ceiling.

The owner provides historical data showing that the conference rooms in the building are occupied for 30 minutes every 90 minutes, on average, and that the break room is occupied only between 11:45 a.m. and 1:15 p.m. each day.

Equation calculates the allowable time-averaging time period:

T = 3ν/Vbz

ν = room volume in cubic feet

v = (400 sq ft x 10 ft)

v = 4,000 ft3

Occupant density = 50 people per 1,000 sq ft (from Table

Default Pz = (50/1,000) x 400

Default Pz = 20 people

Rp = 5 cfm per person (from Table

Az = 400 sq ft

Ra = 0.06 cfm/sq ft (from Table

Vbz = Ra x Az + Rp x Pz

Vbz = 0.06 cfm/sq ft x 400 sq ft + 5 x 50 persons/1,000 sq ft x 400 sq ft

Vbz = 124 cfm

T = 3ν/Vbz

T = 3 x (4,000 ft3)/124 cfm

Result: T = 97 minutes

This result allows the designer to take the average occupancy over a time period up to 97 minutes. 

For the conference room that is occupied for only 30 minutes out of a 90-minute time period, the time-averaged zone population, Pzavg, is the average of the occupancy over that 90-minute time period.

Pzavg = ((20 people x 30 min) + (0 people x 60 min))/90 min

Pzavg = 6.667 (round to 7 people)

Thus, the outdoor airflow required is calculated with Pzavg instead of the default Pz.

Vbz (Time-averaged) = Pzavg x Rp + Az x Ra

Vbz(Time-averaged) = 7 people x 5 cfm/person + 400 sq ft x .06 cfm/sq ft

Result: Vbz(Time-averaged) = 59 cfm

Figure 3: This sample office building shows three different space uses and ASHRAE 62.1 Table occupancy information. Courtesy: WD PartnersIn this example, the required breathing-zone airflow for this conference room has been reduced by 52% from 124 cfm to 59 cfm.

For an office building that contains a large number of sparsely occupied rooms, this reduction in required, outdoor airflow can result in a significant reduction in the total required outdoor airflow that must be heated or cooled.

The two strategies noted above are not exclusive, but they can be used for the same outdoor-air calculation. Combining the results of Example 1 with Example 2, the Vou calculation is further reduced as follows:

Example 3: Combining diversity and time-averaging.

Vou with diversity only: 1,940 cfm

Calculation of Example 1 Vou with diversity and time-averaged Vbz for the conference room:

Vou = D (Rp1 x Pz1 + 5 x Rp2 x Pz2 + Rp3 x Pz3) + (Ra1 x Az1 + 5 x Ra2 x Az2 + Rp3 x Az3)

Vou = 0.40 x (5 cfm/person x 100 people + 5 rooms x 5 cfm/person x 7 people + 5 cfm/person x 50 people) + (0.06 cfm/sq ft x 20,000 sq ft + 5 rooms x 0.06 cfm/sq ft x 400 sq ft + 0.06 cfm/sq ft x 2,000 sq ft)

Result: Vou (time-averaging and diversity) = 1,810 cfm

Vou (neither time-averaging nor diversity) = 2,690 cfm

Percent reduction in standard-required uncorrected outdoor air = 32.7%

The reduction in required outdoor airflow, when applied diligently to HVAC systems design, can result in reduced required cooling capacity in tonnage as well as reductions in fan airflow, duct sizing, and fan horsepower. In addition to the first cost of this equipment, there is also the potential for a reduction in electrical system design sizing, such as smaller wire sizes, smaller circuits ampacity, reduced electric operating-demand kilowatts, as well as electrical energy-consumption reductions.

Such impacts may not be evident immediately, but the use of building energy simulations can demonstrate cumulative effects of a thorough design approach to a system's outdoor airflow. A building owner may not conceptualize the value of such reductions if the engineer simply describes these strategies; however, a diagram will clearly demonstrate the cost savings. Running a rough energy analysis, and presenting the results to the building owner or other stakeholders who facilitate that decision may make a sizable difference in the decision-making process. 

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