Four strategies for implementing ASHRAE 62.1 in HVAC systems
- 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 184.108.40.206, 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 220.127.116.11). 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.
Note that this article focuses on the calculation of uncorrected outdoor airflow values as generally defined in ASHRAE 62.1-2016 Section 18.104.22.168. 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 22.214.171.124.1. 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 126.96.36.199.1 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 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 188.8.131.52, 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)
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)
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%.
Strategy 2: Time averaging
ASHRAE 62.1-2016 Section 184.108.40.206 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 220.127.116.11-1 is the calculation for the averaging time period:
Equation 18.104.22.168-1: 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 22.214.171.124-1 calculates the allowable time-averaging time period:
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 126.96.36.199)
Default Pz = (50/1,000) x 400
Rp = 5 cfm per person (from Table 188.8.131.52)
Az = 400 sq ft
Ra = 0.06 cfm/sq ft (from Table 184.108.40.206)
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
T = 3ν/Vbz
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 = 6.667 (round to 7 people)
Thus, the outdoor airflow required is calculated with Pzavg instead of the default Pz.
Vbz(Time-averaged) = 7 people x 5 cfm/person + 400 sq ft x .06 cfm/sq ft
Result: Vbz(Time-averaged) = 59 cfm
In 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 = 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.
Strategy 3: Real-time air monitoring using CO2 sensing
The next two strategies are variations of what is considered demand-control ventilation (DCV)-or dynamic reset of outdoor airflow, as it is referred to under ASHRAE 62.1-2016 Section 6.2.7-, which is an interactive HVAC system that increases or decreases the real-time requirement for outdoor air as the space air-quality conditions change. These approaches involve the use of sensors that collect data in a specific zone and communicate the information in real time to the HVAC control system, which then makes immediate adjustments in the ventilation airflow. The additional control system complexity that is associated with the use of DCV should be considered in the decision-making process.
Designing the ventilation system with demand-control ventilation and the potential reduction in required outdoor airflows during system operation will also be examined.
The most common application of DCV uses carbon dioxide (CO2) sensors to determine if people who are occupying a room are creating a condition of excessive carbon dioxide and negative oxygen and, if so, remediate it immediately with increased outdoor air. This strategy is a form of dynamic reset, where the air is constantly being sampled and treated, not allowing CO2 a chance to concentrate above a predetermined "high value." With these sensors, if CO2 levels begin to rise, the system resets to a new value of outdoor-intake airflow until the CO2 drops below the control system’s high value. Once this condition is restored, the HVAC control system resets back to its normal setting.
ASHRAE 62.1-2016 220.127.116.11.1 describes the allowance for outdoor airflow reset in response to system population. When using a DCV system, the system designer must determine what airflow range is appropriate for the zone and for the system. Section 18.104.22.168.2 describes the requirement that the value of Vbz cannot be reset lower than the area airflow rate (Ra x Az) for the zone. So when CO2 values are "low" (less than the high value), the designer can set Vbz = Ra x Az, which eliminates the "people airflow" until the CO2 levels increase to the high value. At that time, the Vbz must be recalculated to include people airflow as well as the "area airflow."
It is important to note that the high value should be determined with some knowledge of the ambient concentration of CO2. Often, a value of 1,000 parts per million (ppm) CO2 is considered adequate for the dynamic-reset high CO2 value. The use of 1,000 ppm assumes that the ambient CO2 concentration in outdoor air is well below that value, in the range of 400 to 500 ppm. However, in urban areas, the ambient outdoor-air CO2 concentration may be much closer to 1,000 ppm, and in such cases, the designer should consider how this higher ambient CO2 may impact the high-value setpoint as well as the reset values for outdoor airflow. To demonstrate the value of operating an outdoor-air reset control with DCV, examine the sample building that has been used for the examples within this article and one or all of its 400-sq-ft conference rooms, for 20 people, fitted with CO2 sensors for dynamic reset.
The designer must determine the appropriate design dynamic-reset outdoor airflow rates for the "high" CO2 condition and for the "normal" CO2 condition:
Example 4: Demand control ventilation calculation applied to the sample building’s conference rooms:
Vbz (high CO2) = Ra x Az + Rp x Pz
Result: Vbz (high CO2) = 124 cfm
Vbz (low CO2, reset) = Ra x Az
Result: Vbz (low CO2, reset) = 24 cfm
The values above represent two required minimum HVAC system ventilation operating conditions. The engineer must size the equipment and outdoor- and supply-air ductwork to accommodate both the high and low conditions. Due to this requirement, HVAC system capacity is not reduced, so there is no first-cost advantage to the use of DCV. In fact, DCV will generally be a higher first cost due to the additional controls complexity that is required to operate with DCV.
In example 4, the building control system can reset the outdoor airflow to this room by approximately 80% during times of normal CO2 concentration. During that time period, the HVAC equipment may operate at a reduced system-cooling tonnage demand, and potentially a reduced fan airflow in variable-air-volume supply fan systems, which translates to a lower operating-fan horsepower. Once the room becomes fully occupied, and the CO2 concentration rises above the high value, the building control system adjusts the supply fan speed to increase the outdoor airflow to the room to the Vbz (high CO2) value. Consequently, the cooling or heating demand may increase as a result during the time that the increased outdoor airflow is being supplied to the room.
Ideally, these CO2 sensors are used in areas that have the potential of being densely but intermittently occupied, such as in cafeterias, auditoriums, gymnasiums, and conference rooms. The sensors may not provide a quantifiable benefit for other spaces, such as open-office or reception areas, due to the smaller number of occupants per unit of area. ASHRAE 62.1’s default occupant density for offices is one person per 200 sq ft. There are certainly open-office designs with much higher densities, but generally, spaces are considered densely occupied when there are more than 20 people per 1,000 sq ft. At this higher density, consider a CO2 sensor for the space and use a dynamic-reset/DCV approach.
When they were first introduced more than 25 years ago, CO2 sensors were expensive, unreliable, and in need of regular recalibration. Early generation energy-management systems were problematic and susceptible to bugs. Today, CO2 sensors are more dependable and more cost-effective. Driven by LEED certification and more stringent energy codes, customer demand for CO2 sensors has grown and manufacturers improved the reliability of digital CO2 sensors. As a result, the use of CO2-based DCV has become commonplace in systems of varying design capacities.
Strategy 4: Real-time monitoring through occupancy sensing
The final strategy is using DCV with occupancy sensors. These devices are always monitoring. When there is motion in a space that triggers the sensors, the building automation system responds accordingly. Motion sensors are commonly used in other applications, such as lights that trigger automatically when a customer enters a store restroom or security lights that turn on or brighten when a person walks by.
The approach to outdoor airflows can be the same that is used with CO2 sensors, whereupon the sensing of motion causes the outdoor airflow to reset to the high airflow rate; when there is no motion, the system resets to the low outdoor airflow rate. Alternatively, if the designer chooses to use a dedicated outdoor-air supply (DOAS) system and directly supplies outdoor air to all spaces, there is a new provision in ASHRAE 62.1-2016 that allows for zero required outdoor-air ventilation with occupancy if people aren’t in a particular room, according to Section 22.214.171.124.2 within the exception text. Based upon this exception, there may be opportunities to design the outdoor-air ventilation system supply airflow to vary between zero and Vbz values. Note that a system that uses occupancy sensors as the means of reset will not be able to "count" the number of occupants, so the outdoor airflow is either 0 cfm or Ra x Az + Rp x Pz, depending on the status of the sensor.
At this time, HVAC system designs that incorporate occupancy sensors are not as common in practice as CO2 sensors. Initially, they were developed for commercial applications but really caught on as a way for homeowners to reduce their energy expense. A prime example of this is a thermostat that adjusts the temperature to preset efficiency targets when no one is home, yet adjusts when it senses motion.
The International Energy Conservation Code now requires the engineer to use demand-control ventilation strategies in any space that is at a density equal to or greater than 25 people per 1,000 sq ft, with some exceptions for small systems or specialized HVAC systems. DCV is a good strategy for any space that is expected to be intermittently occupied, regardless of whether code requires it. Major manufacturers of packaged single-zone HVAC equipment are including DCV control modules as a factory option in addition to the more complex and larger air handling unit systems.
Using part or all of these strategies to estimate the total cost savings can be tricky. There are several variables to consider, and any savings has to be calculated on a case-by-case basis. But these strategies are quantifiable energy and money savers, which can be an attractive opportunity for a building owner.
Getting away from the prescriptive formulas to more reality-based formulas and strategies is a smart move. It requires a thorough and critical analysis of the building and its ventilation needs.
The savings that can be realized upon implementation of any or all of the strategies noted above can be in both first cost and operating cost.
Smaller outdoor-air ventilation system size can result in a smaller heating or cooling plant and smaller equipment. Smaller equipment leads to smaller duct and/or piping as well as reductions in electrical wire sizing and circuit breakers.
Operationally, the reduced outdoor airflow saves costs throughout the life of the equipment and the building through smaller operating horsepower of pumps and fans supplying and conditioning the outdoor air.
Regardless, implementing a strategy to maximize energy efficiency in HVAC systems can add up, quickly resulting in appreciable savings for property owners over the long term.
Kari Engen is a senior mechanical engineer at WD Partners. She is a member of ASHRAE and has provided HVAC design and commissioning services for commercial projects for more than 15 years.