Indoor Air Quality Procedure Under ASHRAE Standard 62.1-2019 and -2022

Ventilation is necessary for healthy indoor spaces as human and building generated contaminants would eventually render a space toxic if unmanaged. Building codes across the U.S. and Canada, with few exceptions, reference ASHRAE Standard 62.1 Ventilation and Acceptable Indoor Air Quality for code-compliant ventilation designs.

The foreword in 62.1 defines the duality of the role 62.1 plays for air quality and system design:
ASHRAE Standard 62.1 specifies minimum ventilation rates and other measures intended to provide indoor air quality (IAQ) that is acceptable to human occupants and that minimizes adverse health effects…To signify that indoor air quality goes beyond minimum ventilation requirements—and in recognition of those aspects of building systems (equipment, filtration, controls, and more) that contribute to acceptable IAQ—the title of the standard has been updated to “Ventilation and Acceptable Indoor Air Quality.” -Standard 62.1 Foreword

Standard 62.1 offers three approaches to space ventilation. Mechanical ventilation in most buildings will follow either the Ventilation Rate Procedure (VRP) or the Indoor Air Quality Procedure (IAQP).

Ventilation Rate Procedure. The prescriptive design procedure presented in Section 6.2, in which outdoor air intake rates are determined based on space type/application, occupancy level, and floor area, shall be permitted to be used for any zone or system.

Indoor Air Quality Procedure. The IAQP is an alternative to the VRP used to determine the design rate of outdoor airflow to maintain concentrations of design compounds (DCs) and PM2.5 in the indoor environment to be less than design limits (DLs), based on indoor and outdoor sources, air cleaning, and other variables. These outdoor air requirements shall be calculated with mass-balance equations. Verification of occupant satisfaction and indoor DC concentrations shall be performed after the building is completed.
-ASHRAE 62.1 Section 6.1.1 and 6.1.2

The conventional design approach is the prescriptive Ventilation Rate Procedure, in which a mechanical system is sized to include sufficient outdoor air for contaminant dilution. The approach increases HVAC capacity for heating and cooling and is generally the most energy intensive approach with outdoor air capacity sized for worst-case utilization. Controls may be added to regulate outdoor airflow based on population (or other logic) to achieve some energy savings; however, HVAC equipment capacity is selected for the maximum conditions.

"The VPR is an indirect solution to achieving acceptable IAQ. The IAQP is a more direct approach to the goal.”
ASHRAE 62.1-2019 User’s Manual

ASHRAE established the indoor air quality procedure (IAQP) in 1981 as a direct, engineered approach for a more efficient and cost-effective design. The procedure has evolved to become a well-researched and defined alternative to the ventilation rate procedure (VRP). In some cases, the IAQP is the preferred approach. Under the IAQP, HVAC systems can be simplified and right-sized as the outdoor air load is 30 – 75% lower. With reduced load, the system will have lower operating costs compared to a VRP design. Due to these benefits, IAQP is gaining favor for the efficiency, sustainability, and savings it delivers. Example capacity and energy saving scenarios are in Appendix One.

The IAQP is an engineered approach to acceptable indoor air quality. Engineers apply contaminant source control (air cleaners & filters) in concert with lower outdoor air ventilation to deliver a compliant design. In the past the standard required designers to identify contaminant sources, generation rates, steady state concentrations, and air cleaner removal efficiencies. Now the standard is prescriptive and offers designers the options to supplement or modify contaminants and thresholds.

Successful IAQP designs ensure steady state concentrations as calculated in the mass balance equations are below the maximum levels defined in the standard (or by the engineer). The pivotal decisions are the selection of an outdoor air rate and air cleaner. Air cleaner selection is guided by ASHRAE standards and position documents. Automation for this iterative approach is available in ASHRAE’s calculator or calculators from various manufacturers.[2] A comparison between a VRP design and IAQP design for an elementary school classroom is presented in Appendix Two.

The industry, particularly ASHRAE, is increasing support for IAQP because it is a more energy efficient and effective approach. Lowering outdoor air delivers two potential economic benefits:

• Equipment savings through reduced size and complexity
• Lower operating costs with reduced conditioning demands as demonstrated in Appendix One.

Regarding equipment savings, or initial cost offsets, there are both the direct and indirect benefits of the IAQP to be considered. Initial equipment capacity under the VRP includes the conditioning load for all air in the system, which includes recirculated and outdoor air. It stands to reason that a 50%+ reduction of outdoor air can have a substantial impact on system sizing, particularly considering the summer and/or winter design day. Furthermore, by reducing equipment capacity you reduce equipment size and power draw. In some locations, by reducing outdoor air load on the system, it is possible to avoid energy recovery systems, local code depending.

Projects with higher sustainability goals benefit by reducing conditioning load through the multiplicative benefit of reducing geothermal, solar, wind or other sustainable capacity as it will be less required. Said differently, one Watt-dollar saved on the HVAC via IAQP might be worth tens of Watt-dollars of generation capacity and associated costs.

Multiple calculators, including one from the US Department of Energy may help determine the potential energy savings.[6] This tool was used to determine the kWh/cfm used in Appendix One calculations.

Outdoor air ventilation is the primary means of regulating CO₂ concentration. While CO₂ is not a design compound under 62.1, designers are cognizant of potential CO₂ level rise from human respiration. While outdoor CO₂ levels have increased since the industrial revolution to over 420ppm around the world, it still represents 0.04% of outdoor air. Such low outdoor concentration means indoor concentrations are diluted with a small amount of outdoor air. The definition of “unhealthy” levels of CO₂ is difficult to assess as direction from ASHRAE, third parties, international agencies, National Institute of Occupational Safety and Health (NIOSH) approach the topic differently. ASHRAE does not have an explicit position and does not consider CO₂ a design compound for that reason. [7]

• Indoor CO₂ concentrations do not provide an overall indication of IAQ, but they can be a useful tool in IAQ assessments if users understand the limitations in these applications
• Existing evidence for direct impacts of CO₂ on health, well-being, learning outcomes, and work performance at commonly observed indoor concentrations is inconsistent, and therefore does not currently justify changes in ventilation and IAQ standards, regulations or guidelines.

-ASHRAE Position Document on Indoor Carbon Dioxide

ASHRAE does recommend specific CO₂ thresholds over ambient for activating demand control ventilation [8]. The following points may be helpful, while the industry monitors ASHRAE’s guidance on this topic.

• 1,000 ppm, commonly referenced, for CO₂ was determined by perceived indoor air quality without any controls for TVOC, PM, etc.
• NIOSH CO₂ limit is 5,000 ppm
• Demand control ventilation limits are referenced in 62.1-2022 Addendum AB and have CO₂ increases above ambient of between 600 and 2,100 ppm, or 1,000 to 2,500 ppm steady state.

Mechanical ventilation design is critical for a healthy building. Design choices have meaningful implications for energy, sustainability, and first cost. As designers consider the traditional choice of VRP versus the emerging preference for the IAQP it is helpful to consider how ASHRAE continues to evolve the standard, as documented in Appendix Four.

Advances in technology and standardization support a refreshed evaluation of the IAQP. Owners seek to unlock capital and reduce operating costs through construction and maintenance intervals. In the end, cost effective, code compliant and acceptable indoor air quality is the priority, and the IAQP remains a direct approach to that achieving the goal.

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