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    • ASHRAE 62.1: A review of key requirements and concepts

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    Indoor air quality (IAQ) is probably the most commonly discussed aspect of building indoor environmental quality (IEQ). IAQ has direct effects on our health as well as the perception of an acceptable indoor environment. In the United States, ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality is the most commonly referenced standard to quantify acceptable conditions and appropriate HVAC system design. ASHRAE Standard 62.1 either forms the basis for most mechanical codes or is directly referenced by the codes themselves.

    The standard is best known for its regulation of the amount of ventilation air delivered to each space by HVAC systems through its ventilation rate procedure approach to system design. However, the standard covers many other aspects of building design that are less well known and understood. The key aspects of building design that it covers are: outdoor air quality, envelope design and construction, HVAC system construction, concept of air classes, the ventilation rate procedure, the IAQ procedure, the natural ventilation procedure, and operations and maintenance.

    Before getting into what the standard covers in detail, it is important to also understand what the standard does not cover. Standard 62.1 does not cover low-rise residential buildings and single-family dwellings—those are covered under the companion Standard 62.2. The standard also does not cover air quality in smoking areas, a change to the standard made in the 2010 update. Likewise in 2010 ventilation requirements for healthcare facilities were removed so that they could be solely referenced in ASHRAE Standard 170: Ventilation for Health Care Facilities. The other major areas that are not covered are the requirements for ventilation of industrial, laboratory, or other specific process driven ventilation requirements.

    Outdoor air quality

    Given the general approach within the standard of “dilution is the solution to pollution,” having good outdoor air quality is a very important premise when outdoor air is used as the “fresh” source to dilute the polluted indoor air. The standard requires two key pieces of information to perform the ventilation system design—demonstration of regional compliance with the National Ambient Air Quality Standards and the completion of a local survey of the specific project site. The survey is an important, and an often overlooked, aspect of the outdoor air quality. The survey is expected to focus on local sources of pollution—such as vehicular traffic—as well as other potential pollutant sources on adjacent properties such as boiler exhaust, process exhaust discharges, cooling tower discharges, and generator exhausts.

    Outdoor air is required to be specially treated when the local conditions exceed the national standards. The standard focuses on outdoor air treatment of particulates and ozone. For high particulate areas, HVAC systems are required to provide varied levels of air filtration effectiveness (MERV 6 or MERV 11) depending on the whether the location is in particulate matter (PM) 10 or PM 2.5 noncompliance. For areas with ozone noncompliance, ozone removal systems must be installed. Luckily for most designers, there are only four locations listed that require ozone removal systems and they are all in California (Riverside, Kern, Los Angeles, and San Bernardino counties).

    Envelope design and construction

    Unbeknownst to many architects, ASHRAE 62 includes aspects of architectural design within its scope of requirements—it is not just a standard for mechanical engineers. It is important for mechanical engineering consultants and contractors to educate their architectural colleagues on those requirements. The architectural requirements generally focus on moisture management within the building enclosure to prevent or reduce the risk of mold growth within the building and include requirements for vapor and air barrier constructions.

    While the architect generally relies on the engineer to identify air inlet and exhaust locations, the specification and detailing of the louvers associated with these functions is often by the architect (Division 8 per CSI). In particular, the standard has specific requirements for rain intrusion protection for louvered openings that need to be specified and considered when selecting louvers for a project.

    The other major aspect of the standard that affects architects is the requirement for physical separation of certain spaces—particularly between parking garages and occupied spaces and smoking and nonsmoking areas.

    HVAC system construction

    Like the architectural requirements, much of the focus of the HVAC system requirements deals with moisture management within the system and its components. The main area of focus is on the requirements for drain pan construction and placement. Additional requirements govern the materials used for air conveyance (limiting material susceptibility to moisture damage and mold growth as well as erosion), coil design (limiting coil pressure drop where inadequate access is provided), and insulation requirements to prevent condensation on interior surfaces (noting that specific resistance values are not required—it is up to the designer to determine the required thermal resistance for the specific application).

    Access for proper operations and cleaning is also important to maintaining equipment to allow for continued high levels of IAQ. While many of the requirements are common practice, it is worth reviewing the standard when designing or inspecting systems to ensure adequate access and cleanability is provided.

    Air classes

    The concept of air classes is a fundamental aspect of the standard that can have a significant effect on the basic HVAC system layout and design. ASHRAE divides indoor air into four classes that describe the level of contamination within the air in a given zone. Air classes are important because they govern the ability to recirculate air both within a given space as well as within a given system. Systems that serve spaces that have multiple air classes need to be carefully designed to ensure compliance as when air streams combine, the mixed air stream takes on the air class of the worst stream.

    Class 1 air is considered generally clean and without significant odor and is representative of the air in typical office or classroom areas. Class 1 air can be recirculated to any space type. Class 2 air is considered moderately contaminated or odorous and is restricted in its recirculation. Examples of Class 2 air zones include daycare facilities, dining areas, retail sales areas, and fitness facilities. Class 2 air can be recirculated to other similar Class 2 or 3 areas or Class 4 areas. However, Class 2 air cannot be recirculated to Class 1 spaces. This can have a significant impact on the design of multi-zone HVAC systems. For example, a multi-zone variable air volume (VAV) system serving a school cannot use a common return if the system serves a daycare or pre-school classroom unless that zone is separately exhausted and not returned to the main Class 1 system.

    There is typically less risk of system design errors when it comes to Class 3 and 4 air zones—these are air zones with significant contamination, highly objectionable odors, and potentially dangerous contaminants (in the case of Class 4 air). There is less risk because most designers, contractors, and owners (and codes) clearly recognize those zones and exhaust them by default. These zones include janitor’s closets (3), lab exhaust (4), and commercial kitchen (4) exhaust. The main difference between Class 3 and 4 is that Class 4 air cannot be recirculated even within the space of origin.

    For systems with heat recovery that are exhausting Class 2 and Class 3 spaces, the standard does allow for minor amounts (10% and 5%, respectively) of recirculation due to leakage across the energy recovery device. This allows a dedicated outside air system (DOAS) to effectively recover heat while maintaining acceptable IAQ.

    The three procedures

    The provision of adequate ventilation air to spaces occupies the bulk of the standard and is its most widely known area of scope. ASHRAE has divided the determination of adequate ventilation air and the methods of compliance into three procedures—ventilation rate, IAQ, and natural ventilation.

    By far, the ventilation rate procedure is the most widely used and adopted, and it consists of the fairly familiar requirements for certain quantities of outside air for different space types. The required rates within the ventilation rate procedure have changed over time as the science of IAQ has grown and evolved over time. In the 1970s, at the outset of the standard, the ventilation rates were much lower than those required today. Likewise, the current version of the standard splits the ventilation requirements into two categories—area-based and occupant-based rates. The area-based rates are intended to cover the ventilation required to dilute pollutants generated by the non-occupant loads within the space—such as furniture off-gassing. The occupant-based rate then covers the ventilation required due to occupant-source pollutants, such as CO2 emissions and body odor. A detailed coverage of the ventilation rate procedure is beyond the scope of this article.

    The IAQ procedure is probably the least used procedure in the standard, though it is the most flexible approach to ventilation. It is probably most appropriate when the indoor pollutant sources are very well known and atypical to the standard spaces already documented within the ventilation rate procedure. The IAQ procedure allows the designer to calculate the ventilation requirement based on specific pollutant emissions within the space and the quality of the outdoor air. However, because emissions for many sources are poorly understood and can vary widely, this procedure carries more risk for the designer and a higher burden of knowledge in researching the specific emissions rates of all the possible pollutant sources in the zone.

    The natural ventilation procedure provides rule-of-thumb design parameters for naturally ventilated spaces to guide designers to a basic approach that is suitable for most non-complex single or double-sided natural ventilation schemes. One key aspect of the 2010 edition of the standard is the requirement for naturally ventilated spaces to also have a mechanical ventilation system designed to either the ventilation rate procedure or the IAQ procedure. This requirement is only waived if the system is an “engineered” system that is approved by the authority having jurisdiction and/or has automatic controls that ensure the openings will provide adequate ventilation whenever the space is occupied. This requirement for a backup mechanical system was added to ensure adequate ventilation during period when manually operable windows might otherwise be closed—such as during very cold or hot weather or when outdoor air is objectionable (such as spring allergy season).

    Start-up and operations

    ASHRAE 62.1 also governs system start-up and operations. For sophisticated contractors and operators, the requirements are probably second nature. However, they can serve as a good guideline for less sophisticated operators and also are a good reminder of aspects of operations to design around and include in the operations and maintenance manual.

    ASHRAE’s Standard 62.1 provides the foundation for our understanding of achieving acceptable IAQ. It has a much broader reach into the building design and operations than many realize. It should be noted, though, that the science of IAQ is always evolving and that the requirements of ASHRAE 62.1, like most other standards and codes, represent the minimum requirements for acceptable IAQ. Designers should evaluate the literature, other regional standards (such as U.S. Green Building Council’s LEED and CEN CR 1752), the specifics of their project, and their client’s goals to ensure that systems meet their project’s overall needs.

    Peter Alspach is an associate principal and mechanical engineer in Arup’s Seattle office. His expertise is in HVAC systems design, building physics analysis, and façade engineering. Alspach is a member of the Consulting-Specifying Engineer editorial advisory board, currently serves as the secretary of ASHRAE SSPC 55, and is a board member for IBPSA-USA.


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