ASHRAE 62.1: A review of key requirements and concepts
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.