ASHRAE 62.1: Using the Ventilation Rate Procedure

ASHRAE 62.1 is best known for its regulation of the amount of ventilation air delivered to each space by HVAC systems through its Ventilation Rate Procedure for system design.


Learning Objectives

  • Understand the history, evolution, and organization of ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality.
  • Recognize the equations for the Ventilation Rate Procedure and how to use them.
  • Review example to enhance the understanding of equations in ASHRAE 62.1-2016.

While most mechanical design engineers are familiar with ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality, many are only familiar with the versions they've used through their careers-most use it to satisfy code or permit requirements or to achieve credit compliance for a high-performance building standard. The standard is best known for its Ventilation Rate Procedure (VRP), and its online spreadsheets and calculators have made achieving compliance (on paper) an "easy" step in HVAC design. What is missing, most times, is the understanding behind why ASHRAE 62 is necessary and how the standard evolves to optimize air quality.

Figure 1: In this ASHRAE 62 timeline, the revisions and major focus of each revision are illustrated. All graphics courtesy: Mortenson  In addition to the VRP, there are two additional methods for ventilation discussed in ASHRAE 62: natural ventilation and the indoor air quality procedure (IAQP). These will not be discussed here, but they are important to mention as options within the standard. The natural ventilation method is appropriate for buildings using natural ventilation in lieu of or in addition to mechanical ventilation. IAQP relies on numerical analysis of contaminant sources, contaminant concentration targets, and acceptability targets. For simplicity of a compliance analysis and because the VRP is the most common method for compliance, these methods are not discussed further in this article.

ASHRAE 62 was first published in 1973. It was the first ventilation standard published by ASHRAE and provided a prescriptive approach to minimum and recommended outdoor airflow rates for various indoor spaces. The intent was to provide a comprehensive enforceable method of establishing ventilation rates centered around indoor air quality (IAQ). It provided prescriptive volumetric airflow rates per person and began looking at the concept of acceptable outdoor air quality.

The intent was to provide a comprehensive and defensible method for establishing ventilation rates. The standard has had many revisions since it was first published, each with the intent of improving ventilation in buildings. For existing buildings, it is important to understand the iterations and evolution of the standard to identify how existing buildings might be ventilated based on which version was used at the time of construction or retrofit.

The second version was published in 1981 and reduced minimum outdoor-air rates, included a VRP, as well as introduced the IAQP. It also included provisions for cigarette smoking, recognizing that "higher ventilation rates are specified for spaces where smoking is permitted because tobacco smoke is one of the most difficult contaminants to control at the source." Additionally, recirculation air was addressed as well as provisions for "air cleaners and adequate temperature control" for recirculation air.

The next version, published in 1989, was influenced by a 1984 report from the World Health Organization (WHO) in which the WHO suggested that up to as much as 30% of buildings worldwide registered complaints related to IAQ. Through this, the term "sick building syndrome" was coined and is now synonymous with buildings in which building occupants experience health and comfort effects linked to spending time in a building.

These occupants have no other identifiable illness and report relief upon leaving the building. Common cited causes of sick building syndrome have been identified to be inadequate ventilation, chemical contaminants from outdoor sources, biological contaminants (mold, pollen, viruses), and chemical contaminants from indoor sources. Thus, the 1989 standard significantly increased minimum outdoor-air rates and introduced a requirement for multizone recirculating systems.

Minor changes and clarifications were made in the 1999 and 2001 editions, but these changes did not impact the minimum required outdoor-air rates. In those versions, the provision for thermal comfort was removed and it clarified the intent for 80% satisfaction. The satisfaction rate is achieved with 80% or more occupants expressing satisfaction with IAQ and spaces having no contaminants at harmful concentrations. Further, it recognized the harmful impacts of cigarette smoking and revised language around smoking.

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