Use ASHRAE 62.1 to enhance design of ventilation systems

Mechanical engineers should understand ASHRAE Standard 62.1 and how to apply it

By Consulting-Specifying Engineer February 1, 2024
Courtesy: Amara Rozgus, CFE Media and Technology

ASHRAE 62.1 insights

  • Learn about ASHRAE, International Mechanical Code and other guidelines that drive HVAC system design and specifications.
  • ASHRAE Standard 62.1 establishes national guidelines for indoor air quality (IAQ), incorporating factors such as occupancy rates and space types to determine minimum ventilation requirements, with considerations for energy-efficient dynamic reset and demand control ventilation.

The national consensus standard for outside air ventilation is ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality and its published Addenda. This standard is often incorporated into state and local building codes and specifies the amounts of outside air that must be provided by natural or mechanical ventilation systems to various areas of the building.

While watching the webcast HVAC: IAQ and IEQ is a much more complete overview, reviewing this transcript of the presentation helps define the topic better. It has been edited for length and clarity.

This information was presented by Garett Karalus, PE, CCP, Director of Engineering/Lead Commissioning Agent, IEA Inc., Brooklyn Park, Minnesota.

ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality is the main standard to specify the minimum ventilation rates for buildings to provide indoor quality that’s acceptable to human occupants. This one can be the most simply broken down into the area rate and people rate, a ventilation rate for the type of space and several occupants in a space and then also the type of space and the area of that type of space.

Standard 62.1 Table 6-1 is one of the tables that is used to break up this formula for specifying the middle ventilation rates. The rate for a person in an office setting is going to be different from the rate for somebody in a dance hall, as the activity levels are going to be way different in those two situations. And it’s going to require a different amount of ventilation for an acceptable indoor air quality. This table provides different rates for the people rate and for the area rate and then also has some default values for occupant density and some different space types.

Some other considerations for specifying the minimum ventilation for spaces. One consideration is the zone air distribution effectiveness, which is essentially taken account for the supply and return. And the ultimate heating, ventilation and air conditioning (HVAC) system layout for the space.

After this is accounted for, you can usually assess individual spaces as if they were to serve by a single air handling unit (AHU). But for multizone systems or for more complex systems, there are a few other considerations to consider. Occupant diversity, if the total system population is less than the sum of all the individual space populations, there can be a reduction in the minimum required amount of ventilation.

For example, if you had eight classrooms and only six of the classrooms were occupied at any one time, let’s say one was an art room and one was a music room that got used sparingly, then you’d have a lower system population than the actual sum from each individual space. And then also variable load conditions.

There are some equations to allow the design professional to reduce the minimum required outdoor air flow rates just because a conference room might only be used for 30 minutes or an hour a day or briefly for periods throughout the day. And it’s just accounting for that periodic use and lowering the requirements a little bit due to the variable load conditions. And then outdoor air flow, dynamic reset and demand control ventilation is resetting the outdoor air flow based on changes in operating conditions.

This isn’t necessarily to change the design rate, but it’s to allow control of the system to be more energy efficient and control based off the changing occupant conditions. And this is usually done by occupancy sensors and carbon dioxide (CO2) sensors to indicate rooms that are occupied and to then incorporate that data to provide a ventilation rate that will meet the requirements of the building, as things change, as the people go in and out.

One note to make is that the designer still needs to address the contaminants that aren’t associated with the occupancy levels. And then also that the minimum area of ventilation rate from the table in ASHRAE 62.1 still needs to be met for each zone. Sometimes that the dynamic resets can be incorrectly designed to some arbitrary value instead of making sure that that minimum area rate is maintained from the table.