The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) was founded in 1894 and has now evolved into a global association with more than 50,000 members. ASHRAE standards cover many aspects of heating, ventilation, air conditioning, and refrigeration, and building codes around the world use them as a reference.
The ANSI/ASHRAE 62.1-2016 standard is called “Ventilation for Acceptable Indoor Air Quality.” The standard specifies a design procedure for natural ventilation and two options for mechanical ventilation systems: the ventilation rate procedure (VRP) and the indoor air quality procedure (IAQP).
VRP and IAQP follow different approaches to achieving an adequate level of ventilation. While VRP is based on prescriptive measures and ventilation tables, IAQP is based on performance – providing a ventilation system that effectively controls air pollutants.
Optimize your ventilation design and reduce energy consumption.
Ventilation Rate Procedure: The Prescriptive Approach
The ventilation rate procedure is widely used as it involves standardized calculations that are well known in the HVAC industry. Based on air pollution research, ASHRAE has determined ideal ventilation rates for each building type, which are presented per square meter and per occupant:
- Design airflow per unit area is expressed in cfm per square foot.
- The occupancy component is expressed in cfm per person.
For example, the minimum ventilation rate for a restaurant is 0.18 cfm/sq.ft. and 7.5 cfm/person. If the area is 5,000 square feet and the restaurant is designed for 200 people, the required ventilation rate is as follows:
- Area component = 0.18 cfm/square feet x 5,000 square feet = 900 cfm
- Occupancy component = 7.5 cfm/person x 200 people = 1,500 cfm
- Total airflow = 900 cfm + 1,500 cfm = 2,400 cfm
This value is called Breathing Zone External Airflow in the ASHRAE 62.1 standard. It is then divided by the Zone Air Distribution Effectiveness, to obtain the Zone Outdoor Air Flow that must be supplied by the ventilation system. If the distribution efficiency in the example above is 0.8, the zone's outdoor airflow should be 3,000 cfm (2,400 cfm/0.8).
A single-zone ventilation system requires only an external airflow calculation, and non-recirculating multi-zone designs are based on a simple addition of individual airflows. The most complex scenario is a multi-zone system with recirculation, where ASHRAE provides a detailed stepwise calculation procedure.
Demand-controlled ventilation (DCV) can adjust outdoor airflow according to occupancy, but cannot fall below the area-based airflow component. For example, the 2,400 cfm outdoor airflow calculated above cannot be reduced below 900 cfm.
Indoor Air Quality Procedure: The Performance-Based Approach
The Indoor Air Quality Procedure (IAQP) does not establish a minimum supply of outdoor air. Instead, it provides design guidelines for a ventilation system that maintains pollutant concentrations below a threshold value. In other words, IAQP can achieve lower airflow than VRP, reducing the workload on the HVAC system. Fans consume less energy because they move less air and associated heating and cooling costs are also reduced.
Because IAQP is performance-based, the design process is open and flexible:
- Contaminants of concern (COC) are identified for the project.
- Once the list is available, the next step is to identify your sources and the emission rate from each source.
- A maximum concentration is determined for each air pollutant, based on guidance values from a reliable source.
- The ventilation system is designed in accordance with airflow requirements that will keep pollutants below the specified limit.
- Additionally, an air quality assessment is carried out by the occupants.
The IAQP is limited by the lack of standardized limits for air pollutants and also by the subjective nature of occupant assessments. For this reason, many building codes still do not approve IAQP. The same applies to LEED certification, where only VRP is accepted.
To achieve the benefits of IAQP while meeting building codes and LEED requirements, both approaches can be combined. VRP establishes the minimum outdoor airflow requirement, while IAQP improves air quality without reducing outdoor airflow below VRP limits.
Conclusion
Ventilation systems have a small energy consumption compared to air conditioning and space heating equipment, but their design has a significant impact on the efficiency of the building. Ventilation design determines outdoor airflow, and higher airflow increases heating and cooling loads.
There are a wide variety of HVAC configurations, and choosing the right system for your building is critical to indoor air quality and energy efficiency. A professional MEP engineering firm can specify the ideal equipment and system layout.
