Designing Schools That Achieve ‘Valedictorian of Air Quality’ Status

Promoting Healthy Indoor Air Quality in the Classroom

As HVAC professionals, one of our primary goals is to promote healthy indoor air quality in all spaces, and there are fewer applications where this is more important than in classrooms. It is estimated that approximately half of public school districts need to update or replace the HVAC systems in their schools.¹

Active Chilled Beam Cabinets (ACBCs) retrofitted to the perimeter of a classroom deliver displacement ventilation to occupants while conditioning exterior windows

An overwhelming body of research has shown that improving air quality in classrooms increases student performance.² Benefits include increased cognitive function,³ reduced absenteeism⁴ and improved test scores.⁵ One study found that for every additional 2 cfm (1 L/s) of ventilation air per student, test scores increased by roughly 3%.⁶ These improvements extend to teacher satisfaction and benefit school systems that rely on student performance for funding.

How Do We Quantify “Good” Air Quality?

With the exceptions of pollen and pollution, outdoor air is often considered the gold standard of air quality. This is because outside air is free of the unwanted substances that build up indoors, including CO₂, aerosols, volatile organic compounds (VOCs) and fine, inhalable particulate matter (PM_2.5). ASHRAE 62.1-2022, Ventilation and Acceptable Indoor Air Quality specifically outlines the minimum outdoor air requirements for indoor spaces.⁷

Minimum outdoor airflow rates for select spaces at design occupancy according to ASHRAE 62.1-2022; a 1,000 sq. ft. classroom requires a minimum of 630 cfm of outdoor air

The need for outdoor air is often at odds with the costs of conditioning that air for an indoor environment. Moist air (relative humidity greater than 60%) increases the risk of mold issues, while dry air (relative humidity less than 30%) can result in dry eyes and throats and a longer hang time for airborne particles. Room temperature should be maintained between 72°F and 77°F for classrooms.⁸ The cost of conditioning outdoor air incentivizes HVAC designs to utilize less outdoor air to be more energy efficient.

What Makes Schools Challenging?

Educational facilities have a high occupant density, resulting in increased demand for cooling and outdoor air relative to typical commercial spaces. Additionally, many existing public schools are aging, and their HVAC systems were designed to older design standards with lower air quality requirements. Unfortunately, implementing quick fixes within an existing HVAC system, such as upgrading to high-efficiency filters or increasing outdoor airflow, is not always possible.

Methods for Improving Indoor Air Quality

The minimum outdoor air requirements for classrooms in ASHRAE 62.1-2022 are just that – minimums; it is often preferable to go beyond this threshold. One rule of thumb when determining ventilation rates for schools is to achieve six air changes per hour (ACH) of fresh air to the breathing zone of occupants (ACH_e). This is achieved with a combination of outdoor air (ACH_oa), filtering (ACH_f), in-duct air cleaning (ACH_e,c), in-room air purifiers (ACH_ir) and improved ventilation effectiveness (E_z), expressed by the following equation:⁹

ACH_e = E_z (ACH_oa + ACH_f + ACH_e,c ) + ACH_ir

For a 1,000 sq. ft. classroom with a 9 ft. ceiling, this equates to 900 cfm. Higher airflow rates, such as 10 or 12 ACH, will result in better air quality, but they face diminishing returns relative to the increased cost of the mechanical system.

Distribution of CO2 levels using displacement versus mixed air ventilation

There are three primary methods to achieve this ventilation rate:

1. Design the Mechanical System With Greater Airflow and Filtration: It is important to ensure air jets from grilles and diffusers are sufficient to mix air in the room and are supplying enough fresh air. This includes checking that the outdoor air component of the mixed air meets ASHRAE standards and recirculated air goes through MERV 13 filters (or better). MERV 13 filters are rated to capture approximately 77% of infectious aerosols¹⁰ and 85% of PM2.5,¹¹ meaning approximately 80% of the recirculated air can count toward this fresh air requirement. MERV 8 filters, while helpful for capturing larger particulates, are insufficient for capturing infectious aerosols and submicron particulates. Note that implementing higher-efficiency filters in an existing mechanical system may be costly and require replacement of the mechanical equipment.
   
   
2. Add Targeted Air Purification Devices: Air purification equipment can provide additional filtered airflow to a space independently of the comfort cooling system. A single Price Overhead Air Purifier (OAP) can achieve an additional 3 ACH (500 cfm) in a typical classroom without exceeding a noise level of NC 35. These can be considered for spaces such as a nurse’s office or gymnasium where occupant conditions and loads warrant additional air purification. Air purification devices only require a 120V power supply and the occasional filter change. The recirculated air from air purification devices does improve indoor air quality, but this does not count toward the minimum outdoor air requirements in ASHRAE 62.1-2022.
   
   
3. Move the Air More Effectively With Displacement Ventilation: Case studies have shown that displacement ventilation (DV) can decrease CO₂ levels relative to traditional mixed air ventilation systems (MV) in a real-life classroom setting.¹² Research performed at Price Research Center North in Winnipeg, MB, has demonstrated that seated occupants are exposed to 58% fewer aerosols generated by neighboring occupants in a DV system compared to MV at 6 ACH.¹³ ASHRAE 62.1-2022 allows up to a 33% decrease in outdoor air for designs using DV.
   
   DV systems function by allowing heat sources in a room to drive the room air up to the ceiling using natural convection. This results in a vertical thermal plume above each student that individually carries particles up to the ceiling and improves ventilation effectiveness. Wall-mounted displacement solutions, such as the Price Displacement Flow In-Wall Diffuser (DFW) and Quarter-Round 90° Displacement Diffuser (DR90), fit seamlessly into the wall spaces of classrooms. And challenging perimeter spaces with old unit ventilators can be easily retrofitted with Active Chilled Beam Cabinets (ACBCs).

For more information about improving air quality in schools, and the air distribution solutions that Price has to offer, visit the Price High-Performance Schools website at www.pricevm.com/schools.

References

1. “K-12 Education: School Districts Frequently Identified Multiple Building Systems Needing Updates or Replacement,” US Government Accountability Office, published June 4, 2020.
2. J. Mendell and G. A. Heath, “Do Indoor Pollutants and Thermal Conditions in Schools Influence Student Performance? A Critical Review of the Literature,” Indoor Air, 15, 1 (2005): 27–52.
3. G. Allen et al., “Associations of Cognitive Function Scores With CO₂, Ventilation and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments,” Environmental Health Perspectives, 124, 6 (2015): 805.
4. G. Shendell et al., “Associations Between Classroom CO₂ Concentrations and Student Attendance in Washington and Idaho,” Indoor Air, 14, 5 (2005): 333–341.
5. J. Shaughnessy et al., “A Preliminary Study on the Association Between Ventilation Rates in Classrooms and Student Performance,” Indoor Air, 16, 6 (2006): 465–468.
6. Haverinen-Shaughnessy et al., “Association Between Substandard Classroom Ventilation Rates and Students’ Academic Achievement,” Indoor Air, 21, 2 (2011): 121–131.
7. American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), ANSI/ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality (2022).
8. ASHRAE, “Educational Facilities,” chap. 8 in ASHRAE Handbook – HVAC Applications (2023).
9. Price Industries, “Air Purification,” chap. 22 in Price Engineer’s HVAC Handbook, 2nd ed. (2023).
10. ASHRAE, ASHRAE Standard 241-2023, Control of Infectious Aerosols (2023).
11. ASHRAE, ANSI/ASHRAE Standard 52.2-2017, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size (2017).
12. Eley et al., “Displacement Ventilation in Action: Performance Monitoring of Demonstration Classrooms,” ACEE Summer Study on Energy Efficiency in Buildings (2006).
13. Ryan Johnson and Chris Burroughs, “Reducing Airborne Particulates Using Displacement Ventilation,” ASHRAE Journal, December 2022, 38–48.

Ryan Johnson is Manager of Engineering Programs for Price's Innovation and Learning team. He is based out of Price's facility in Suwanee, GA.