September 30, 2021

School ventilation strategies for a pandemic

Riedel

Haugk

How could future schools be designed so they are safer during a pandemic? By planning ahead, strategically selecting proven systems, and implementing best practices, it is possible to construct new schools with reduced potential for viruses to spread.

One of the most effective defenses against airborne pathogens is ventilation. Teachers and students spend most of the day in classrooms with upwards of 30 other people, giving communicable illnesses significant potential. Some factors that affect the spread of an airborne virus in classrooms (other than viral load), include how much outside air is delivered, how the air is mixed in the room, how much air is returned, and how well the air is treated before being returned to the classroom. When creating a new school, we have an opportunity to provide a healthier indoor environment.

Images courtesy of NAC Architecture [enlarge]

At Stevens Creek Elementary School in Lake Stevens, white displacement ventilation grilles are visible under the bench seating in the open library.

Traditional ventilation systems provide airflow at the ceiling, relying on mixing for controlling temperature within the space. By contrast, thermal displacement ventilation introduces air low in a room and at low velocity so that it does not mix. The body temperature of each occupant heats the air around them, creating an individual thermal plume that rises toward the ceiling. This warmer air is then returned at the ceiling level to an air handling unit to be partially exhausted, filtered, and then partially recirculated. By not mixing the air, when a student breathes or coughs there is less chance for the potentially contaminated air to be inhaled by other students. School districts with these systems have confirmed through testing and data collection that absenteeism prior to the pandemic dropped by 4-8%. You can see a simulation of the effectiveness of displacement here: https://tinyurl.com/ykahjh59

NAC Architecture and Hargis Engineers pioneered the design of displacement ventilation in schools in Washington in 2003, with the Phase 1 project at Silas High School in Tacoma and at Little Cedars Elementary School in Snohomish. To avoid potential maintenance issues with floor vents, the toe kick of the classrooms’ perimeter casework are used as a supply air plenum. The skylight shafts in each classroom are used to bring air back to the mechanical penthouse, where it passes through a heat exchanger before being exhausted to the outside. In occupied mode, this is a 100% outside air system, further increasing ventilation effectiveness by not returning any air back to the classroom during occupied periods.

Knowledge of these systems has positioned our state ahead of the curve. In subsequent years, both displacement ventilation and heat recovery have become fairly commonplace in new Washington state schools, with a variety of iterations on the specific designs. Because COVID-19 has increased awareness of the disruptive potential of viruses, we anticipate this air delivery method will become even more common. The heat recovery portion allows for 100% outside and exhaust air while capturing heat to avoid wasting energy.

In addition to indoor air quality benefits, the slow movement of air helps lower mechanical noise in classrooms, creating a better learning environment. The effectiveness of displacement ventilation also reduces fan energy, as less air is needed to heat or cool the space.

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The displacement ventilation system at Silas High School is integrated into the building design, using toe kick grilles at the perimeter casework to supply air and the open daylight shafts to return it for heat exchange before exhausting to the outside.

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Filter ratings starting at MERV 13 are effective at removing bacteria and virus particles.

While planning a school with displacement ventilation, it is important to accommodate the shafts early in the design process to ensure they do not pose issues for classroom layout. Note that the lower velocity air requires increased duct sizes. While these factors typically make it difficult to accommodate displacement ventilation in a limited remodel, there are numerous local examples of successfully incorporating them in school designs. With the increased benefits of displacement ventilation, manufacturers of grilles have a variety of options to work within any space. Some are available to install at the ceiling level or within a wall cavity that provide the same effectiveness.

There are other options to consider for improving air quality in schools, either in addition to displacement or with traditional mixing systems. These include improved air filtration (MERV 13+) for central air-handling equipment, humidification, modified control sequences, and air disinfection devices like UV lights, ionizers and electrostatic precipitators. Proper filtration can effectively capture many airborne viruses. Be sure to consult with your architect and mechanical engineer for current knowledge regarding the safety and effectiveness of these components.

Philip Riedel is a principal and PK-12 practice leader at NAC Architecture, as well as the Pacific Northwest director for the International Board of A4LE. Brian Haugk is a mechanical engineer and principal at Hargis Engineers.

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