As the vaccination rate continues to rise, the promise of returning to normal routines edges closer for many office workers. Already, schools in hundreds of districts around the country are fully open; New York City schools are due to reopen on March 22 and Chicago’s target date is April 19. But when they return, the majority of employees and schoolchildren will go back to old-fashioned ventilation systems, whose designs will do little to reduce coronavirus transmission by aerosol: that is, droplets that may linger in the air in enclosed spaces for some time after they leave a person’s body.
Thermal displacement ventilation, a system that’s been popular in Europe for more than 30 years but slow in adoption in the U.S., could help rid rooms of those virus particles more effectively. Though existing buildings can’t be retrofitted with the system, it could be a key design principle going forward, to improve not only indoor air quality, but also energy efficiency.
“We’re changing the paradigm for so many other things about our built environment in the wake of COVID,” says Simon Turner, an indoor air quality consultant who’s spent decades assessing buildings for their indoor environmental performance. “We should absolutely be looking at thermal displacement.”
Commercial buildings in the U.S. conventionally use “mixing ventilation” systems, where the air both enters and exits a room via the ceiling. As fresh air is pumped in, it relies on natural turbulence in the room to mix it with the existing, and possibly contaminated, air. The result is that the pollution concentration of the air becomes pretty uniform around the whole room.
“Everyone is dosed with the same pollutants,” Turner says, which could now include coronavirus particles. “So no wonder people get sick from other people, because you’re being exposed to their aerosols.” (That’s if the system works as intended; sometimes, it can “short circuit”: the fresh air pumped in leaves the nearby ceiling exhaust before it even mixes in the room, leaving the existing air as contaminated as before.)
In a thermal displacement system, air supply comes not from the ceiling, but from diffusers installed at a lower level. As the cool air circulates around knee height, it naturally finds heat sources—principally human bodies—and creates “thermal plumes” around those objects. As that air warms, it rises around each individual, giving every individual a “personal chimney” of fresh air. As it then passes up around people’s noses and mouths, the warm air picks up pollutant particles along the way and carries them out through the ceiling exhaust.
Because of that stratified system, whereby warmer, polluted air stays at the very top level of the room, away from people, multiple studies and reports have suggested that, if designed properly, thermal displacement has the potential to be more effective in preventing exposure to airborne particles than in the traditional setup, including studies that have considered SARS-CoV-2 specifically. The Environmental Protection Agency has also recommended that “designers should investigate” the system because it “successfully uses natural convection forces to reduce fan energy and carefully lift air contaminants up and away from the breathing zone.”
Turner suggests it’s a particularly good system for school classrooms, where infectious particles continuously circulate at all levels. The room also remains cool, because only warm air is exiting. (The temperature difference at different heights is too small to notice for the purposes of comfort.)
Notable buildings in the U.S. that use the system include Hearst Tower in New York, the Newseum in Washington, D.C., the Clinton Presidential Center in Arkansas, and the Monterey Aquarium in California. But it hasn’t been adopted more universally in everyday commercial spaces like in Europe, especially Scandinavia, where it’s found in malls, gyms, restaurants, theaters, and auditoriums.
That’s largely due to a culture of conservatism that pervades American building design. “We tend to be a bit conventional,” Turner says. “We tend not to like to go out on a limb.” Because it’s a relatively unusual concept in the U.S., engineers are reluctant to design new buildings around thermal displacement, mainly for fear of litigation in case of problems. “So most conventional engineers don’t even do it,” Turner says. “They don’t even give it a second thought.”
That doesn’t mean it’s hard, or even that it’s revolutionary. “It’s not rocket science,” he says. “This is not new technology, by any means.” Besides, it likely works out cheaper for building owners in the long run, because of the “spectacular energy savings,” he adds. That’s because it takes advantage of basic physics: cool air falls and warm air rises. Instead of disrupting the natural plumes of warm objects, the system harnesses them. And it uses a warmer supply of air, which is pumped in at 65 degrees, versus 55 in the mixing system, and much less airflow is needed.
Thermal displacement doesn’t come without challenges. It provides only cooling, so colder climates need separately engineered heating apparatuses, producing additional costs. (The typical solution is to install perimeter heating that warms up a space once, early in the morning, then goes off and lets the cooling structure alone balance out temperatures.)
Some studies have also warned of the “lock-up effect,” whereby warm, exhaled air, potentially polluted, may temporarily linger at an intermediate level, closer to a person’s head, before traveling farther up toward the ceiling. The largest drawback is that retrofitting older buildings is not an option, due to a fundamentally different setup and distinct equipment. But when assessing new buildings, planners should be intrigued by the long-term savings from the energy efficiency, particularly school districts constrained by budgets.
Turner thinks the pandemic could finally be the incentive for reluctant engineers to go to bat for thermal displacement. In the current economic climate, they’re likely to be more receptive to the idea. “[COVID-19] was such an existential threat to the commercial real estate industry,” he says. “Nothing is off the table anymore.”