Tyrone Burke, November 16, 2020
Photo credit: Luther Caverly, file photos

Occupancy-based ventilation could help reduce airborne transmission of COVID-19 in indoor environments

The primary purpose of heating and cooling systems in commercial buildings is temperature control. They usually accomplish this by controlling the flow of air, but they were not designed to limit the spread of an airborne virus.

“COVID-19 moves quite differently in indoor environments than it does outdoors,” says Burak Gunay, an Assistant Professor of Building Science in Carleton’s Department of Civil and Environmental Engineering.

“Staying two metres apart might be adequate in an outdoor setting, but might not be in a commercial building. We are trying to understand the deficiencies that common heating, ventilation and air conditioning (HVAC) configurations have when it comes to delivering and maintaining sufficient indoor air quality.”

Gunay has been awarded an NSERC Alliance Grant for this research, which he’s conducting with Sensible Building Science and Cisco.

Most of the air in commercial buildings is recirculated. Typically, only 20-40%  is fresh air from outside. Increasing the proportion of outdoor air could help reduce the possibility of the virus remaining aloft in the indoor air, but extreme outdoor temperatures present a challenge.

“You could maximize ventilation, and try to deliver 100% fresh air. This might be possible in moderate climates, but not in Canada,” says Tareq Abuimara, who recently completed his PhD in Civil Engineering and is a researcher on the project.

“It would be a very energy intensive thing to do — you could easily triple the amount of energy use if you decided to ventilate with 100% fresh air during winter. It could also damage HVAC equipment, which isn’t designed to operate with that much frigid air. So Abuimara is using computer modelling to determine how to keep the air fresh, without overloading HVAC systems with cold winter air.

“By using measured room-level occupancy data from several case study buildings, I’m trying to understand how outdoor air is distributed to different parts of a building and to analyze the effectiveness of different occupancy-centric ventilation strategies,” he says.

“Pre-COVID, we wanted all occupants to get at least about ten litres of fresh air per second. In order to achieve higher ventilation rates for COVID, we have explored the use of occupancy-based ventilation that targets fresh air to the parts of a building where there are people. This dilutes existing recirculated air and lowers the risk of infection. Our findings indicate that we need to at least to triple the per person ventilation rate, but we don’t need to use 100% fresh air.”

Abuimara worked with occupancy data and analysis from Brodie Hobson, who is currently a PhD student in Civil Engineering at Carleton.

“There is sometimes an assumption that people are evenly distributed throughout a building, but there can be a mismatch between the number of people that a room was designed for, and the number of people who actually use it,” says Hobson. “For example, our building has a large computer lab that is often filled with undergraduate students because it has the software that they need to do their assignments. But the rest of the floor is relatively empty. Many rooms may be entirely vacant. Redirecting the fresh air intended for vacant spaces to spaces where all the people are could help us increase ventilation while also saving energy.”

It isn’t always obvious which parts of a building are most heavily used, so Hobson has used occupancy data to identify use patterns.

“We don’t have radio tags on people, or security cameras in every room. To keep track of where people are going, we need to work with the data we already have. The obvious answer is Wi-Fi. It’s a very good proxy for the number of people who are in the building,” Hobson says.

“We’ve found a correlation between the number of devices, and how many people are actually there. Within the Canal Building, we found an average of about 1.2 devices per person. There are several Wi-Fi access points on the floor, and we looked at the number of devices connected to each access point. Then, based on the floor geometry and additional sensor data in the building, we can figure out how many are in a room.”

Data from motion detectors and carbon dioxide sensors – both common features of modern office buildings – can also help determine how a building is being occupied. Once a building’s occupancy is known, its HVAC system can be programmed to direct fresh air to the most heavily used areas.

Their findings could help develop recommendations for how building ventilation systems can be managed, such as those made by the American Society for Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE), which established an epidemic task force in March 2020.

“This research is quite scalable. It uses existing technologies and HVAC configurations,” says Gunay.

“COVID-19 is a big challenge for the commercial building sector, to make sure that COVID-19 outbreaks do not emerge. The solution is really occupancy sensing and occupancy-based ventilation. In the past, the priority in office buildings was never in indoor air quality, it was heating and cooling. I think we will see changes in the design of indoor air quality standards after COVID-19.”

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