With the renewed focus on increasing ventilation rates, natural ventilation specialist, Thomas Bamber, discusses how to strike a balance between airflow and comfort – in winter and summer.
As an HVAC engineer who keeps descriptions of what I do fairly lean, it’s bizarre how ventilation systems are suddenly such a popular topic. It’s hard not to smile when your stepmom is suddenly passionate about HEPA filters. Never have people had such strong opinions about which HVAC systems operate in their homes, schools, and offices. And with good reason.
We have understood that the coronavirus can become aerosolized and remain suspended in the air for hours for over a year. As was highlighted in this influential SFGate article and graphic from The BMJ, social distancing rules are superseded by new guidelines informing whether an event or situation is low, medium, or high risk. These guidelines are intended to assess transmission risk when all parties are asymptomatic, and it should go without saying, in fall and winter 2021, we stay home when we have symptoms, masks are worn, and hands are washed frequently.
These charts conceptualize the virus transmission risk of the “4 °C’s” (close, closed, crowded, and continuous) when a mask is worn and not worn. Note this only applies to presymptomatic/asymptomatic persons. © BMJ
Front and center, now, is the contest between ventilation rates and the creation of airborne COVID-19 particles through speech. Intrinsic to this relationship are those twin factors; the duration and occupant density of events.
I have found myself referring to this graphic with some frequency, for example, when recently entering a restaurant in Vancouver. The restaurant was low-occupancy, and most were servers wearing masks. However, the room was poorly ventilated, and due to the loud music playing, people raised their voices – a factor not helped by masks. Whether this was a medium- or high-risk space would be determined by whether contact was for a short or prolonged duration.
If key metrics against asymptomatic transmission are wearing masks, occupant density, contact duration, the volume of speech, and ventilation level, what options do we have at school, at work, and in other public settings such as restaurants? Our collective mental health, education, and livelihood are at stake, and most people will find it necessary to have prolonged contact with more than a handful of individuals. The most desirable option is to make our environments as safe as possible so that we can function as normally as possible. This brings us back to building ventilation systems.
How much airflow do we need?
What is still unknown and will likely remain unknown for some time is the rate at which exhaled droplets become airborne vs. the viral load required for COVID-19 transmission to occur, as well as other factors (e.g., the characteristics of people and situations).
Aerosolized infections have been studied for other respiratory viruses, including the common cold, and these studies can provide hypothetical guidance on the role of ventilation. An ASHRAE Journal from 2014 publishes a study by Sun, et al. that found a lower incidence of common cold infections were associated with higher ventilation rates in college student dormitories.
The results are presented in the graph below, and the correlation between airflow, or lack of, and transmission is startling. The most ventilated group of dormitories that were between 1.5-7 Air Changes per Hour (ACH) experienced 75% fewer incidences of the common cold than the worst-ventilated ones (less than 0.5ACH).
To minimize the risk for COVID-19 re-entry, the World Health Organization has recommended 6ACH as the target for indoor ventilation rates, i.e., replace the air in a space with outdoor or filtered air six times every hour. The Harvard T.H. Chan School of Public Health likewise recommends 4-6ACH, which achieves a 98.2-99.8% dilution rate.
How do we increase ventilation to make systems safer?
What makes this question challenging is that buildings and gathering spaces are all very different. There are enough ventilation, heating, and cooling systems out there that, even after ten years of designing, we are still coming across new and unique approaches.
Most new buildings provide around 1ACH as their base rate of outdoor air; to supply oxygen, remove CO₂ and body odor. Most air-handling systems can have their controls systems adjusted to maximize airflow part-way or, in some cases, all the way to 6ACH, either providing 100% outdoor air or through recirculating air through the Air Handling Unit (AHU). The latter can still be made safe for COVID-19 if appropriate filtration such as MERV-15 is provided.
The unfortunate reality for some buildings is that our mechanical systems will fall well short of 6ACH. The recommended solution here is to use in-space air scrubbers, which “clean” the air. This excellent webinar by Perkins&Will highlights some of the challenges and solutions for buildings and climates where natural ventilation is not an option. There are many air cleaners available; the simplest and most cost-effective are indoor HEPA filtration units which are 99% effective at removing airborne pathogens and do not generate any undesirable secondary chemical products.
The ability to open the windows is powerful. Buildings designed for summertime comfort with natural ventilation typically provide 6ACH as their base airflow rate on hot, windless days. If there’s a small amount of wind pressure, this can quickly increase to 10ACH, 15ACH, and more. You generate far more airflow by opening windows than you do by driving air down narrow ducts, using fans.
How to achieve natural ventilation comfort in summer
There is plenty of natural ventilation science, and there are ways to apply it well and poorly. Many building owners won’t go near-natural ventilation systems, having been stung by older systems that failed to achieve comfort or were too complex for occupants to control. However, if done properly, natural ventilation creates unique healthy spaces connected to the outdoors, which makes us more productive.
The basic principles for summertime comfort are:
- Provide generous airflows to remove all heat gains. A single window often isn’t enough; cross-ventilation is much more effective.
- Make sure heat gains are low, to begin with. In particular, ensure the space is well shaded.
A good example of a recent project is Integral Group’s Clayton Community Centre in Vancouver, BC. Key project principles included cross-ventilation and large openings to generate 6ACH across large volumes of space on warm days. (During a 2021 summer heatwave in British Columbia, Clayton Community Centre also acted as an Extreme Heat Cooling Centre for Surrey residents needing relief.)
What about natural ventilation in winter?
In many quarters, governments and health experts are urging us to keep windows and doors open this winter and send kids to school wrapped in blankets. Is there a way we can regulate air intake and limit the worst of cold discomfort? To achieve good airflows with smaller openings in winter, compared to summer. Cold air is denser and quickly floods into a heated space.
The Clayton Heights library space was designed with 16 m² of window openings. On a warm summer’s day, that provides 6ACH. Open the windows fully when it’s -1°C out, generating 20ACH of cold air, even with no wind. Position the openings at one-third, on the other hand, and this brings us to 6-7ACH. There is still a lot of chilly air, but with the heating systems on full blast, you have a better chance of keeping the space temperate. Local plug-in heaters, radiant heaters, and hot water bottles can all help – there is a consensus that energy efficiency can, pardon the pun, go out the window this winter.
There are two big caveats here. The first is that, yes, higher levels of ventilation do reduce COVID-19 transmission risk further. And second, many spaces have openings that barely achieve 2ACH in summer and would want to be fully open in winter.
As ventilation expert Dr. Shelly Miller wrote in this Fast Company article, it is difficult to measure air change rates in space, but there is a good proxy for doing so. CO₂ sensors are relatively cheap and will tell you if a space has low or high levels of exhaled air. External levels of CO₂ sit at 400 parts per million (ppm), and a well-ventilated space typically has CO₂ levels of 800ppm. To maintain optimum ventilation levels relative to occupants during COVID-19 re-occupancy, Dr. Miller recommends targeting below 600ppm.
Natural Ventilation Retrofits
Understanding that high ventilation levels are important in the fight against COVID-19 has had some developments back-peddling to incorporate natural ventilation midway through construction. For existing buildings in a suitable climate, there’s hope as well. Renovation projects like the Hawkins\Brown project ARK Academy Putney in London show that even very poorly constructed 1970’s buildings can be adapted to achieve excellent thermal comfort with natural ventilation.
Key innovations for this renovation included:
- Replacing the whole curtain walling with air-tight, solar controlling glazing
- Removing t-bar ceiling to expose beneficial thermal mass
- Cross-vents to corridors and fans help provide boost flow on the hottest days of the year
Even spaces with limited access to the outdoors can find ways to achieve 6ACH. This warehouse space below is my previous office, which was a deep plan and single aspect only. Ducts from the back of the space draw air to the outside, and the fans are low-energy as it’s a short length of duct only.
Short-term or long-term thinking
A question that prompts plenty of discussions is whether the changes we make now will be beneficial beyond COVID-19. Many more drastic solutions, such as in-space treatment systems, provide limited future benefits beyond the next pandemic. Improving filtration systems is more easily justified given the benefits to occupant health and, in many parts of the world, increasing the risk of poor air quality from forest fires.
The benefits of having the option to open the window go well beyond this pandemic. As we discuss in our IG VISION webinar, mixed-mode buildings (i.e., have the option of mechanical and/or natural ventilation and cooling) are far more resilient and well-placed to deal with any manner of future events. As we have learned during COVID-19, having that crucial connection to the outdoors in our homes, offices, and schools, we will become and stay more resilient in the future.
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