Until recently the guidance for mitigating airborne virus spread has largely focused on maximizing outside air (OA) ventilation. For months, enVerid and other industry experts have highlighted the huge energy toll (not to mention the carbon emissions impact) of this strategy, pointing to research that demonstrates high-efficiency filtration is as effective from a relative risk standpoint as increasing ventilation rates, but far less costly from an energy and system standpoint.
Recently, in an under-reported change to its , ASHRAE’s Epidemic Task Force acknowledged the energy penalty associated with increased OA ventilation, and offered an alternative approach. ASHRAE recommends that those concerned about the high energy spend associated with maximum OA ventilation instead use minimal OA ventilation combined with high-efficiency filtration.
While this published guidance is new, as early as June ASHRAE’s ETF was sharing its findings that high-efficiency filtration offers equivalent efficacy without adding significant expenditure.
In a June 18th presentation, “Fundamentals of COVID-19 Risk Management,” William Bahnfleth, Chair of ASHRAE’s Epidemic Task Force and professor of Architectural Engineering at The Pennsylvania State University, shared research showing that the mean relative risk of infection can be reduced by filtration or by ventilation, and that using filtration to achieve the same infection risk as an equivalent ventilation rate increase is much more cost effective. This research and additional studies are also explored in our recent
Professor Bahnfleth stated that “going from lower filter efficiency to higher filter efficiency leads to a significant reduction in infection risk with not much change in annual cost” and “achieving the same relative risk of infection using ventilation is much more costly, primarily due to the energy cost of increased ventilation rates.”
Key recommendations in the updated ASHRAE ETF Commercial Guide include:
- If there are significant energy impacts from increasing outside air for ventilation, use minimum outside air as required by Standard 62.1 with a minimum MERV-13 filter.
- If outdoor air quality is not healthy per Section 4 of Standards 62.1-2019, especially high particulate matters, do not open windows or increase ventilation without using the proper filters.
- Maintain relative humidity between 40% and 60% where possible.
- Consider portable air cleaners for conference rooms, private offices, and stairways; and when a room has insufficient outside air per code requirements and filtration is below MERV-13.
- Consider UVC light as an enhancement where spaces require additional measures, g. spaces that serve vulnerable occupants, or when MERV-13 filtration or 100% outside air are not possible.
Bob Ioanna, a senior principal at Syska Hennessy summed up the shift in guidance in his comments in our press statement:
“While building owners and tenants want to do everything in their power to mitigate the risk of transmission of COVID-19 in their commercial settings, the high costs and system impacts of maximum OA ventilation guidance were a major concern,” said Robert Ioanna, Senior Principal at Syska Hennessy. “ASHRAE’s updated guidance provides helpful clarification that using minimum outside air as required by ASHRAE 62.1 with MERV-13 or higher efficiency filters is the preferred approach when there are significant energy impacts from increasing outside air for ventilation and/or when there are concerns about humidity control or outdoor air quality.”
Ioanna makes an important point that another big concern about the maximum OA ventilation strategy is outside air quality and the high occurrence of dangerous particulate matter (PM 2.5). Pollution is always a concern in cities. Writ large in the West right now are the tragic California wildfires that are not only threatening the communities near them, but those downwind are confronting dangerous air quality, some rated the worst in the world last week.
To simplify a complicated and technical discussion, we’ve created a video that should help both the lay person and the expert make the argument for high-efficiency filtration as the preferred HVAC strategy for addressing airborne transmission of COVID-19. Watch it below.
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