Late Fall 2020 ushered in a change in climate and a shift in expert guidance on HVAC approaches for mitigating airborne transmission of COVID-19. Initial HVAC recommendations from ASHRAE and others rightly received a lot of attention. However, like so much about this novel coronavirus, scientific research and insights advance weekly. It is paramount that those responsible for occupant safety in our built environment understand the important evolutions in guidance and are well informed on the range of HVAC IAQ strategies and tools at their disposal for curtailing airborne virus transmission.
Scientists have converged around the understanding that the primary means of transmission for SARS-CoV-2 is likely respiratory droplets and airborne viral particles, not fomites as previously thought, including the CDC.¹ ASHRAE’s Epidemic Task Force (ETF) has been at the forefront of providing actionable recommendations for mitigating the airborne transmission of COVID-19 in buildings. Initially ASHRAE recommended maximizing the outside air intake of mechanical systems and minimizing recirculation. It encouraged opening windows and increasing ventilation with fans. High-efficiency filtration was an additional key consideration, with guidance urging an upgrade to a minimum of MERV 13 filters for central HVAC systems. In the absence of central systems, or if they could not accommodate high-efficiency filtration, then it was recommended that “demonstrated safe and effective” in-room air cleaners be employed.
Since the Spring, however, several developments have contributed to a shift in thinking regarding early guidance. Dr. Bill Bahnfleth, Professor of Architectural Engineering at Penn State and Chair of the ASHRAE ETF, recently shared in a widely attended October webinar that early guidance from the ETF in April was driven by perceived risk and possible effectiveness. He referred to the initial recommendations as “conservative” and that they “did not factor in cost and operational impacts.” Also, he outlined key factors that have prompted a re-evaluation of the guidance, and previewed new core recommendations that will be forthcoming from ASHRAE.
Risk estimations. Increased use of tools that estimate airborne transmission offers a quantitative prediction of risk. There are a number of estimators, but the most widely used is The COVID Airborne Transmission Estimator,² created by Professor Jimenez of the CIRES (Cooperative Institute for Research in Environmental Sciences) at CU Boulder. This estimator, and others, rely on the Wells-Riley model, an accepted industry standard approach for assessing the risk of airborne infection in buildings. The Wells-Riley model takes into account factors such as the size of the building, number of occupants, number of infected people, and the amount of time infected individuals spend in the building.
Equivalent filtration effectiveness. In numerous presentations, Prof. Bahnfleth has emphasized high efficiency filtration, and reiterated that guidance in the recent webinar, “We have evidence that better filtration is as effective as ventilation and lower cost.” He referenced this analysis conducted by Prof. Brent Stephens and Parham Azimi for the Built Environment Research Group at Illinois Tech that shows the relative cost of outside air ventilation in different U.S. locations versus filtration.
The graph below shows that the most cost-effective and efficient approach to reducing infection rates is using a MERV 13 filter, and that outdoor air increases cost at least four times as much to achieve the same benefit.
Energy, economic, and operational considerations. Prof. Bahnfleth also spoke to the fact that ASHRAE has taken a closer look at the energy, economic, and operational impacts of HVAC strategies for COVID mitigation, particularly its guidance on ventilation. As many in the industry have pointed out, maximizing outside air ventilation is expensive and energy intensive—the often-cited “energy penalty” associated with conditioning increased amounts of outdoor air. It is even more of a concern during the energy-intensive winter months. Additionally, there are other environmental considerations that need to be factored in: PM 2.5 from pollution and the smoke from wildfires plaguing the western U.S. are two significant concerns. Relative humidity also plays a role, as research emphasizes³ the need to keep relative humidity between 40-60% to both reduce the transmission risk of the virus and to support the respiratory health of building occupants.
Equivalent outdoor air changes. The Air Change Rate, e.g., the rate at which the air in a specific space is replaced with clean air is typically expressed in Air Changes per Hour (ACH). ACH has been assumed to refer to outdoor air exchanges. However, with limitations of HVAC systems, the superiority of highly filtered air given environmental issues such as pollution/smoke, and new consideration of energy impacts, experts are now promoting “equivalent outdoor air” changes. These equivalencies can be achieved by deploying high-efficiency filtration, in-room HEPA filters, as well as innovative approaches such as sorbent-based filtration.
A helpful tool for school facility managers is the Harvard-CU Boulder Portable Air Cleaner Calculator for Schools. It simplifies decision making around deploying in-room air cleaners, and can also be adapted for office and retail settings. Experts from the Harvard T.H. Chan School for Public Health recommend 5 ACH per hour, but provide ranges, with 4-5 ACH as Good, 5-6 as Excellent and 6 ACH as Ideal.
The ASHRAE ETF will likely share new core recommendations before the end of 2020, with the goal of updating guidance to reflect the above factors, and to simplify recommendations across its various subcommittees. From the preview Prof. Bahnfleth offered, we anticipate the update will likely emphasize:
With the renewed focus on cost from increased energy consumption, there is a need for a tool to help calculate costs of various ventilation and filtration approaches. While risk is the paramount consideration, models providing a more complete picture allows for more informed decision-making.
My company, enVerid, has released the enVerid COVID-19 Energy Estimator, an open-source, vetted tool building upon Prof. Jimenez’s COVID Airborne Transmission Estimator. This tool performs predictive calculations for energy expenditures. In addition to enabling a comparison of energy use among HVAC strategies, it offers insight into anticipated carbon emissions associated with each approach. For example, building engineers evaluating a 50,000 square foot office in Boston with 250 occupants and design supply air of 50,000 CFM can compare two approaches—switching to 100% outside air (OA) or upgrading to MERV 13 high-efficiency filters and bringing in minimum OA as per ASHRAE’s Indoor Air Quality Procedure (IAQP).
As shown in the table below, the Energy Estimator shows both strategies will deliver over five effective air changes per hour (ACH), considered excellent by the Harvard School of Public Health, but the 100% OA strategy will cost $85,827 per year compared to $12,261 per year for the MERV13/IAQP approach. When reviewing the carbon impacts of the two approaches, the Energy Estimator shows the 100% OA strategy will generate 325 metric tons of CO2 per year versus 28 metric tons for the MERV 13/IAQP approach.
Given the current focus on IAQ, it makes sense that building management and engineers take a close look at ASHRAE’s Indoor Air Quality Procedure. In its Standard 62.1, ASHRAE defines two procedures for mechanical ventilation: the Ventilation Rate Procedure (VRP) and the Indoor Air Quality Procedure (IAQP). VRP defines ventilation requirements based on space size and occupancy without factoring in the efficiency benefits afforded by air cleaning technologies. Alternatively, IAQP is a performance-based ventilation approach that achieves the same results with less outside air, and therefore a reduction in energy expenses, particularly when employing air scrubbing technology.
Designing HVAC systems using IAQP combined with sorbent-based air cleaning and high-efficiency filtration significantly reduces first and operating costs without increasing the risk of airborne transmission of viruses. When put to the test utilizing the Wells-Riley equation described above, it was found that same relative risk of airborne transmission can be achieved with improved filtration and reduced ventilation when ASHRAE’s 62.1 IAQP is used with sorbent-based air cleaning technology. Using this approach, savings from reduced ventilation can more than offset the cost of increasing filtration (see graph below).
There are other important IAQ considerations that arise from utilizing high-efficiency filtration and air scrubbing technology:
COVID-19 has thrust IAQ issues into the spotlight. Controlling airborne transmission of the virus is the most pressing task, yet there are a host of IAQ considerations that need to be addressed. Facilities need to employ effective and energy efficient solutions to respond to the pandemic today, and to future proof for the next crisis. Additionally, for many, addressing IAQ must also be managed within the context of lowering a building’s carbon footprint. This is a tall order, but all within reach.
Engel is SVP, sales and marketing for enVerid Systems, a Westwood-MA provider of commercial indoor air quality products including localized high-filtration devices and the award-winning HVAC Load Reduction® (HLR®) module.
Do you have a comment? Share your thoughts in the Comments section below, or send an e-mail to the Editor at [email protected]