Mitigating Pathogens to Create Healthy Buildings by Increasing Outdoor Air Specification

by brittney_cutler_2 | October 1, 2021 12:00 am

Images and figures courtesy Mortar Net Solutions

by Nick Agopian

The renewed interest of outdoor air’s importance to commercial building occupant health and indoor air quality (IAQ) is one major positive rising out of a pandemic that has negatively impacted the globe for nearly two years.

As waves of COVID-19 and its variant mutations continually reappear in spots across the globe, increasing outdoor air is arguably one of the most proactive foolproof mitigation methods. Both the American Society of Heating Refrigerating and Air-Conditioning Engineers (ASHRAE)[2] and Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA) recommended increased outdoor air as early as March 2020 when the pandemic was developing.

Diluting indoor air with outdoor air is considered to be effective because severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, is primarily an airborne contaminant, according to Public Health Ontario (PHO). PHO’s “What We Know So Far” documents provide a rapid review of the evidence. It maintains the virus is transmitted primarily at short range through respiratory particles ranging in size from large droplets to smaller droplets (aerosols)[3]. Depending on size and weight, the particles either drop to a surface or stay suspended in the occupied breathing zone for extended time periods.

It is a well-known premise among consulting engineers that outdoor air can mitigate contaminants, but introducing unconditioned air is energy intensive. Therefore, how much outdoor air is enough, especially during a pandemic?

Many organizations are studying this unknown; however, the WELL Building Institute is a fairly new building certification geared toward comprehensive occupant health, safety and well-being that might suggest helpful strategies. The WELL Health Safety Seal is an emblem a building displays that confirms its achieved rating as derived from the WELL Building Standard (WELL). WELL is a performance-based system for measuring, certifying, and monitoring features of the built environment that impact human health and well-being, through categories of air, water, nourishment, light, movement, thermal comfort, sound, material, mind, and community. The seal and the standard are programs created by the International WELL Building Institute (IWBI), which was formed in 2013 as a public benefit corporation that works to advance a global culture of health through better buildings, more vibrant communities, and stronger organizations to everyone’s benefit.

Outdoor air induction and building performance

HVAC and indoor air quality (IAQ) via outdoor air induction is a significant factor in a building performance standard, such as WELL, which has attained 11,173 certified and rated projects, plus 18,588 enrolled projects for a total of 29,761 projects with 250.8 million m2 (2.72 billion sf) in nearly 100 countries. Canada has been an active WELL country and boasts dozens of buildings and several educational institutions totaling millions of square feet that are WELL certified, such as:

• Oxford Properties’ 25,734-m2 (277,000-sf) MNP Tower in British Columbia;

• Ernst & Young’s 23,226-m2 (250,000-sf) EY building in Toronto;

• Royal Bank of Canada with several properties totaling hundreds of thousands of square feet;

• The Toronto 24,619-m2 (265,000-sf) Arcade Building and 38,368-m2 (413,000-sf) State Street building are just two of Dream Office Management Corp’s certified properties; and

• The University of New Brunswick’s (UNB’s) 5574-m2 (60,000-sf) Faculty of Kinesiology building.

Buildings earn points that are applied toward gold, silver, or platinum certifications. Under the enhanced ventilation category[4] applicants earn one or two points by increasing outdoor air by 30 per cent and 60 per cent over American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 62.1-2010 recommendations.

Carbon dioxide (CO2) reduction is also emphasized in WELL and awarded points (under the implement demand-controlled ventilation category). Studies reveal that human cognitive function, productivity, and more is adversely affected by high concentrations of CO2 levels[5].4 It is not unusual for CO2 levels to rise to 3000 ppm in occupied spaces. The National Institute of Health (NIH) and ASHRAE recommend CO2 levels below 1000 ppm. Consequently, WELL awards one, two, or three points for reducing CO2 levels to 900, 750, and 600 ppm, respectively. Incidentally, outdoor CO2 levels are approximately 425 ppm; therefore, diluting indoor air with outdoor air can significantly dilute indoor CO2 levels.

This is similar to Leadership in Energy and Environmental Design (LEED), however, WELL certification must be validated annually by outside evaluators to maintain its health and safety status.

That said, WELL’s enhanced ventilation recommendation could be a criteria for minimizing COVID-19 spread. While viruses are invisible and untraceable with conventional sensor equipment, CO2 is easily monitored by sensors that can report to building management systems (BMS) or self-controlled HVAC systems and accompanying or self-contained energy recovery equipment that accompanies them. If there is potential for viruses (i.e. human occupancy), whether it is a coronavirus pandemic or influenza and the common cold, diluting CO2 to recommended levels is a good substitute in a scenario where there are few alternatives. The potential for high viral loads is greatly reduced if CO2 levels are also reduced through outdoor air dilution.

PHO echoes this sentiment with the “Heating, Ventilation and Air Conditioning (HVAC) Systems in Buildings and COVID-19” document[6]. The document specifically states, “high indoor CO2 levels can potentially identify spaces with poor ventilation rates,” but it also cautions that “CO2 is not an indicator of COVID-19 transmission risk.”

Adding outdoor air without raising energy costs

Conventional HVAC systems are sized and designed to provide desired cooling and heating set points when less than 25 per cent of outdoor air—as per compliance with ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality—is mixed with recirculated air[7]. Outdoor air is important, however these systems fall short or even fail to provide indoor temperature set point goals when outdoor air is doubled or tripled to dilute indoor airborne contaminants. Doubling or tripling outdoor air will certainly surpass ASHRAE 62.1 IAQ compliances but will fall well short of complying with the global energy performance benchmark ASHRAE 90.1-2019, Energy Standard for Buildings Except Low-Rise Residential Buildings.

When providing building owners with the flexibility of boosting outdoor air when needed, consulting engineers add energy recovery ventilation (ERV) equipment to both new construction and retrofit designs. Generally, ERVs extract heat from the exhaust air and transfer it to the supply air. This handles both latent and sensible loads and allows more outdoor air without overdriving the existing HVAC units’ cooling or heating capabilities. ERVs can be applied as supplemental outdoor air units to existing air handling units (AHUs), both separately or connected to the AHU itself. Depending on the size, configuration, and number of ERVs, any building with a conventional roof top unit (RTU) can be converted to 100 per cent outdoor air.

Cooling coil performance with Varying Outside Air (OA) Outdoor air is important, however conventional air conditioning systems fall short or even fail to provide indoor temperature set point goals when outdoor air is doubled or tripled to dilute indoor airborne contaminants.

Canadian facility improves resident health and cuts energy costs

Le Centre d’hébergement des Pensées in Saguenay Lac-Saint-Jean, Quebec, is a good example of reducing CO2 and energy costs, while improving overall IAQ. The 99-room long-term care centre’s original rooftop packaged HVAC system had reached the end of its lifecycle; therefore, consulting engineer Patrick Barriault, with Martin Roy & Associés in Quebec City, combined a new rooftop with a 2973-L/s (6300-cfm) ERV. The four-storey facility’s first use of an ERV increased the facility’s outdoor air versus the original unit. Annual heating and humidifying energy costs and carbon emissions were reduced by $25,000 and 10,886 kg (12 tons), respectively.

The ERV receives return air via new ductwork connected by mechanical contractor M.G.S. Metal, Jonquiere, to positions where five former ventilator exhaust fans were located. The ERV pre-heats, pre-cools, dehumidifies, or humidifies the outdoor air before sending it to the new rooftop. Temperature and pressure sensors modulate the outdoor airflow to maintain operating conditions without wintertime frost formation. Total project cost was $393,000 and the surcharge attributable to energy efficiency measures was $130,000. Payback on the ERV equipment was 4.7 years, which also also included an “Energy Efficiency Studies and Incentives Program – Implementation Incentives Component” grant by diversified energy company, Energir, a natural gas distribution firm in Quebec that also supplies electricity through wind power.

Another example of limiting COVID-19 is the efforts of the University of Colorado-Boulder. One of many tasks, which also included social distancing, mandatory masking, and increased surface cleaning, was increasing outdoor air in the sprawling university’s 1.2 million m2 (13 million sf) of buildings. The facilities task force, a group born out of the facilities management department to help mitigate COVID-19, adapted every possible air handler on campus for increased outdoor air[9].

Choosing the best energy recovery method for the application

ERVs are reactive in nature. They do not generate cooling or heating. Instead, they are limited to transferring whatever the exhaust air temperature is to the supply air. What they transfer may be sufficient on moderate climate days; however, they are usually reduced to preheating or precooling on extreme days.

Additional flexibility can be provided by dedicated outdoor air systems (DOAS). These units perform the same heat recovery tasks of ERVs, but when temperature set points cannot be met, they proactively make up the difference with their own on-board cooling or heating modules.

Since IAQ equipment failure is not an option during a pandemic when IAQ safety is paramount, ERV and DOAS selection becomes critically important. That said, not all ERV equipment is the same. The future maintenance required is a critical specification factor. Products with many moving parts will need maintenance and periodic monitoring for specification performance. More importantly, most energy recovery equipment does not send alarm emails if there is an operational failure. Organizations such as ASHRAE and REHVA recommend ERV and DOAS equipment, but it is with the assumption they will retain their intended performance specification throughout their lifecycles[10].

The University of Colorado-Boulder upgraded its air handling systems providing HVAC to a 1.2-million-m2 (13-million-sf) campus for outdoor air and added MERV-13 filters in several specific cases.

There are several methods of energy recovery air-to-air transfer; the most popular are energy wheels and static plate (enthalpy cores) heat exchangers.

Energy wheels are mechanical and depend on belts or gears, a motor, and a drive, all which need periodic maintenance. Static plate cores have no moving parts and only need filter changes and annual core face vacuuming.

Depending on the method and its capabilities, energy recovery can be a boon for mitigating a building’s COVID-19 potential, but they can also do more harm than good if they leak contaminated return air into the supply air during thermal transfer, which is called exhaust air transfer rate (EATR)[12].

Energy (rotary) wheels without purge capabilities, for example, should not be used during pandemics, because of their cross-contamination potential. Purge models represent about 10 per cent of the energy wheel industry. These newer, higher priced wheels with purge modes, triple seals, and other premium features can minimize cross contamination when correctly calibrated, installed, and maintained.

The most common fault among older energy wheel models occurs with fans mounted in such a way that higher pressure on the exhaust air side is created. This will cause extract air leakage into the supply air. The degree of uncontrolled polluted extract air transfer can, in these cases, be up to an unacceptable 20 per cent in older, non-purge models. According to REHVA, if energy wheels cannot reach a near-zero cross contamination rate, which typically requires solid seals to eliminate bypass between the wheel substrate and the HVAC system, they should not be operated during a pandemic[13]. REHVA also recommends maintenance guidelines for energy wheels.

When specifying energy recovery equipment, it is important to consider the maintenance the end-user will be required to provide for the unit. While energy wheels have many moving parts i.e. rotor, wheel, belts, or gears, etc., static plate enthalpy cores do not.

In terms of cross contamination, static plate enthalpy cores keep air streams separate, according to AHRI Standard 1060, Performance Rating of Air-to-Air Exchangers for Energy Recovery Ventilation Equipment, which is used to certify equipment. Static plate enthalpy cores offer near-zero per cent cross-contamination when operating with a properly balanced airflow. Instead of the energy wheel strategy of rotating through the supply and return airstreams with the same media, static plate technology completely separates the return air from the supply air during the energy exchange process. Many models have a leakage rate of 0.1 per cent. REHVA backs this up with a recent position paper stating “virus particle transmission via heat recovery devices is thought not to be an issue when a HVAC system is equipped with a twin-coil unit or another heat recovery device” for  separating return and supply side air stream.

Since static plate systems are tested and rated as near-zero cross contamination, their EATRs are significantly better than wheel technology. However, seals around the plates could have been installed incorrectly or aged in older HVAC systems. Therefore, seals need examination if the static plate-style ERVs are used during a pandemic. Neutral balancing between the return and supply air is also recommended.

COVID-19 will hopefully dissipate to non-pandemic levels someday, however, experts predict it may be part of everyday life similar to influenza. Therefore, ventilation is still the best solution for mitigating contaminants in occupied buildings, whether it is for current and future viruses, mould, mildew, allergens, or a host of other potential airborne contaminants such as CO2 and volatile organic compounds (VOCs).

However, if the air conditioning system cannot handle the additional outdoor air’s heating or cooling loads, then an ERV or DOAS is perhaps one of the best ways to rectify any shortcomings.

Nick Agopian is vice-president of sales and marketing at RenewAire in Waunakee, Wisconsin, which specializes in enhancing indoor air quality (IAQ) in commercial/residential buildings of all sizes through high-efficiency, enthalpic-core, static-plate energy recovery ventilation (ERV) and dedicated outdoor air systems (DOAS). Agopian is a 38-year veteran of the HVAC industry, holds a building systems engineering degree from Vanier College and an MBA from Duke University. He’s a member of ASHRAE TC 2.3 “Gaseous Air Contaminants and Gas Contaminant Removal Equipment” and ASHRAE SPC 62.1 “Ventilation for Acceptable Indoor Air Quality” committees. He can be reached at

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