Disclaimer:

The following reflects our best understanding at this time. As the understanding of SAR-CoV-2 is rapidly evolving, it is critically important that the reader consult with best available authorities especially those having jurisdiction over design and operation practices.

The World Health Organization (WHO) defines airborne transmission as the transport of microbes from an infected human to an un-infected human within (water) droplets, considered to be ≤ 5μm in diameter (ASHRAE calls these “aerosols” and defines them as ≤ 10 μm (10 microns) in diameter). These can remain airborne for several hours. Water droplets larger than this fall out of the air within seconds and are not transported by air even in well-ventilated spaces. According to the WHO, it is not yet clear whether airborne transmission of Covid-19 occurs. For example, as of March 29, 2020 WHO reported that “In an analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported." However, ASHRAE considers that “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled”.

ASHRAE, April 14, 2020, ASHRAE Position Document on Infectious Aerosols.

World Health Organization, 29 March 2020, Modes of Transmission of Virus Causing COVID-19: Implications for IPC Precaution Recommendations.

World Health Organization, Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19) 16-24, February 2020, [Internet]. Geneva: World Health Organization, 2020.

The RenewAire enthalpy-plate exchanger does not transfer viruses or bacteria between airstreams. The exchanger transfers water vapor in the form of individual molecules absorbed into the membrane, transported across the membrane as bound water molecules, and desorbed into the other airstream. Viruses are much larger and cannot be carried along. However, exchangers and units typically do have small leak paths and if static pressures in the exhaust air compartments of the unit are higher than in the supply, some recapture of exhaust air is possible.

Coronaviruses in general have been reported by REHVA to be 80-160 nanometers, or 0.08 to 0.16 microns. The SARS-CoV-2 virus has been reported to be 70-90 nanometers, or 0.07 to 0.09 microns.

REHVA, April 3, 2020, REHVA COVID-19 Guidance Document, (May 8, 2020).

Jeong-Min Kim et al, Identification of Coronavirus Isolated from a Patient in Korea with COVID-19, Osong Public Health and Research Perspectives, 2020;11(1):3-7.

ASHRAE recommends that air filtration be improved to “MERV-13 or the highest compatible with the filter rack, and [to] seal edges of the filter (rack) to limit bypass.” Only HEPA filters can provide 99%+ capture of the virus particles themselves. But MERV 13 should be able to reduce aerosol transmission because they can capture some of the aerosol particles in the 0.3 to 1.0 micron size range. The National Air Filtration Association cautiously states that “Filtration in building (HVAC) systems can be a part of an overall risk mitigation approach but is not generally regarded as a solution by itself. There is no direct scientific evidence of benefit, but some reduced exposure can reasonably be inferred based on the ability of some filters to remove particles that contain a SARS-CoV-2 virus.”

This table below shows particle size (capture) efficiency for MERV 8 and MERV 13 filters. Keeping in mind that aerosol droplets range from as large as 10 microns, down to, possibly, as small as the viruses themselves, or about 0.1 microns, we can see that MERV 13 is much more effective than MERV 8, but not good enough to stop the smallest aerosols.

MERV VALUE Average Particle Size Efficiency, % in size range
Particle Size Range 1:
0.3 to 1.0 microns
Particle Size Range 2:
1.0 to 3.0 microns
Particle Size Range 2:
3.0 to 10.0 microns
MERV 8 None ≤ 20% ≤ 70%
MERV 13 ≤ 50% ≤ 85% ≤ 90%

ASHRAE, 2020, Guidance For Building Operations During COVID-19 Pandemic, (May 8, 2020).

National Air Filtration Association, COVID-19 (Corona Virus) and Air Filtration Frequently Asked Questions (FAQs), (May 8, 2020).

National Air Filtration Association, Understanding MERV | NAFA User’s Guide to ANSI/ASHRAE 52.2, (May 8,2020)

No. The filter racks in RenewAire units are intended for maximum MERV-13 filters and are not provided with the specialized seals needed for HEPA filters.

Special precautions are needed ONLY if it is known or suspected that viruses have been generated in the space. In that case both exchangers and filters should be handled as if they could have captured viruses or other microbial agents; wear a N95 respirator, use gloves, ventilate the space, and bag and dispose of the filters. Since viruses on the core, if any, will lose viability over time, the core does not need to be replaced if it is otherwise in good condition.

If the alternative is no ventilation, in most cases it will be better to continue to use the ERV to deliver outside air to the space. Even if, as is true for some devices, it has high leakage rates, this only reduces its rate of dilution of indoor air contaminants.

ASHRAE, Covid-19 Preparedness Resources, Residences FAQ #11, (June 17, 2020)

ASHRAE, June 9, 2020, TC5.5 Practical Guidance for Epidemic Operation of Energy Recovery Ventilation Systems.

The fixed-plate core used in RenewAire units is inherently less leaky than, for example, energy recovery wheels. However, there is at least some leakage potential in any ERV unit. The difficulty in following the advice in the Position Document was that the Position Document does not tell the reader how to determine whether a specific system leaks, and whether that leakage is into the supply air or the exhaust air. ASHRAE TC5.5 Air-to-Air Energy Recovery has developed Practical Guidance for Epidemic Operation of ERVs, which provides ASHRAE’s more detailed guidance on all types of ERVs and is the basis for RenewAire’s recommendations. See also FAQ #8.

ASHRAE, April 14, 2020, ASHRAE Position Document on Infectious Aerosols.

ASHRAE, June 9, 2020, TC5.5 Practical Guidance for Epidemic Operation of Energy Recovery Ventilation Systems.

ASHRAE, REHVA and others take the approach that covid-19 can be transmitted between people by so-called “airborne or aerosol transmission”, while stating that the most important transmission mechanisms, which are direct ingestion of virus-laden droplets coughed, sneezed, breathed or even talked out by infectious people, or person-to-surface-to-person. Aerosol transmission refers specifically to the transport of viruses from infectious people to healthy people in water droplets so small (no larger than 10 microns in diameter) that they can stay suspended for several hours.

Therefore, it is not impossible that virus-laden aerosols could reach the face of the cores in our ERVs, since the MERV-8 filters on the unit return are only about 70% effective with particles between 3 and 10 microns, and some aerosols will be smaller (See also FAQ #3). What aerosols get past the filter will be, by definition, airborne, and the laminar flow inside the core means there is no reason to expect them to attach to the interior of the exchanger. However, some may come to rest on the face of the exchanger.

ASHRAE, April 14, 2020, ASHRAE Position Document on Infectious Aerosols.

REHVA, April 3, 2020, REHVA COVID-19 Guidance Document, (May 8, 2020).

 

Viruses are not alive as such, and they lose viability over a period of time depending on many factors including the surface, if any, they are on. They cannot reproduce without getting into a human or body (or the bodies of certain animals) where they can penetrate a host cell and hijack its machinery and metabolism to create copies of the virus.

One study (below) reported the median half-life of SARS-CoV-2 (covid-19) as 1.2 hours in aerosols, about 3.5 hrs on cardboard, and about 5.5 hrs on stainless steel. Based on these and similar findings we expect that any virus on the surface of the core will continue to lose viability over time.

New England Journal of Medicine, “Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1.

ASHRAE, April 14, 2020, ASHRAE Position Document on Infectious Aerosols says HEPA filtration and/or UV treatment can be effective when properly applied.

ASHRAE, April 14, 2020, ASHRAE Position Document on Infectious Aerosols.

Authoritative bodies including ASHRAE and REHVA agree that it is possible that virus-containing aerosols could be carried by ventilation systems and captured by filters. In an energy-recovery unit it is possible that aerosols also could be captured on the faces of the energy exchange core and other surfaces with dust build-up; the same can be said of surfaces in the rest of the heating and cooling system.  ASHRAE, REHVA and the National Air Filtration association agree on how to handle air filters. For example, according to Prof. Jeffrey Siegel, a professor in the Department of Civil & Mineral Engineering at the University of Toronto and an expert in indoor air quality, “In general, it is wise to assume that filters have active microbiological material on them. Whether this represents an important infectious disease risk from viruses is not known, but the precautionary principle would suggest that care should be taken.  This becomes particularly important in any building (including a home) where there are known or likely cases of any infectious disease including COVID-19 and also extends to portable air cleaner filters and vehicle cabin air filters.  Filters should be changed with the system turned off, while wearing gloves, with respiratory protection if available, outdoors if possible, and disposed of in a sealed bag.

One possible precaution would be for the homeowner to perform the maintenance themselves (assuming they agree that they themselves are the only potential virus sources in their homes). If they have access to a HEPA vac, they should use it to clean the interior of the ERV, as well as the faces of the core. Otherwise, they may choose to turn off the energy-recovery ventilator for a period of three days or more to allow any active viruses to expire, and then use an ordinary vacuum cleaner. They should use the same precautions (N95 mask, gloves, wiping down adjacent surfaces, bagging used filters) as described for service technicians (below) Of course an effort should be made to provide sufficient ventilation during that period.

If a technician is called in to perform the service, the first priority is to assume that both you and they are asymptomatic-but-contagious. This means N95 masks, gloves and distancing for everyone, and single-use disposable booties for the technician. Extending the above recommendations about filters, all filters and all dust or condensate in the ventilation, heating, or cooling systems, need to be handled as if they could have active microbiological material on them, and should be disposed of in sealed bags. An article in the ACHR News outlines the type of precautions that the homeowner should request. If the homeowner is uncertain that the filters have been handled without potential to exposure before being brought to the home, they should ask the contractor to leave the unit turned off for three days after installing the filters.

ASHRAE, April 14, 2020, ASHRAE Position Document on Infectious Aerosols.

World Health Organization, 29 March 2020, Modes of Transmission of Virus Causing COVID-19: Implications for IPC Precaution Recommendations.

REHVA, April 3, 2020, REHVA COVID-19 Guidance Document, (May 8, 2020).

National Air Filtration Association, COVID-19 (Corona Virus) and Air Filtration Frequently Asked Questions (FAQs), (May 8, 2020).

The NEWS, March 24, 2020, Comprehensive Guide: HVAC Service Calls During COVID-19.