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Hugh Crowther
2020-11-23 08:30

Size of SARS-CoV-2 Virus

Did you know the size of the SARS-CoV-2 virus that causes COVID-19 is only 100 nanometers? That’s 0.1 Microns (a micron is a micrometer or μm) or 0.00000393 inches. What kind of filter would it take to arrest a SARS-CoV-2 virus? Before we can answer that we need to consider how the virus transmitted. COVID-19 spreads from an infected person to another through respiratory droplets created when the infected person coughs, sneezes, sings, shouts or talks. The World Health Organization (WHO) and the Center for Disease Control (CDC) define a droplet as particles larger than 5 μm and aerosols as less than 5 μm. Droplets are large enough hand heavy enough to fall and settle on surfaces within 6 feet of the infected person hence the need to stay 6 feet apart and to constantly wash your hands that may touch surfaces where the droplets have settled. 

Droplet Size

Aerosols are small and light enough to stay airborne for a long time. By increasing the ventilation rate the concentration of aerosols is diluted and the chance of transference reduced. This is why ASHRAE, REVHA and other organizations recommend increasing the ventilation rate as long as the equipment can maintain acceptable thermal comfort (A thermally stressed occupant is more susceptible to infection).

Aerosols can also be returned to the air handling unit or be re-entrained in an ERV so the filtration performance in this equipment becomes important to the wellbeing of the occupants.

MERV 8  Filter
MERV 13 Filter

Mechanical filters have media with tangled fibers that arrest particles. The finer the spaces between fibers the smaller the particle the filter can arrest. As filters become dirty, their effectiveness improves because the openings get smaller. At some point of filter loading, the pressure drop increases to the point where airflow is diminished and the filters need to be changed. 

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Mechanical Air Filters

Three are gas-phase filters such as carbon filters, electronic filters such as electrostatic filters, ultraviolet energy such as UVGI lamps and mechanical filters. Mechanical filters arrest airborne particles that can inhibit the equipment’s performance and lower indoor air quality.

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ASHRAE Std 52.2-2017 Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size rates filter performance using Minimum Efficiency Reporting  Value or MERV.  The higher the MERV the small the particle a filter can capture. 

ASHRAE Std 52.2 MERV Ratings

ASHRAE Std 62.1-2019  – Ventilation for Acceptable Indoor Air Quality requires a minimum of MERV 8 filters so many HVAC units are designed to have racks for this level of filtration.  This is usually a 2 inch filter rack where the filters are slid in from the side or face loaded.

Looking at the MERV 8 rating in the table above it can be seen the filter is less than 20 % efficient between 1.0 to 3.0 microns and 70% efficient between 3.0 to 10.0 microns. A MERV 8 filter will definitely help but will have limited capability for arresting aerosols that are less than 5 microns.

Filter Efficiency

The recommendation from ASHRAE, REVHA and other organizations during epidemics such as COVID-19 is to increase the filter efficiency up to MERV 13 (ISO ePM1) or greater because they are effective at capturing airborne viruses. A MERV 13 filter is 50% efficient between 0.3 and 1.0 microns, 85 % efficient between 1.0 to 3.0 microns and 90% efficient between 3.0 to 10.0 microns. 

Swegon GOLD With MERV 13 Filters in supply and Return Air Paths

The increased performance of MERV 13 filters is why they are standard for Swegon GOLD ventilation units both on the supply air path as well as the return air path (where an aerosolized virus may be found). Including MERV 13 filtration in the return air path of an ERV can help reduce any possible cross-contamination through the heat recovery device as the air will be filtered prior to entering the heat recovery device.  

The effectiveness of a filter is heavily influenced by the filter rack.  The air must pass through the filter in order for the filter to do its job. For high performance filtration systems (healthcare and lab systems) the filter is typically face loaded into a frame with gaskets that seal the air on all 4 sides. The harder the air pressure pushes the filter into the gasket, the better the frame seals. To face load a filter requires an access section in front of the filter and the unit needs to be big enough to enter.

The alternative to face loading filters is side loading. Side loaded filters are slid into the air handling unit in a track. Side loaded filters do not require access in front of the filter rack air handling unit and work well for smaller units. The issue is getting a seal between the filters. If multiple filters are just slid down a track, air will easily pass between the filters resulting in poor filtration.

Swegon GOLD Side Load 2 inch MERV 8 Prefilter Rack (2) and MERV 13 Main Filter Rack (1)

Side loaded filter racks such as the one used in the Swegon GOLD unit use a track with a cam lock system that pulls the filters forward into a gasketed frame that seals on all 4 sides. This ensures the MERV rating of the filter is what will actually be experienced in the building.

Eurovent air handling unit test start and EN 1886 rates several performance parameters including filter rack leakage. Leakage here means ventilation air is not being cleaned by the filters. The test measures the leakage rate at 400 Pa (1.6 inches w.c.). The GOLD filter leakage class is F9, the highest class, which is less than 0.5% of nominal airflow rate.

During epidemics such as COVID-19, it is recommended that the filter change frequency be reduced. Changing the filters (which can be contaminated) requires the service technician to wear PPE so fewer changes are better. Filter efficiency improves with time so, from a filtration point of view, the occupant’s wellbeing is not being compromised.

GOLD Unit Filter Monitoring Setup

Some air handling units and ERV controllers monitor the pressure drop across the filter bank(s) for scheduling service. During Epidemic periods it is better to avoid filter replacement based on a schedule and instead operate the filters to the maximum air pressure drop recommended by the filter manufacturer.

Swegon GOLD Fan with Integral Airflow Sensor

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Supply Airflow Control

Maintaining correct ventilation airflow is critical to maintaining good indoor air quality (IAQ) and proper building pressurization. Filter loading and air density changes through the seasons mean that even a properly balanced and commissioned system will see airflow changes if the fan speed is fixed. Swegon GOLD units in constant airflow mode are controlled to the operator inputted airflow setpoint (in cfm) so filter loading will not impact ventilation airflow rate.

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One issue with operating filters to the maximum pressure drop is the impact on airflow. For air handling units or ERVs with fixed speed fans, an increase in filter pressure drop will decrease airflow. If the fans are variable speed and the controls can be set up to support it, the fan control should be set up to deliver the required airflow as opposed to operating the fans are fixed speed. This generally means an airflow sensor is required. As the filter pressure drop increases, the fan speed is increased to offset the larger pressure drop so the occupant’s wellbeing is not impacted by reduced ventilation airflow.

In summary, increasing the filters to MERV 13 or better, extending the replacement period, making sure the filters are sealed so the air passes through the media and maintaining the highest ventilation rate possible will improve the wellbeing of the building occupants.



Guidance for Building Operations During the COVID-19 Pandemic, ASHRAE Journal, May 2020

ASHRAE Building Readiness, www/ASHRAE.org

Practical Guidance for Epidemic Operation of ERVs, ASHRAE TC 5.5.

ASHRAE Position Document on Infectious Aerosols


ASHRAE Epidemic Task Force – Filtration and Disinfection PPT, Oct 20, 2020

covid, gold, merv, and Std 62.1