How It Works: Using Mechanical Air Filtration to Prevent the Spread Of COVID-19

Understanding how your air filtration system works is key to creating a safer space for your occupants.

You walk into a new mixed retail space that's just opened, excited to see what you didn't know you needed, but it turns out you simply must have it before you walk out. You glance around and appreciate the open views of the outside and the airy feeling created by the interior layout, welcoming you into the space and encouraging you to start shopping.

But what you didn't actively notice (at least before the COVID-19 pandemic), was how clean and fresh the air seemed, a reprieve from the harsh-for-early-June heat outside but not harshly blowing you around at the entry door. There's a lot to this feeling of a well-conditioned space, but one portion that has become increasingly important is the use of mechanical filtration.

There are far more benefits than just those focused on health, which is obviously at the forefront of everyone's minds these days, but there are tangible benefits there. Let's dive in and see the role it plays in this welcoming (and also healthy) space.

Particulate Filters

Particulate filters are nothing new, and as such, their importance can easily be overlooked as a staple of any HVAC system. They are often neglected as part of a recommended maintenance routine, even when it's evident to the naked eye that they're accomplishing something great for the space. Keeping up with them also saves the owner from costly cleaning and repair when the unit or system begins under-performing and creates a comfort issue. What's not known by most is that those filters, in proper spec, can be continually improving health and potentially saving lives from an unseen microbial danger.

MERV Ratings for Mitigating the Spread of COVID-19

ASHRAE has officially recommended MERV (Minimum Efficiency Reporting Value) 13 or higher particulate filters to mitigate the spread of viral particulates (including the coronavirus family associated with COVID-19, among others. (Source: ASHRAE) The design point at which filters are tested to receive these ratings are for a particle size of 0.3 microns. This value is important because it is slightly larger than the "most penetrating particle size," a particle that is massive enough to carry momentum through a filter, but not so large that it is easily captured by the filter media, about 0.1 microns.

MERV ratings are given out based on a range of efficiency at capturing those slightly larger 0.3-micron particles. The more efficient, the higher the rating. The coronavirus family resides in the 0.1-micron range; however, virus particles alone aren't just emitted from an infected host. Instead, they are contained in droplets or aerosols containing water, saliva, other organic materials, and proteins, which altogether are at or well over 1 micron in size. These aerosolized droplets are easily captured by MERV 13 filters and above, with higher MERV ratings capturing slightly smaller particles but with much higher financial and pressure loss costs.

HEPA vs. MERV Filters

There are several modes of capture in mechanical filtration but let’s simplify them by not simply thinking of it like a mesh that captures in one direction, but where particles are forced to bounce around until they are moving slowly enough to attach to the filter media. The commonly touted HEPA (High-Efficiency Particulate Air) filter (not so impressive, once you break down the name, huh?) works in the same way that any standard filter with a MERV rating does, but is larger, more effectively sealed (both the filter element and the frame in the air handling cabinet or ductwork that it resides) and tested to meet or exceed standards set forth by both the Institute of Environmental Sciences and Technology (IEST) and the International Organization for Standardization (ISO).

Due to this high level of detail in construction and required testing, HEPA filters are both expensive and have a higher pressure drop across the filter, and those same traits apply to higher-rated MERV filters as well just to a lesser extent. This could limit their application in an existing system whose fan does not have sufficient power to overcome this additional pressure drop. This is why we can't slap a HEPA filter on any system and be done with it. Before installing any higher-level filtration (HEPA or higher MERV rated filters) in an existing system, it should be analyzed to determine that performance will not be compromised.

An additional consideration is size: MERV 14 and above are commonly found in a 4" filter cartridge form factor. At the same time, most small-to-medium-sized commercially packaged RTUs accept 2" filter media as standard. This makes it harder to find and more costly to get MERV 14 in a 2" filter. Generally, more surface area (usually achieved by adding pleats) is required to improve capture efficiency, which in turn makes the filter element itself larger in size.  

These considerations led us at HFA to elect to upgrade to MERV 13 where we could in Bentonville, but also illustrates the limitation to doing so in our Franklin office (fan performance due to increased static pressure and maintenance concerns). To learn more about how we're mitigating the risk of COVID-19 in our offices, check out our blog post on the topic.

Contact Nathan Mehan today to learn more about how we can help you!

Written by
Nathan Mehan