Since early 2020, it seems just about everyone has an opinion on viruses, disease transmission and what should be done as a society, as a business owner, and as a private citizen. We here are Barton Associates are not healthcare workers, epidemiologists, or even medical students. We are just a group of engineers who believe in fact-based approaches to solving complex building design and operational issues. The following article is part of a series intended to provide building owners the information needed to make an educated decision on how to approach facility air system design and operation in this new environment where airborne viruses change from something that few systems address, to the forefront of environmental systems.
From a building design and operation standpoint, there are several ways to reduce the prevalence of airborne contaminants, and subsequently surface contamination. While occupant density, building use, housekeeping, and personal hygiene are all factors in the air quality of a facility, we are focusing on our area of expertise which is design, construction, and operation of building systems. The strategies covered here include:
- Standard Filtration
- Enhanced Filtration Techniques
- Ultra-Violet HVAC Components
- Thermal Environment
- Restarting, operation and maintenance
This issue focuses on mechanical filtration in HVAC air systems. Mechanical filtration is a key component to effective operation of an air system. Filters are intended to prevent large particles from attaching to heat transfer surfaces, reducing buildup on coils and maintaining heat transfer. Filters can arrest particles of sizes down to 0.3 microns or less, which is enough to capture a range of harmful substances, including some viruses and most bacteria. However, mechanical filtration comes at a significant operational cost with increased levels of filtration. As filtration increases, so does air resistance. As air resistance increases, so does the fan horsepower requirement. This typically means most facilities that were not designed for high levels of filtration may not be candidates for an increase in filtration level without upgrades to the equipment.
Filters used to be defined by a percent filtration or percent arrestance (30% prefilter is still a common reference). Unfortunately, there was no standard accepted by all manufacturers, which resulted in manipulation of performance numbers to attain a price point. This often resulted in filters that didn’t really filter until they were dirty, then, of course, they were changed out for new ones. Think of the bargain-bin filters at your local home store. If you can see through it, it’s not doing much.
To combat this issue, ASHRAE and ANSI developed Standard 52.2 in 1999 to standardize how filtration devices are rated based on particulate size. The result of this research is the MERV rating system that is now uniformly applied to filtration ratings. According to the latest iteration, Standard 52.2-2017, there are three particle “Size Range Groups” that are used to determine the filter’s performance.
- 3.0 – 10 microns (includes pollen, mold spores, hairspray)
- 1.0 – 3.0 microns (Includes legionella, many dusts, dust mites)
- 0.3 – 1.0 microns (includes bacteria, sneezes, most smokes)
Each MERV rating is based on a percent arrestance (filtered particles) for each of these three size range groups. The most common commercial air filter is the MERV 8. A MERV 8 filter is typically a pleated synthetic material housed in a cardboard frame, and is generally between 2” and 4” in thickness. Unless your facility was designed with a special purpose, or for a LEED rating, this is most likely where your filtration level lives. Unfortunately, the MERV 8 (<20%, <20%, 70-85%) filter is only designed to stop less than 20% of particles between 0.3 to 1.0 microns, less than 20% of particles between 1.0 to 3.0 microns, and 70-85% of particles of 0.3 to 10 microns. Which means it does a great job of filtering pollen, mold, large dust particles, and human hair, but it is not rated to capture particles smaller than 0.3 microns. Meaning air systems with MERV 8 filtration are still capable of recirculating bacteria, dust mites, viruses, and even dust.
If your facility was designed around the LEED for new construction standards, you likely have MERV 14 (75-84%, 90% or better, 95% or better) filters, which results in filtering many more contaminants, including most bacteria and some viruses.
Finally, some healthcare and research applications have a higher stage of filtration, known as HEPA. HEPA filters can have a rating of 99.97% for particles of 0.3 micron or higher, depending on the level of filtration desired.
The question of what filtration level is appropriate is an important one, and not just because of the novel Coronavirus (SARS-COV2), but for overall building health and operational efficiency. More importantly, there is no “one-size fits all” approach. While it is safe to say that all mechanical equipment should have some level of filtration, its equally safe to say that not all buildings require HEPA level filtration. Most educational, commercial and other facilities could benefit from utilizing a MERV 8 prefilter and a MERV 11 – 14 final filter in their large HVAC equipment. The prefilters can be changed regularly to filter out large contaminants based on a time interval or when a differential pressure sensor indicates heavy filter loading. The MERV 11-14 filter, which is more expensive, can be changed out less frequently, and will filter out the smaller contaminants. This approach reduces buildup on cooling coils, energy recovery devices, fans and ductwork. As an added benefit, it may reduce airborne particulate in the space, providing less opportunity for bacteria, mold and viruses to find a place to attach, and can increase the effectiveness of your housekeeping staff’s cleaning efforts.
The consideration of filtration level must include the capabilities of the existing HVAC equipment. If there is not space for the desired filters in the air handler, fan horsepower to handle the increased resistance, or preventative maintenance programs to adequately maintain these systems, the cost and effort to enhance these air systems may be wasted or even detrimental to overall system performance. Barton Associates has the in-house expertise to recommend a comprehensive design, operation and maintenance plan to achieve your goals to provide a healthy and safe environment for your occupants.
In the coming weeks, please look for our articles on enhanced filtration opportunities, Ultraviolet HVAC components, and ventilation. While there is no one solution to the problem of airborne contaminants in building systems, choosing the right partner provide a sensible approach to combating these issues can make all the difference. If you have any questions, please do not hesitate to contact Stephen E. Oskin, PE, LEED AP at 814-880-6070 or firstname.lastname@example.org.