As our nation begins the journey back to work and school, we carry a heightened sense of responsibility to our fellow coworkers, students and educators to do what we can to mitigate the spread of COVID-19. We have all seen a higher level of sanitation of surfaces and have been instructed to wear masks when indoors, or when social distancing is not possible. However, there are other preventative measures that can help slow the spread of COVID-19. One of the simplest ways building owners can slow the spread of COVID-19 is to provide the appropriate indoor air filtration.
On August 20th the ASHRAE Epidemic Task Force updated the Building Readiness Guidelines f, recommending a minimum mechanical filter efficiency of MERV 13 and preferably MERV 14 or more. The higher the MERV number the better the filter is at removing particles in the 0.3-micron to 1-micron diameter size. The increase in filter efficiency comes with stipulations that must be overcome. The MERV 13 filters are better at removing particles and require a larger pressure drop to force the air through the filter. Since most existing HVAC systems were designed to operate with MERV 8 filters, they will need to be evaluated to see if it can still deliver the design airflow with the added pressure drop.
To evaluate increasing the efficiency of the filters, McKenney’s would review the design airflow of the HVAC unit and determine the velocity through the filter rack. We would then analyze the performance data of the installed MERV 8 filter when they are first installed (clean) at the design velocity.
For our evaluation of the filter upgrade, let’s assume the existing MERV 8 filters were selected with an initial (clean) pressure drop 0.25” at 500 fpm, and over time they load up to their final pressure drop (dirty) of 0.50”, giving us a filter loading of 0.25” (filter loading = dirty filter pressure drop – clean filter pressure drop). For our example, we are using double the clean pressure drop which is the minimum filter loading required by ASHRAE 52.2, however the final filter pressure drop is based on equipment selections and for large equipment typically is 1.0”.
Fan performance is impacted by system pressure. As pressure drop increases, airflow decreases for a given fan. In order to avoid HVAC unit fan motor upgrades, , we need to select a new MERV 13 filter with an initial (clean) pressure drop as close to the existing MERV 8 filters as possible, which will allow us to load the MERV 13 filter similarly to that of the MERV 8 filter. Make sure you check with multiple filter manufactures as some have better pressure drop performances than others. Some examples are below:
- Camfil AP Thirteen MERV 13 filter with an initial pressure drop of 0.33” at 500 fpm, would allow us to load the filter 0.17” before we needed to change the filters.
- Koch Multi-Pleat Green 13 MERV 13 filter with an initial pressure drop 0.27” at 500 fpm, would allow us to load the filter 0.23” before we needed to change the filters (only 0.002” less than the originally installed MERV 8 filters).
Keep in mind that larger units operating at low-brake horsepower might have enough fan horsepower left to overcome the increase in pressure drop, but should be evaluated on a case-by-case basis.
McKenney’s is aware that every facility is unique and is faced with different operational challenges. For more specific feedback on your building or particulars of your system, please contact us to discuss.
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ADDITIONAL RESOURCES:
- The American Society of Refrigeration and Air-Conditioning Engineers (ASHRAE) developed COVID-19 Preparedness Resources and has established the ASHRAE Epidemic Task Force to respond to the current global COVID-19 pandemic and provide guidance on how to prepare for future epidemics. This task force will address the challenges as it relates to the effects of heating, ventilation, and air-conditioning systems on disease transmission in healthcare facilities, the workplace, home, public and recreational environments. The task force will also provide recommendations for setting up temporary field hospitals in convention centers, arenas, and indoor stadia to deal with surges.
- ASHRAE Position Document on Filtration and Air Cleaning
- ASHRAE Standard 52.2-2017 Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size
