This email is the second in our Returning to Work series, now that NC is starting to allow businesses to reopen. In case you missed the first, here is a link.
Filter Rating System
Air filters used in HVAC systems are rated according to the MERV – Minimum Efficiency Reporting Values. MERV is a rating system created by ASHRAE (the standard setting body in the US for heating, AC and refrigeration). This standard rates filters based on how effective they are at filtering particles of various sizes. The requirements are laid out in ASHRAE standard 52.2, which also provides the test methods that filter manufacturers can use to test their filters.
The higher the MERV rating the smaller particles the filter is capable of catching. Higher rated filters also catch a larger percentage of those small particles.
How Does the Virus Spread?
According to the CDC, one of the main ways diseases similar to COVID-19 spread is via respiratory droplets. These droplets range in size from 1 to 5 microns. If you saw our post from last week, ASHRAE has said that HVAC systems are likely to spread these droplets. We outlined some operational measures that you can take with your systems in that post.
In HVAC systems the air is filtered as it returns from the space to the HVAC Unit – either by filters mounted at the unit or in return air grills in the space. If these filters are effective enough they can prevent respiratory droplets from being circulated.
What type of Filtration is Needed?
Referencing the chart above, once filters reach MERV13 or higher, they start to be effective at removing particles in the size range of respiratory droplets. But this is where things get a little bit complicated.
The capabilities of the HVAC system need to be considered in determining the type of filtration that system can handle. Some blowers do not have the power to pull air through the extra resistance of more effective filters.
Many manufacturers offer kits to allow the use of thicker filters, for instance going from a 2 inch thick filter in MERV8 to a 4 inch thick MERV13 can keep the pressure drop from the filters to levels the unit can handle. Because the 4 inch thick filter has more surface area for the air to pass through, that compensates for the higher resistance of the filter media.
Because the more efficient filters gather more particles, they may need to be changed more frequently.
Finally, when changing filters, they should be treated as hazardous, gloves and masks should be worn and they should be placed into sealed bags before being carried through the building and disposed of.
Give us a call to learn more! 919-773-9223