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MERV A: Exposed!

June 12 small image

By: Robert Martin, Associate Category Manager  |   06.12.14

Many pleated air filters (especially at commonly used MERV 8) have very low efficiencies in capturing submicron particles. That is why it’s crucial to look beyond a filter’s MERV and evaluate the E1, E2 and E3 efficiencies of the filter as measured under the ASHRAE 52.2 Test. But don’t be swayed by 52.2’s non-standard Appendix J optional test or you could end up with lower filtration performance than you bargained for.

We’re all familiar with the two common types of air filter media. One type relies solely on its mechanical structure to filter the air. The other utilizes both its mechanical structure and the added benefit of an electret charge. The mechanical efficiency provides for sustained filtration efficiency and the electret charge increases initial efficiency and is particularly useful in increasing capture efficiency for E1 and E2 submicron particles. 

Manufacturers that only offer filters using mechanical media may try to divert attention from their poor submicron particle capture performance by instead focusing on the filter’s MERV rating only or the non-standard MERV-A test. They may also work with filter distributors to get these poorer performers “spec’d in.” 

Appendix J Does Not Reflect Real-World Conditions

When ASHRAE published its latest update to Standard 52.2, it included an Appendix not found in the previous standard: Appendix J: Optional Method of Conditioning a Filter Using Fine KCL Particles. The Appendix was created to address the interest of critics of ASHRAE 52.2, who were concerned that air filters featuring an electret charge performed at high filtration efficiencies during initial testing while their filtration efficiencies could decline during actual. Thus, they argued, the resulting MERV of the filter (as indicated by that initial test) would not represent the true minimum filtration efficiency of the filter. 

The critics’ solution was to “level the playing field” by masking an electret filter’s charge. The MERV-A test subjects the filters to extreme loads of very fine KCl (potassium chloride) particles – many times what the filter would be exposed to over its real-world, installed life. 

While the stated goal of this was to represent real-world conditions, it does not represent real-world conditions at all. In fact, the test was designed only to drive down the efficiency of electret filters. It represents a “worst-case” scenario that is likely to never happen. In addition, differences in environmental conditions and lab-to-lab variances have also been uncovered, leading to the conclusion that techniques which “condition” the filters are not repeatable. Moreover, these same conditioning techniques have been shown to decrease the filtration efficiency of certain mechanical-only filters as well. 

These are some of the reasons the electret masking step was not added to the 52.2 Standard as a mandatory part of the test but was included as an option only. This optional test method may be conducted at the filter manufacturer’s discretion, but only if the standard ANSI-certified 52.2 test method is conducted to determine the filter’s true performance. 

Avoid Confusion

It’s important to remember that it is impossible to isolate the structural and physical properties of an electret-charged filter media from the charge distribution without impacting other filtration mechanisms and/or other filtration properties. 

Be careful not to confuse the results of testing under the optional Appendix J, which should be reported as MERV A and should not be confused with the filters’ E1, E2, and E3 Fine Particle Efficiency as a way to measure filter performance. Ask to see the full ASHRAE 52.2 test report and request an energy cost analysis of the filter with mechanical-electret filter media versus one of the same MERV rating that utilized mechanical-only filters.


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