The American Society for Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) 52.2-2017 published this standard for evaluating air filters that is applied in HVAC systems. The standard describes a method for laboratory testing for measuring the performance of general ventilation air filters. It provides filters initial efficiency as a function of particle size as well as a numeric value called as Minimum Efficiency Reporting Value (MERV) for filter selection. MERV is a single number expressed on a 16-point scale which helps in easy filter selection.
The filters performance over a range of 12 particle sizes is determined. The twelve size ranges are placed in three larger groups E1, E2 and E3 (Composite average Particle size efficiency). The table below shows the particle size in Micro meter and the sizes assigned to each group.
The average efficiency of these three groups is referenced against the MERV parameters and the appropriate MERV value is selected for the filters. For assigning a MERV rating all three specific range of parameters for E1, E2 & E3 needs to be satisfied. If anyone parameter is not met, the filter will go down in rating.
With the introduction of ASHARE 52.2, the standards committee had recognized the need to provide a standard that allows the user to select a filter based on specific contaminant based upon that contaminant’s particle size. ASHRAE has also given importance to respirable contaminant and tried to address the same using MERV ratings. Another important criterion in developing this Standard was the goal of providing a ‘lowest point filter efficiency’ or, in most cases, initial filter efficiency. In this manner, the user would know how efficient the filter is at its lowest point, which in most cases is as soon as it is installed in the system.
Minimum Efficiency Reporting Value (MERV) Parameters Table
Appendix J
There were many studies conducted universally that displayed a loss in efficiency in some filters as they are exposed to sub-micron particles, due to the discharge of electrostatic charge from the media. Appendix J was added to ASHRAE Standard 52.2 in 2008 as a non-ANSI approved, optional conditioning step to provide a method of identification of the drop in efficiency. The reported value per Appendix J is referred to as MERV ’A’. Filters tested per Standard 52.2 with the Appendix J option have both a MERV and a MERV ‘A.’
A motion at the ASHRAE meetings in New York City in 2014 to make appendix J a mandatory part of the standard was subsequently voted down. For the time being, it remains an optional appendix. Note that the ISO 16890 standard now in use in Europe has a conditioning step as a mandatory part of the test methodology.
(All above information & Data are derived from Book records of Filtration Standards & Other related standards, research papers and its related webpages)