Understanding MERV

The ASHRAE Standard 52.2

ANSI/ASHRAE Standard 52.2 features many improvements over the 52.1 standard. In 2009 ANSI/ASHRAE 52.1 “Dust Spot” efficiency testing was removed from the standards while two other parts of the 52.1 Standard were adopted into the 52.2 Standard, allowing for the 52.1 to be retired. The two parts of 52.1 adopted by 52.2 are “Arrestance for determining MERV 1-4 and “Dust Holding Capacity.” It should be noted that the ANSI/ASHRAE standard makes it clear that Dust Holding Capacity is reported as the total weight of synthetic loading dust captured by the air cleaning device over all of the incremental dust loading steps. This value should not be used to calculate the expected life of the device in use.

Some of the improvements found in the ANSI/ASHRAE 52.2 standard include:

  • The use of mandatory (code) language, which enables the standard to be referenced by other codes that are developed.
  • Where 52.1 expressed efficiency as an overall percentage, 52.2 expresses efficiency as a function of specific particle size.
  • Seventy-two (72) data points are reduced into a single curve that typifies the minimum efficiency of a filter.

Standard 52.2 Test Procedure: How Data is Obtained:

An air filter’s performance is determined by measuring the particle counts upstream and downstream of the air-cleaning device being tested.

Particle counts are taken over the range of particle sizes six times, beginning with a clean filter and then after the addition of standard synthetic ASHRAE dust loadings for five additional measurement cycles.

A laboratory aerosol generator, which operates much like a paint sprayer, is used to create a challenge aerosol of known particle size in the air stream. This will generate particles covering the 12 required particle size ranges for the test (See Table 2).

The challenge aerosol is injected into the test duct and particle counts are taken for each of the size data points.

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