With today’s complex IAQ challenges, how do you select the best air filtration for a building? A new standard known as ASHRAE 52.2,“Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size,”should help to pave the way.

This standard allows hvac contractors to evaluate filters based upon controlled and repeatable laboratory testing, providing much more reliable comparisons.

It establishes minimum efficiencies for filters, a more stringent and conservative measure than the previously used average efficiency.

And, for the first time, it allows filter selection based on offending contaminants and their particle sizes, making the selection process much more specific and targeted than ever before.

An overview of 52.2

Air filter testing has previously been based on ASHRAE Standard 52.1-1992. Designed to complement — not replace — the older standard, ASHRAE 52.2 offers several advances. Those advances are outlined below.

  • It uses a highly controlled method of laboratory testing. The older standard measures “dust spot” efficiency using atmospheric air — an uncontrolled test aerosol that does not yield accurate, repeatable comparisons among different laboratories and filter manufacturers.

    Outside weather conditions can also affect test results. The new ASHRAE 52.2 test method, by comparison, uses a dry, solid-phase aerosol, potassium chloride (KCl), for more consistent results than atmospheric dust.

    Using this aerosol, several test cycles are performed, and efficiency and pressure drop across the filter are measured after each dust loading.

  • It measures minimum efficiency instead of average efficiency. ASHRAE 52.1 measures the average efficiency of an air filter over its service life.

    For most media filters, efficiency is lowest just after the filter is installed, and it increases as the filter loads with dust. Average efficiency is therefore not an accurate measure of filter performance, because it exaggerates that performance for part of the filter’s actual service life.

    The new standard shows a filter’s minimum performance throughout its life, allowing the contractor or building owner to select filters knowing their “worst-case” efficiency.

  • It measures a filter’s ability to remove particles of specific sizes. The old standard does not tell you a filter’s efficiency in removing specific particle sizes (such as respirable particles).

    By comparison, with the ASHRAE 52.2 test, particle counters measure the number of airborne particles with diameters of 0.3 to 10.0 microns, both upstream and downstream of the air filter. Using this information, it becomes possible to take a highly targeted approach to filter selection.

  • It establishes a useful Minimum Efficiency Reporting Value (MERV) system. After the test is completed, the filter’s minimum efficiency values at various particle sizes are recorded. These efficiency values are then used to assign a MERV to the filter.

    Designations range from MERV 1 (typically a low-efficiency, throwaway filter) up to MERV 16 (a 95%-plus ASHRAE filter). The new MERV system is much more comprehensive than previous systems, and it enables you to compare efficiencies of filters at a glance.

    10 tips for using ASHRAE 52.2

    When a contractor first learns about ASHRAE 52.2, the tendency is to want to select filters based strictly on the MERV numbers. While this system provides an accurate way to compare relative efficiencies of most air filters, it’s unwise to select filters by MERV alone.

    Following are some basic guidelines for proper application of the new standard.

    1. Be aware of the new test’s limitations regarding synthetic media filters.

    Preliminary testing has shown that air filters with electrostatically charged media do not always yield accurate test results.

    Most synthetic media filters use an electrostatic charge to enhance efficiency. As the filter loads with dust, the charge dissipates, leading to a drop in efficiency. As the filter continues to load, efficiency rises again. This situation can generate misleading test data.

    The ASHRAE 52.2 Standard Committee is working on a conditioning step that will compensate for the effect of electrostatic charge dissipation. Until the situation is resolved, be aware that the MERV number assigned to such filters may not be fully accurate.

    For critical applications, it may therefore be preferable to select an alternative type of media. If you’re uncertain whether a particular filter media is electrostatically enhanced, ask the manufacturer.

    2. To determine the required filter efficiency, start with the particle size of the target contaminant.

    Standard 52.2 groups airborne particles into 12 different size ranges, from 0.3 to 10.0 microns in diameter.

    To target a specific contaminant, you must first know its size range before you can locate the corresponding MERV. Table 1 (page 8) matches up various common contaminants with the correct MERV. Additional information on particle sizes of contaminants is available from ASHRAE and from leading filter manufacturers.

    3. In many cases, you may not be targeting one specific contaminant. For these situations, ASHRAE 52.2 organizes particle sizes into three simplified efficiency ranges — E1, E2, and E3.

    The first group, E1, is best addressed by what we currently refer to as high-efficiency filters. These filters would be used to target small particles of 0.3 to 1.0 micron.

    To target medium particles of 1.0 to 3.0 microns in size, you would choose a medium-efficiency filter with optimum efficiencies in the E2 range.

    And for large particles (3.0 to 10.0 micron), a low-efficiency filter with removal efficiencies in the E3 range would be the appropriate choice.

    4. Include MERV designations in your filter specifications.

    A comprehensive specification might be worded as follows: “Filter shall be MERV 15 and shall have minimum efficiency values of 85% (E1), 90% (E2), and 95% (E3).” After these values have been specified, you can match the filter to the job.

    5. In replacement and retrofit applications, where there is no specified MERV, you may still use the reporting system as a guide for comparing products.

    6. Be aware that final resistance is another important barometer of filter performance.

    Table 1 shows minimum final resistance of filters, measured in inches of water gauge pressure (in. wg). For a filter to earn a MERV 12 designation, it must not only achieve 80% efficiency on 1.0- to 3.0-micron particles, it must also run to a minimum final resistance of 1.0 in. wg, or twice the initial resistance (whichever is greater).

    Final resistance helps determine whether a filter will do the required job over its full service life, and it is therefore an important factor in determining the filter’s proper MERV designation.

    7. Refer to the older standard (ASHRAE 52.1) when you need information on arrestance (the percentage of test dust, by weight, that a filter can capture).

    The new standard does not measure arrestance, which is a useful measurement for comparing low-efficiency filters only.

    8. Also refer to ASHRAE 52.1 for data on the dust-holding capacity (DHC) of filters.

    DHC allows you to compare the relative service life of filters of similar design — a very important factor in virtually every filtration decision. Thus, ASHRAE 52.1 will continue to be a useful tool because it addresses some filter performance aspects that are not covered under the new standard.

    9. Examine other desired performance criteria.

    Do you need a filter with high moisture resistance? This will impact media selection. Does the application require a filter that is easy to dispose of or incinerate? This may dictate the use of a non-metallic product.

    What is the desired energy performance? Pressure drop information must be reviewed carefully, as this will have an impact on energy costs.

    By now it should be clear that there is more to a filter than its MERV. Individual performance criteria are equally important in helping you to arrive at the best filter choice.

    10. Finally, always use the new standard when selecting filters for IAQ control.

    Until now, filter selection was based all too often on previous experience, insufficient comparison data, or guesswork.

    Now, by using ASHRAE 52.2 in concert with ASHRAE 52.1 and individual performance criteria, filter selection can be a science, not an art.