Filter Selection: A Standard Procedure (June 2000)
An Overview of ASHRAE 52.2-1999Air filter testing has previously been based on ASHRAE Standard 52.1-1992. Designed to complement, not to replace, the older standard, ASHRAE 52.2-1999 offers several advances (Table 1):
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 different filter manufacturers. Outside weather conditions can also affect test results. The new ASHRAE 52.2-1999 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 52.1-1992 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 through 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 lung-damaging respirable particles). By comparison, with the ASHRAE 52.2-1999 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.
Guidelines for Using ASHRAE 52.2-1999While the MERV 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.
Be aware of the new test’s limitations regarding synthetic media filters. Most of these filters incorporate an electrostatic charge to enhance efficiency. The test standard uses an artificial test dust, which is loaded at a high rate to the filter. The rate of delivery and makeup of the test dust can mask the dissipation of the electrostatic charge that typically occurs in actual service. This situation can generate misleading test data for charged filters. Some industry experts call this syndrome “MERV in a box” – referring to the fact that a charged filter’s MERV may reflect its performance when it’s still clean (“in the box”) more accurately than it reflects real-life performance!
The ASHRAE 52.2-1999 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 numbers assigned to such filters may not be accurate. For critical applications, it may be preferable to select an alternative type of media. (If you’re uncertain whether a particular filter media is electrostatically enhanced, ask the manufacturer.)
To determine the required filter efficiency, start with the particle size of the target contaminant. ASHRAE Standard 52.2-1999 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 2 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.
In many cases, you may not be targeting one specific contaminant. For these situations, ASHRAE 52.2-1999 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 (3.0 to 10.0 micron) particles, a “low efficiency” filter with removal efficiencies in the E3 range would be the appropriate choice.
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 90% (E3).” After these values have been specified, you can begin to match up the right filter to the job. In replacement and retrofit applications, where there is no specified MERV, you may still use the reporting system as a guide for comparing products.
Be aware that final resistance is another important barometer of filter performance. Resistance is the loss of static pressure caused by the device operating at its rated airflow, measured in inches of water gauge (wg) pressure. Final resistance is the point at which the filter should be replaced. Since final resistance helps determine whether a filter will do the required job over its full service life, it is therefore an important factor in determining the filter’s proper MERV designation. The standard requires testing through a filter’s simulated life to a measured resistance to airflow that is either equal to the number shown in Table 2, or double the filter’s initial resistance – whichever is greater. This is to discourage manufacturers from only testing a clean filter. Take the example of a filter with an initial resistance of 0.85 in. wg. 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 final resistance of 1.7 in. wg (2 x 0.85 in.), since this number is greater than 1.0 in. wg (the number listed in Table 2).
As shown in Table 1, refer to the older standard (ASHRAE 52.1-1992) when you need information on arrestance – defined as 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. Also refer to ASHRAE 52.1-1992 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 52.1-1992 will continue to be a useful tool because it addresses some filter performance aspects that are not covered under the new standard.
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 or incinerate? This may dictate use of a product with non-metallic construction. 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.
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 old-fashioned guesswork. Now – by using ASHRAE 52.2-1999 in concert with ASHRAE 52.1-1992 and individual performance criteria – filter selection can be a science, not an art. The benefits? Enhanced indoor air quality, better energy performance, and lower filter service costs!