MERV, or Minimum Efficiency Reporting Value, is a number from 1 to 16 that is relative to an air filter’s efficiency. The higher the MERV, the more efficient the air filter is at removing particles. At the lower end of the efficiency spectrum a fiberglass panel filter may have a MERV of 4 or 5. At the higher end, a MERV 14 filter is typically the filter of choice for critical areas of a hospital (to prevent transfer of bacteria and infectious diseases). Higher MERV filters are also capable of removing higher quantities of extremely small contaminant (particles as small as 1/300 the diameter of a human hair). A higher MERV creates more resistance to airflow because the filter media becomes denser as efficiency increases. For the cleanest air, a user should select the highest MERV filter that their unit is capable of forcing air through based on the limit of the unit’s fan power
Most filters become more efficient as the filter is used in the system. Care should be taken when considering filters that incorporate an electrostatic charge. Although offering a reasonable MERV value these filters will actually drop in efficiency as the filter loads with contaminant. A number of electrostatic filters are presently being offered for residential as well as the commercial/industrial applications. Refer to the filter packaging or literature to determine if the filter you are purchasing relies on an electrostatic charge to boost its MERV. If it does, request that the manufacturer provide you with the "discharged MERV” for the filter to determine its actual in-use performances
An air filter’s performance is determined by measuring the particle counts upstream and downstream of the filter being tested. Particle counts are taken over the range of particles six times. One begins with a clean filter and then uses that same filter 5 additional times.
The particle generator creates particles of a known size in the air stream. The objective is to create particles of sufficient numbers to obtain meaningful counts in all of the measured particle ranges which are the following:
|Range||Size (in microns)||Group|
|1||0.30 to 0.40||E1|
|2||0.40 to 0.55||E1|
|3||0.55 to 0.70||E1|
|4||0.70 to 1.00||E1|
|5||1.00 to 1.30||E2|
|6||1.30 to 1.60||E2|
|7||1.60 to 2.20||E2|
|8||2.20 to 3.00||E2|
|9||3.00 to 4.00||E3|
|10||4.00 to 5.50||E3|
|11||5.50 to 7.00||E3|
|12||7.00 to 10.00||E3|
The efficiency of the filter is then measured on each of the 12 particle size ranges for each of the 6 cycles. Efficiency is measured as the percentage of particles captured by the filter. The lowest of the 6 readings is then taken to determine the Composite Minimum Efficiency Curve.
The twelve size ranges are then placed in three larger groups (E1, E2, E3) and the percentages in each group are averaged. This average is called the Particle Size Efficiency and these are used to determine the Minimum Efficiency Reporting Value (MERV).
|ERV Value||Group 1
Av. Eff. %
(0.30 to 1.00)
Av. Eff. %
(1.00 to 3.00)
Av. Eff. %
(3.00 to 10.00)
When using the above table you move up each Group until you arrive at a true statement. This will correspond with a MERV number. The lowest MERV of the three groups is the MERV rating for the filter.
Another important consideration is the speed of the airflow during the test. There are seven approved speeds:
|1.||118 Feet Per Minute (FPM)||0.60 meters/second (m/s)|
|2.||246 FPM||1.25 m/s|
|3.||295 FPM||1.50 m/s|
|4.||374 FPM||1.90 m/s|
|5.||492 FPM||2.50 m/s|
|6.||630 FPM||3.20 m/s|
|7.||748 FPM||3.80 m/s|
The Minimum Efficiency Reporting Value (MERV) must be stated with the speed of the airflow at which the filter was tested.
One of the challenges of having both an ASHRAE 52.1 test and an ASHRAE 52.2 test is trying to make some comparisons between the two. The table below may help in this regard:
|MERV Rating||Efficiency||Particle Size||Applications||Filter Type|
|5-8||<20 to 35%||3 to 10 um||Commercial||Pleated Filters|
|9-12||40 to 75%||1 to 3 um||Res. – Best||Best Pleated|
|13-16||80 to 95%||0.3 to 1 um||Smoke Removal||Rigid Cell|
These comparisons are not meant to be definitive. They are guidelines only.
As a practical matter, let’s see how we can use the MERV rating. One example would be mold spores. What filter would we recommend if we were concerned with pulling out a high percentage of mold? Mold varies in size from about 4 microns to 40 microns. The greatest number of mold spores are less than 10 microns in diameter. If we look at our chart a MERV 8 filter will pull out at least an average of 70% of the particles down to 3 microns. Not a bad recommendation. However, for just a little more money pleated filters are available in a MERV 11. The MERV 11 would give you an average of at least 85% mold removal. For all those but the most mold sensitive this should be adequate.