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Frequently Asked Questions

Will Dirty Air Filters Really Increase My Utility Costs?

Definitely! As your air filters collect more particulate, referred to as dirty, they begin to restrict the air flowing through the whole system. This causes the HVAC unit to work harder to maintain a sufficient air flow. When your HVAC unit is working harder and longer, it is using more electricity, which over time will dramatically increase your utility costs. Additionally, dirt acts as an insulator disrupting the heat transfer of the air flowing through the unit.

Filter Services of Indiana can perform a technical analysis on your system and produce energy savings calculations and filter life cycle reports that will determine the optimum time to change your filters. This allows our partners to save money on their air filtration products without jeopardizing additional energy cost to run the HVAC unit.

Additionally, filters that are not changed at the proper time can lead to costly mechanical breakdowns on your expensive HVAC equipment.
Developing a compressive filter change program that considers all of the aforementioned, is the best way to reduce your filter replacement and energy cost.

Filter Services of Indiana personnel are your experts in Indoor Air Quality (IAQ) and we are committed to protecting your HVAC equipment while
providing a clean environment for your employees, customers, vendors, family and friends!

What Does Efficiency Mean?

(1) The ability of a device to remove particulate or gaseous material from an airstream by measuring the concentration of the material upstream and downstream of the device.
(2) In the ASHRAE 52.1 Standard test method, it is a measure of the ability of a filter to remove the staining portion of atmospheric dust from the test air. This is officially termed Atmospheric Dust Spot Efficiency.

For air filters, efficiency is determined by the relative size of the filter fibers, the density of the fibers in the filter, the adhesive, if applied, and the velocity at which the air is moving through the filter. Generally, large filter fibers are effective on large particles at high velocities and small filter fibers work best on small particles at low velocities. Proper filter design, including choice of fiber diameter, density of media, and the proper adhesive, are necessary for a filter to function properly. With proper design it is possible for a filter to function well beyond its nominal theoretical efficiency range. For example, a properly designed synthetic media filter, with the proper adhesive, will function as well or better in the medium efficiency range than a dry fine fibered filter.

If the filter fibers are designed to remove large particles, they will not be effective on smaller particles regardless of the depth of the filter. For this reason, properly designed filters must be used to stop fine dust particles. Also, many filter fibers will initially stop dust particles, but will release them after unless the proper adhesive is employed to permanently trap the particles.

What is Arrestance?

A measure of the ability of an air filtration device to remove a synthetic dust from the air. ASHRAE arrestance is a measure of the ability of a device to remove ASHRAE dust from test air.

What is a MERV Rating?

An “overall” reporting value of a 52.2-evaluated air filter is the expression of the Minimum Efficiency Reporting Value (MERV). The MERV is a single number that is used along with the air velocity at which the test was performed; to simplify the extensive data generated by the method of testing. MERV is expressed on a 16 point scale and is derived from the particle size ranges.

The ASHRAE Standard 52.2

ASHRAE Standard 52.2 features many improvements over the 52.1 standard. Data such as average efficiency, arrestance and dust holding capacity
which are provided by 52.1 will remain important performance characteristics (see Table 1 for Application Guidelines for the two standards).

Some of the improvements found in the 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 sizes.
  • The 52.2 method of test will create results that are reliable and verifiable.
  • Seventy-two (72) data points are reduced into a single curve that typifies the minimum efficiency of a filter.

MERV Std 52.2

Average ASHRAE Dust Spot Efficiency Std 52.1

Average ASHRAE Arrestance Std 52.1

Particle Size Ranges

Typical Applications

Typical Filter Type


< 20%

60 to 80%

> 10.0 µm

Residential / Minimum Light / Commercial Minimum / Equipment

Permanent / Self Charging (passive) Washable / Metal, Foam /
Synthetics Disposable Panels Fiberglass / Synthetics


< 20 to 35%

80 to 95%

3.0 – 10.0 µm

Industrial Workplaces Commercial Better / Residential Paint
Booth / Finishing

Pleated Filters Extended Surface Filters Media Panel Filters


40 to 75%

> 95 to 98%

1.0 – 3.0 µm

Superior / Residential Better / Industrial Workplaces Better /
Commercial Buildings

Non-Supported / Bag Rigid Box Rigid Cell / Cartridge


80 to 95% +

> 98 to 99%

0.30 – 1.0 µm

Smoke Removal General Surgery Hospitals & Health Care
Superior / Commercial Buildings

Rigid Cell / Cartridge Rigid Box Non-Supported / Bag






£ 0.30 µm

Clean Rooms High Risk Surgery Hazardous Materials


Note: This table is intended to be a general guide to filter use and does not address specific applications or individual filter performance in a given application. Refer to manufacturer test results for additional information.
(1) Reserved for future classifications
(2) DOP Efficiency


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.

The filter’s performance, on each of the twelve particle sizes, during the six test cycles (a total of 72 measurements) is determined. For each
measurement, the filtration efficiency is stated as a ratio of the downstream-to-upstream particle count. The lowest values over the six test
cycles are then used to determine the Composite Minimum Efficiency Curve (Note: in many cases, this will be the initial reading before the five dust loads). Using the lowest measured efficiency avoids the fiction of averaging and provides a “worst case” experience over the entire test.

Table 2: ASHRAE 52.2 Particle Size Ranges





0.30 to 0.40



0.40 to 0.55


0.55 to 0.70


0.70 to 1.00


1.00 to 1.30



1.30 to 1.60


1.60 to 2.20


2.20 to 3.00


3.00 to 4.00



4.00 to 5.50


5.50 to 7.00


7.00 to 10.00

The twelve size ranges are placed in three larger groups according to the following schedule: ranges 1-4 (or E1, which is 0.3 to 1.0 ìm), ranges 5-8 (or E2, which is 1.0 to 3.0 ìm), and ranges 9-12 (or E3, which is 3.0 to 10.0 ìm). Averaging the Composite Minimum Efficiency for each of these groups will calculate the average Particle Size Efficiency (PSE), and the resulting three percentages (E1, E2, E3) are then used to determine the MERV.

Table 3: MERV Parameters

The average PSE for each of the three groups (E1, E2 and E3) is referenced against the Minimum Efficiency Reporting Value Parameters (see Table
3: MERV Parameters). Move up the appropriate Range Group (E1, E2 and E3) on Table 3 and record the MERV to the left of the first true statement. Do this for all three groups.

Standard Test Airflow Rates

The Minimum Efficiency Reporting Value (MERV) must be stated with the air velocity at which the filter was tested. For example, if the filter was tested with an air velocity of 492 FPM and was found to be MERV 10, the filter’s Minimum Efficiency Reporting Value would be MERV 10 @ 492 FPM. ASHRAE Standard 52.2 tests are to be conducted at one of seven airflow rates:

118 FPM (0.60 m/s)
246 FPM (1.25 m/s)
295 FPM (1.50 m/s)
374 FPM (1.90 m/s)
492 FPM (2.50 m/s)
630 FPM (3.20 m/s)
748 FPM (3.80 m/s)

Minimum Final Resistance

Final resistance must be at least twice the initial resistance at the test airflow rate, or the values in Table 3, whichever is greater.

Average Arrestance By Standard 52.1

Filters with an efficiency of less than 20% in E3 (MERV 1 through MERV 4) must be tested per the arrestance test of ASHRAE Standard 52.1