HVAC Filter Selection and MERV Ratings:
What Does It All Mean?
By David Klenk, P.E.
Mechanical Engineer
In 1968, American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) created
Standard 52.1 to test air filters using established techniques employed by the U.S. National Bureau of
Standards and the Air Filter Institute. During the course of the past 40 years, this standard has been
modified several times to reflect changing air filtration testing requirements. The version in use today,
Standard 52.1-1992, evaluates filters based upon three key measurements:
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Arrestance. Designated as a percentage, the arrestance of the filter, or ability of a filter to
remove synthetic dust, is calculated based upon the known weight of dust fed into a test unit
under controlled conditions and the amount of weight added or captured by the filter.
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Atmospheric Dust Spot Efficiency. Designated as a percentage, two procedures can be used
to calculate dust spot efficiency, which is the ability of a filter to remove atmospheric dust from the
air. Both the intermittent flow method and the constant flow method rely upon white filter paper as
a target and the ratio of light transmission. Sampling of atmospheric air is conducted at 25%,
50%, and 75% of dust loading and at final resistance. Efficiency is determined by an equation that
incorporates total airflow through the upstream and downstream targets and opacity of dust spot
on the upstream and downstream targets. Traditionally, engineers and designers have used the
terms 30%, 60%, and 85% filters to indicate which type of filter to install. These terms refer to the
dust spot efficiency of the filters.
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Dust Holding Capacity. Expressed in grams, dust holding capacity measures the amount of dust
that a filter can hold. Synthetic test dust is fed into a filter and the final capacity is based upon the
amount of dust held by the filter.
Standard 52.1 does not evaluate particle size and efficiency of the filter for a particular particle. It is not
known what type and size of particles will be captured by the filter. For instance, a 30% filter (based on
arrestance) means that 30% of the weight of the dust has been captured. Depending on the size of the
dust particles the actual number of particles (and hence the percentage of particles) captured may be
substantially less.
MERV Defined
To address the issue of particle size, ASHRAE 52.2 was issued in 1999 and modified in 2007 to
complement, not replace, Standard 52.1, by enabling engineers/designers to select a filter based upon
the actual known particle size of the contaminant desired to filter. Standard 52.2 indicates the lowest
point of filter efficiency (which is normally right at the time of installation1). Filters are evaluated based
upon initial efficiency as a function of one of 12 possible particle size ranges. Based on this data, a
numerical value is assigned to a filter. This numerical value is designated as MERV: Minimum Efficiency
Reporting Value. MERV ratings are based upon specific airflow speeds which must be noted on the
manufacturer’s data.
1
Note that synthetic filters are typically electrostatically charged and will have a greater efficiency at time of installation but will lose
efficiency quickly as the charge is dissipated. ASHRAE 52.2 currently does not address the dissipation of the electric charge. Refer
to the filter manufacturer literature or ask your local representative to determine if a filter is electrostatically charged.
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HVAC Filter Selection and MERV Ratings:
What Does It All Mean?
Testing for MERV
MERV ratings are created using the following testing process:
1.
2.
Initial resistance versus airflow is measured.
An optical particle counter measures and counts the particles in 12 equally distributed particle
size ranges before and after the filter.
3.
ASHRAE test dust is introduced to the filter in
High Efficiency Particulate Air
five predetermined steps (pressure drop across
(HEPA) Filters
filter increases by 0.04” water gauge (w.g.), 25%
of manufacturer’s recommended final pressure
Six types of high-efficiency filters are
drop, 50% of manufacturer’s recommended final
available; however only Type A filters are
pressure
drop,
75%
of
manufacturer’s
typically used in GMP facilities.
recommended final pressure drop and 100% of
manufacturer’s recommended final pressure
1. Type A filters. Commonly known as
drop).
Industrial Grade, Type A filters are
4.
Efficiency performance curves are developed
used for GMP/industrial processing,
based upon the optical particle counter data for
hospitals, and non-critical areas.
each of the six readings (one initial, and five
Unlike the other filter types described
loading steps).
below, which are tested both at the
5.
A composite minimum efficiency curve with the
manufacturing facility and after
lowest efficiency reading for the six readings for
installation, Type A filters are tested at
12 particle size ranges is developed.
the manufacturing facility only.
6.
Twelve particle size ranges are placed in three
2. Type B filters. Commonly known as
larger groups (E1, E2, and E3) and the
Nuclear Grade, these filters are used
percentages of each group are averaged. This
for Department of Energy nuclear
average is deemed the Particle Size Efficiency
programs (test reactors, R & D, and
which is the value used to determine the MERV
weapons) and commercial reactors.
rating based upon ASHRAE data. (See the
3.
Type C filters. Commonly known as
ASHRAE Application Guidelines chart listed on
Laminar
Flow Grade, Type C filters
page 4 for additional information.)
are used for laminar flow work
stations, clean rooms, semiconductor
Filter Selection
and drug manufacturers.
4.
Type D filters. Commonly known as
Proper filter selection requires an understanding of
Very
Large Scale Integrated (VLSI)
several factors: the type and size of particle/contaminant
filters,
these filters are used for
to be removed; expected filter service life; and pressure
semiconductor
and drug
drop. ASHRAE 52.1 and 52.2 should be used in
manufacturers.
conjunction with one another to make a final filter
5. Type E filters. Commonly known as
selection. To help in the selection process, the following
Bio/Hazard Grade, Type E filters are
information will be required:
used for hazardous biological
containment facilities.
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Physical size of particulate. The chart on page 4
6.
Type F filters. Commonly known as
should be helpful in determining the MERV rating
Ultra
Low Penetration Air (ULPA)
of the filter once the particulate size has been
Grade,
these filters are used at semidetermined. For example, if bacteria ranges from
conductor
facilities.
0.3 microns to 4 microns, the chart indicates a
MERV 13 would be the minimum rating to select,
and higher MERV-rated filters should be selected
with the end users’ level of comfort and
economics in mind.
HEPA filter efficiencies are based upon
0.30 µm particle sizes.
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HVAC Filter Selection and MERV Ratings:
What Does It All Mean?
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Service life of filters. Review the dust holding capacity in both ASHRAE 52.1 and the
manufacturer’s product data to compare the relative service life of comparable filters. Additional
service life can be obtained by simply increasing the depth of the filter (e.g., 2” deep to 4” deep).
Initial and final pressure drop of filters. Review manufacturing product data to determine initial
and final pressure resistance. The system should be able to provide design airflow throughout the
entire loading process of the filters. Note that increasing the number of filters, and, therefore, the
total surface area, will increase the service life of the filters but will not affect the final system
pressure drop.
Filter frame material. Determine if the filter will be in a wet or dry environment, and how it will be
disposed. (For example, if incineration will be used, a metal frame would not be advisable.)
Design airflow. Pre-filters and final filters should be matched for airflow compatibility and pressure
drop.
Physical space allotted for filter bank. Allow space for duct transition before and after the filter
bank.
Remember, the filter rating is based upon the filter only and is tested during ideal lab conditions.
JKRrating
spec:must
Primary
filter - the filter housing. The filter housing must be matched to
Determination of the system
include
Synthetic
provide the proper sealingAverage
of filters
to the Dust
filter frame. An inexpensive, poor quality filter frame negates
spending funds on a high-rated
MERV
filter.
Weight arrestance 85%,
Note: Ashrae 52.1 has been withdrawn
Dust Spot Efficiency 30%
and replaced with Ashrae 52.2
ASHRAE Application Guidelines
Std 52.2
Particle size efficiency
(%)
Std 52.2
Minimum
Efficiency
Reporting Value
(MERV)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Std 52.1 Dust
Spot Efficiency
(%)
Std 52.1
Average
Arrestance
Efficiency
(%)
E1
0.30 to 1.0 µm
E2
1.0 to 3.0 µm
E3
3.0 to 10.0 µm
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
E < 75
75 ≤ E < 85
85 ≤ E < 95
95 ≤ E
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
E < 50
50 ≤ E < 65
65 ≤ E < 80
80 ≤ E
90 ≤ E
90 ≤ E
90 ≤ E
90 ≤ E
E < 20
E < 20
E < 20
E < 20
20 ≤ E < 35
35 ≤ E < 50
50 ≤ E < 70
70 ≤ E
85 ≤ E
85 ≤ E
85 ≤ E
90 ≤ E
90 ≤ E
70 ≤ E
90 ≤ E
95 ≤ E
< 20
< 20
< 20
< 20
< 20
< 20
25 ≤ E < 30
30 ≤ E < 35
40 ≤ E < 45
50 ≤ E < 55
60 ≤ E < 65
70 ≤ E < 75
80 ≤ E < 90
90 ≤ E < 95
E > 95
n/a
< 65
65 ≤ E < 70
70 ≤ E < 75
75 ≤ E < 80
80 ≤ E < 85
85 ≤ E < 90
E > 90
E > 90
E > 90
E > 95
E > 95
E > 95
E > 98
E > 98
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
17
99.97 < E
18
99.99 < E
19
99.999 < E
20
99.999 < E
"E" indicates efficiency percentage
JKR spec: Secondary filter 3
Efficiency is 90-95%
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Email: aei@hixson-inc.com Website: www.hixson-inc.com
HVAC Filter Selection and MERV Ratings:
What Does It All Mean?
References
ANSI/ASHRAE. “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by
Particle Size.” ANSI/ASHRAE Standard 52.2. 2007.
ASHRAE. “Chapter 24: HVAC Systems and Equipment.” ASHRAE Handbook. 2004.
Camfil Farr. “ASHRAE Testing for HVAC Air Filtration: A Review of Standards 52.1-1992 & 52.2-1999.”
Technical Services Bulletin January 2001.
Flanders Corporation. “HEPA Filters and Filter Testing: PB-2007-1023.” May 2004.
Thornburg, Don. “Filter Selection: A Standard Procedure.” Engineered Systems June 2000: 74-80.
Advocacy: Hixson is a "No-Bias" Company
At Hixson, we seek to serve as extended team members of our clients’ organizations, not as representatives of
companies with outside concerns. Therefore, we do not accept income or incentives – or sell/represent products –
from equipment suppliers, contractors or any company that could improperly influence our opinion. Because of this,
Hixson ensures that the information we present is unbiased and has been developed in the best interests of our
clients.
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