ASHRAE Journal - December 2020 - 19

TECHNICAL FEATURE

difference in test results for the ﬁlters. This portion of the
TABLE 1 Standard 52.2-2017 Table 12-1 MERV parameters.
table needed to be revised by smoothing the table out in
COMPOSITE AVERAGE PARTICLE SIZE EFFICIENCY,
ways that made more sense for actual ﬁlters.
STANDARD 52.2 MINIMUM
PERCENT IN SIZE RANGE, µM
EFFICIENCY REPORTING
Over time, people began to consider the removal of
RANGE 1
RANGE 2
RANGE 3
AVERAGE
VALUE (MERV)
0.30 - 1.0
1.0 - 3.0
3.0 - 10.0 ARRESTANCE (%)
smaller particles as important to indoor air quality and
1
N/A
N/A
E3 < 20
Aavg < 65
looked to reduce their concentrations using HVAC ﬁlters.
65 ≤ Aavg
2
N/A
N/A
E3 < 20
However, for MERV 11 to MERV 12 ﬁlters, which were a
step up for many applications, the MERV table did not
3
N/A
N/A
E3 < 20
70 ≤ Aavg
deﬁne any lower efﬁciency limits for the smaller particles.
4
N/A
N/A
E3 < 20
75 ≤ Aavg
To address these issues the MERV table was revised in
5
N/A
N/A
20 ≤ E3
N/A
2015. Reference 3 details the proposed changes ultimately
N/A
6
N/A
N/A
35 ≤ E3
added to the Standard 52.2 method in 2015. These changes
N/A
7
N/A
N/A
50 ≤ E3
helped deﬁne the changes in the E3 range for MERV 8
70 ≤ E3
N/A
8
N/A
20 ≤ E2
and up. But they also served to ensure deﬁned levels of
75 ≤ E3
N/A
9
N/A
35 ≤ E2
efﬁciency for the E2 range for MERV 8 and MERV 9 and for
80 ≤ E3
N/A
10
N/A
50 ≤ E2
the small particles with E1 requirements for MERV 11 and
65 ≤ E2
85 ≤ E3
N/A
11
20 ≤ E1
higher. In 2020, with the push in the market to buy ﬁlters
80 ≤ E2
90 ≤ E3
N/A
12
35 ≤ E1
that remove the SARS-CoV-2 virus, having clearer delinea85 ≤ E2
90 ≤ E3
N/A
13
50 ≤ E1
tion in the MERV table helps buyers and sellers understand
90 ≤ E2
95 ≤ E3
N/A
14
75 ≤ E1
what their ﬁlter can do. Table 2 shows the changes.
90 ≤ E2
95 ≤ E3
N/A
15
85 ≤ E1
In addition to the changes to the MERV table that led to
95 ≤ E2
95 ≤ E3
N/A
16
95 ≤ E1
more sensible and smoother assignment of the MERV to
ﬁlters, ASHRAE RP-1088, completed in 2005,
TABLE 2 2015 changes to Standard 52.2 Table 12-1 MERV parameters.3
led to changes to Standard 52.2-2012 to improve
COMPOSITE AVERAGE PARTICLE SIZE EFFICIENCY, PERCENT IN SIZE RANGE, µM
STANDARD 52.2 MINIMUM
repeatability and reproducibility. It tested ﬁlters
EFFICIENCY REPORTING
RANGE 1
RANGE 2
RANGE 3
VALUE (MERV)
in multiple laboratories and found signiﬁcant
0.30 - 1.0
1.0 - 3.0
3.0 - 10.0
variability. Based on examination of the data,
8
N/A => 20 ≤ E2
70 ≤ E3
changes were proposed and implemented to
9
E2<50 => 35 ≤ E2 85 ≤ E3 => 75 ≤ E3
reduce this variability. These changes included
10
85 ≤ E3 => 80 ≤ E3
tightening the allowed relative humidity range to
11
N/A => 20 ≤ E1
make sure the aerosol is dry and in its stable size
12
N/A => 35 ≤ E1
distribution when it reaches the ﬁlter.4 Follow-on
13
E1<75 => 50 ≤ E1
90 ≤ E2 => 85 ≤ E2
project 1287-RP was instigated to determine, in
14
90 ≤ E3 => 95 ≤ E3
large part, the inﬂuence of the optical particle
counter (OPC) speciﬁcation on the efﬁciency
15
90 ≤ E3 => 95 ≤ E3
result.5 New OPC speciﬁcations resulted from
16
this project and were added to ASHRAE Standard
52.2-2012 to improve the test procedure and
(HEPA) ﬁlters6 and hypothetical MERV 17 to MERV 20.
The intent was only to show how MERV 15 and MERV 16
equipment speciﬁcations to yield improved precision.
designations merge smoothly into HEPA designations.
Why You Need to Forget MERV 17 to MERV 20
Unfortunately, some people took this information to
Neither table shows particle removal efﬁciencies above
mean Standard 52.2 could be used to test the perforMERV 16. This is because there has never been a MERV
mance of HEPA ﬁlters. This is incorrect (indeed the body
higher than 16! In Standard 52.2-1999, a table in an infor- of the standard contains no information about how data
mation-only appendix included correlations between
from such tests might be interpreted or used).
Institute of Environmental Sciences and Technologies
Filters with HEPA-level efﬁciencies are often used in
(IEST) classiﬁcations for high-efﬁciency particulate air
critical applications such as cleanrooms and hospitals,
D ECEM BER 2020

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ASHRAE Journal - December 2020

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