ASHRAE Journal - May 2022 - 56

COLUMN DATA CENTERS
TABLE 2 Evolution of recommended air-cooled data center environmental conditions.
CLASS
FIRST EDITION
2004
Recommended Temperature
(A1 to A4)
Recommended Temperature
(H1)c
Recommended Relative
Humidity
(All Classes)d
Recommended Dew Point
(All Classes)d
N/A
68°F to 77°F
N/A
40% to 55% RH
SECOND EDITION
2008
THIRD EDITIONa
2012
64.4°F to 80.6°F
N/A
60% RH Max.
(See Dew Point for Min.)
41.9°F to 59°F
64.4°F to 80.6°F
N/A
60% RH Max.
(See Dew Point for Min.)
41.9°F to 59°F
FOURTH EDITION
2015
64.4°F to 80.6°F
N/A
60% RH Max.
(See Dew Point for Min.)
15.8°F to 59°F
FIFTH EDITION
2021
64.4°F to 80.6°F
64.4°F to 71.6°F
70% RH Max.b
Or
50% Max.b
15.8°F to 59°F
aThe Third Edition did not have any changes to the recommended or Class A1 allowable conditions. Rather, environmental classes A3 and A4
were added to the guidelines, with broader operating ranges.
bIf testing with silver or copper coupons results in values less than 200 and 300 Anstroms/month, respectively, then operating up to
70% RH is acceptable. If testing shows corrosion levels exceed these limits, then catalyst-type pollutants are probably present and RH
should be driven to 50% or lower.
cClass H1 has a recommended temperature operating range that is different than all other equipment classes.
dRecommended Relative Humidity and Dewpoint are only applicable to Class H1 in the Fifth Edition, since Class H1 was not defined
earlier.
to the original Class 1 and 2, respectively. Classes A3
and A4 were added in the Third Edition to facilitate
wider adoption of cooling without mechanical refrigeration.
In the Fifth Edition, a new high-density class H1
was added to align with new high-density ITE products
entering the market. The original Class 3 and Class 4
(renamed Class B and Class C in the Third Edition) are
not " data center environments " and are not addressed
in this column.
Description of Air-Cooled Changes for Subsequent Editions
Second Edition Changes
The environmental envelope changes in the Second
Edition4 can be found outlined in blue in the third
column of Table 2. Changes were made to both the
recommended temperature and humidity ranges. For
temperature, the recommended range was expanded
from 68.0°F to 77.0°F (20°C to 25°C) to 64.4°F to 80.6°F
(18°C to 27°C). The recommended humidity range was
also adjusted. For relative humidity, the low-end RH
threshold was eliminated and the high end was raised
from 55% to 60% RH. A recommended dew-point
temperature range was also added, with a range from
41.9°F to 59.0°F (5.5°C to 15°C) The most restrictive of
the RH vs. dew-point temperatures defines the envelope.
No changes to the allowable envelopes were made
in the Second Edition.
56
ASHRAE JOURNAL ashrae.o rg M AY 2022
Third Edition Changes
The environmental envelope changes in the Third
Edition5 can be found outlined in blue in the fourth
column of Table 3. There were no changes to the recommended
range in the Third Edition. For Class A1 and A2,
there were also no changes to the allowable environment,
but two new environmental classes were added:
A3 and A4. Class A3 had an allowable temperature range
from 41°F to 104°F (5°C to 40°C), while Class A4 had an
allowable range from 41°F to 113°F (5°C to 45°C). The
allowable humidity environment for Classes A3 and A4
was broader than for existing Classes A1 and A2.
These new classes were created in large part due
to the desire to provide data center cooling without
mechanical refrigeration. The thought was that
equipment designed to Class A3 could be placed in a
data center that could be cooled without mechanical
refrigeration-using water evaporation through cooling
towers or other adiabatic heat rejection pathways
to the atmosphere. Similarly, equipment designed to
Class A4 could be placed in an air-cooled data center
that uses outdoor air (with appropriate filtration and
humidity control) to provide 100% of the cooling and
heat rejection.
While not specifically covered in this column, the
Third Edition also provided industry data on ITE failure
rates as a function of ITE inlet temperature. This could

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