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Rev 2.0
63-7048 1998 Honeywell Inc.
Page 3 of 22
Defining a Comfortable Humidity Level
Much research has been conducted on what combination of temperature and humidity
creates the most comfortable indoor environment. The 1997 ASHRAE Fundamentals
Handbook
1
defines how comfort is perceived as a function of temperature and humidity.
Figure 2 shows a summary of the ASHRAE conclusions.
Figure 2 – ASHRAE Winter Comfort Zone
In designing a humidifier control, what is of particular interest is the lower region of the
comfort zone. Figure 2 shows that a comfortable humidity level is defined in terms of
dewpoint, rather than relative humidity. The lower end of the winter comfort zone is a
dewpoint of 36
F, for dry bulb temperatures ranging from about 69
F to 76
F. This
strongly supports the notion that a comfortable humidity level is best described by
dewpoint, not relative humidity. In fact within the temperature range of 69
F to 76
F,
the relative humidity can vary from 23 % to 81% and still be considered to be in the
comfort zone, as ASHRAE has defined it! You can begin to see the problem in trying to
use relative humidity to control humidity level. Relative humidity is not a direct
measurement of the amount of moisture in the air. As shown in Figure 1, it is very
dependent on the temperature of the air being measured.
1
"Conditions for Thermal Comfort", 1997 ASHRAE Fundamentals Handbook, page 8.12,
Dry Bulb Temperature ( F)
Comfort Zone
Dewpoint ( F)
60
70
80
10
30
40
50
45
55
60
20
65
10 % RH
20 % RH
30 % RH
40 % RH
50 % RH
60 % RH
70 % RH80 % RH90 % RH
65
75