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between or outside of the listed values in Table 2. In these cases,
you can only approximate your home’s actual indoor RH.
Manual Mode:
(Automatic Mode – see pg 3)
Your Aprilaire Humidifier Control is installed in the cold air return. It
is important to anticipate a drop in outdoor temperature and reduce
the setting accordingly to avoid excessive condensation. For exam-
ple, with an outdoor temperature of 20° the correct setting will be
35% relative humidity. If the temperature is expected to fall to 0° that
evening, then merely reduce the setting to 25% several hours prior to
the temperature change.
The recommended settings on the humidistat are based on
years of research (see table 3) and experience as to what is best for
the average home. These settings represent a compromise between
RH levels that would be most desirable for comfort
reasons and humidity levels that are suitable for protection of your
home and to avoid condensation on your windows. For example, a
wintertime indoor RH of 50% may be considered ideal, but unfortu-
nately, it probably would result in damage to your home.
Observance of the recommended RH levels on your humidistat,
therefore, is an important safeguard. Condensation of water on
inside windows in the form of fogging or frost is usually an indica-
As an example, if the outdoor temperature is 20º F, and, following step 3, the
humidifier turns off at “5” on the dial range, then the RH in your home is 35%.
(Table 2) % Relative Humidity Guide
-10 0 10 20 30 40
1 101010152025
2 101015202530
3 101520253035
4 152025303540
520253035 40 45
6 253035404545
7 303540454545
Outdoor Temperature (ºF)
Dial Setting