Aprilaire 220 - Manual (Page 7)

Manual Humìdistat:
(Automatic Humidifier Control - see pg 3)
Your new Aprilaire Humidifier is controlled by a humidistat
installed either in the living area or 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
example, 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. For example, a wintertime indoor RH of 50% may be
considered ideal, but unfortunately, 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 indication of too high relative humidities.
This same condensation can take place in other areas in your
home with the possibility of damage resulting.
(Table 3) Outdoor-Indoor Relative Humidity
Outside Temperature Recommended RH
+40“ 45%
+30“ 40%
+20“ 35%
+10“ 30%
0“ 25%
-10“ 20%
-20“ 15%
The humidistat is a precision instrument that can be used
to determine the RH accurately in your home during the winter.
Turn the dial to the lower setting, then reverse the dial direction
slowly until a “click” is heard. At this point, read the RH on the
dial. This will be very close to the actual RH in your home.
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