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4. Auto and Fan Humidify
The 2 choices of AUTO and FAN can be selected together.
This provides both functions simultaneously.
5. Humidify Off
The humidify function can be turned off completely. This does
not require changing existing set points.
To Select Humidification (Between NORMAL, FAN, AUTO,
FAN and AUTO, or OFF) (See Table 4.)
Press HUMIDITY button to bring up humidity select screen. It is
indicated by "hu" or "dhu" in clock display. Successive presses
will change between "hu" and "dhu" in clock display. Select "hu"
for humidity functions. The large display shows actual humidity
level. The smaller display (in heat set point location) shows
humidity set point (Humidify Set Point) or OF (off). When
Humidity Set Point value is displayed, its value can be changed
with up and down buttons. Successive presses of MODE button
will move between 5 choices described above and each will be
indicated as shown in Table 4.
To exit humidity select screen, press END button.
Additional Humidify Comments
The humidifier is actually turned on when humidity is 2 percent
below set point and turned off when it reaches 2 percent above set
point. This built-in hysteresis prevents humidify output from
toggling on and off when humidity level is near set point.
The vacation mode provides some additional humidification func-
tions. These are specially designed to provide protection for an
unoccupied home while simultaneously minimizing energy use.
Refer to "Vacation" section for additional information.
DEHUMIDIFICATION
Dehumidification is done only during cooling. Depending on type
of equipment used, compressor speed, blower speed, set point
adjustment, and equipment cycling are modified to provide added
dehumidification. A dehumidification set point (separate from
humidification set point) is available to the homeowner. It can
range from 50 to 90 percent relative humidity. When actual
humidity is higher than set point, a dehumidification demand
exists. The Thermidistat Control responds by activating its dehu-
midify output. It may also control the compressor and blower,
depending on equipment type and dehumidify selection choice.
The 3 available selections are described below.
The amount of extra dehumidification available is very dependent
on the type of equipment in the home. Without a variable-speed
blower, the system’s ability to adjust dehumidification is very
limited.
1. Normal Dehumidify Operation
When normal dehumidify is selected, the compressor will not
turn on without a cooling demand. If dehumidify demand
exists while cooling, dehumidify output will also be active
(24vac removed). This output commands variable-speed
blowers to reduce their airflow, which improves water re-
moval from the cooled air.
2. Cool to Dehumidify
The cool to dehumidify selection tells the system to operate
the compressor, within limits, when there is a dehumidify
demand even if there is no cooling demand. The limits are that
the system may overcool up to 3°, but no more, while
attempting to satisfy a dehumidify demand. Within this 3°
range, there is an additional balance between overcooling and
humidity satisfaction. When overcooling must occur, the
dehumidify set point is adjusted upward by 2 percent per
degree of overcooling. For example, a cooling set point of
76°F and a dehumidify set point of 60 percent is equivalent to
a cooling set point of 75°F and a dehumidify set point of 62
percent. This dehumidify set point change is internal to the
Thermidistat Control and is not shown on the display.
During cool-to-dehumidify demand, the compressor runs a
maximum of 10 minutes on, followed by 10 minutes off.
When the compressor turns off, the fan (G output) is also
turned off immediately. The immediate fan shutoff prevents
re-evaporation of water on the coil, improving dehumidifica-
tion. If fan is set to continuous, the G (fan on) signal is
removed for 5 minutes starting when compressor turns off.
In most furnaces and fan coils, the blower operates for 90 sec
after both Y and G disappear. This 90-sec delay should be
removed, if possible, for maximum dehumidification perfor-
mance. Consult furnace or fan coil Installation Instructions to
see if delay can be disabled.
3. Dehumidify Off
Dehumidification can be turned off completely. This can be
done without changing existing set points.
To Select Dehumidification (Between NORMAL, COOL TO,
and OFF)
Press HUMIDITY button to bring up humidity selections. Succes-
sive presses will show "hu" or "dhu" in clock display. Select "dhu"
for dehumidify selections. The large display shows actual humidity
level. The smaller display (in cool set point location) shows
dehumidify set point (DSP) or OF (off). When DSP value is
displayed, its value can be changed with up and down buttons.
Successive presses of MODE button moves between 3 choices
above and each will be indicated as in Table 5.
Superdehumidify (With Cool to Dehumidify)
This selection only affects cool-to-dehumidify operation. It is part
of the installer setup (see Configuration Option No. 7) and must be
made by installer. The Homeowner’s Guide does not cover this
selection. A requirement is the use of a variable-speed indoor unit
with superdehumidify capability. During cool-to-dehumidify call,
it provides maximum dehumidification by reducing airflow to a
minimum. The actual superdehumidify command from Thermidis-
tat Control to the indoor unit is a Y signal without a G signal in
addition to dehumidify signal. The indoor unit responds to this
combination by reducing the airflow to a minimum. All other
characteristics of cool to dehumidify are the same.
Additional Dehumidify Comments
Dehumidification can be enhanced (with some efficiency loss) by
turning blower off immediately at the end of each cooling cycle
(eliminating normal 90 sec blower off delay). Where maximum
Table 4—Humidification Selections
SELECTION MODE DISPLAY FAN DISPLAY
HUMIDIFY SET POINT
DISPLAY
Normal Humidify Blank Blank Humidify Set Point Value
Fan Humidify Blank FAN Humidify Set Point Value
Auto Humidify AUTO Blank Humidify Set Point Value
Auto and Fan AUTO FAN Humidify Set Point Value
Humidify Off Blank Blank OF
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