Ingersoll-Rand ReliaTel, Manual, Page: 147 / 176

RT-SVD03L-EN 147
Dehumidification
Hot Gas Reheat 3 to 25 Tons
Factory installed hot gas reheat allows application of
dehumidification. Initiation of reheat is performed by
an input to the RTOM module from either a relative
humidity sensor or a humidistat type device. These
connections are made through the low voltage terminal
strip located in the unit control panel.
Actuation of the reheat is accomplished by energizing a
valve that allows refrigerant hot gas to flow through
the hot gas reheat coil.
Dehumidification is allowed only when the outside air
temperature is above 40°F and below 100°F.
Dehumidification is also not allowed if there is an
active call for heating or a request for greater than 50%
cooling.
If, during a dehumidification cycle, there is a call for
heating or cooling, the dehumidification cycle is
terminated. The economizer outside air damper is also
driven to minimum position during dehumidification.
A humidity sensor that is capable of providing a 4 - 20
ma output can be used. The RH setpoint is established
by adjusting the R41 potentiometer (labeled Dehumid
SP) on the RTOM module. The range can be set from 40
to 60%. In the absence of a zone humidity sensor input,
an on/off input from a zone humidistat is used to
initiate or terminate the dehumidification cycle.
N No otte e:: If a non-Trane® humidity sensor is provided
from another source, it may be necessary for a
wiring modification to the RTOM. Refer to HVAC
case5284.
A relative humidity sensor takes priority over a
humidistat. Dehumidification takes priority over a call
for one stage cooling. Heating or 2 stage cooling takes
priority over dehumidification.
Sequence of Operation
Single Compressor Units
On a call for dehumidification, the reheat valve is
energized and the compressor is turned on. When the
humidity control setpoint is satisfied, the valve is de-
energized and the compressor is turned off. If there is a
call for cooling or heating from the space temperature
controller, i.e. zone sensor or thermostat, while in
reheat, the reheat valve is de-energized and the
compressor continues to run, or the heat is turned on.
The three-minute compressor on and off times are still
active during compressor operation.
Dual Compressor Units
On a call for dehumidification, the reheat valve is
energized and both compressors are turned on. When
the humidity control setpoint is satisfied, the valve is
de-energized and both compressors are turned off. If
there is a call for 1st stage cooling from the zone sensor
or thermostat while in the dehumidification mode, no
action takes place.
If there is a call for heating or 2nd stage cooling, the
reheat valve is de-energized, and the unit reverts to the
cooling or heating mode. If 2nd stage cooling is
satisfied and there is still a call for dehumidification,
the reheat valve will once again be energized, and the
second compressor will again be started. The three-
minute compressor on and off times are still active
during compressor operation.
If both compressors are running, and there is a call for
dehumidification, the dehumidification call is ignored
and unit stays in the cooling mode.
Refer to appropriate unit wiring diagrams for control
connections.
For reliability reasons, a purge time function was
added to ensure that adequate oil is being returned to
the compressor.
During normal cooling mode or during
dehumidification mode, if the unit stays in a mode for
90 continuous minutes without switching to the other
mode, a three minutes purge time will initiate.
If the unit has been in cooling mode for 90 minutes, the
purge sequence will switch to the dehumidification
mode for three minutes. If there is still a call for
cooling, the unit will once again switch back to cooling.
The same sequence is also true if unit is in the
dehumidification mode. It will switch to cooling for
three minutes and then back to dehumidification.
Figure 94. Relative humidity sensor locations
A unit with dehumidification installed will operate
normally if a humidity sensor or humidistat is not
installed. The compressor will switch to the
dehumidification mode for a total of three minutes
after 90 minutes of compressor run time in the cooling
mode.