RT-SVD03G-EN 125
Dehumidification with Hot Gas Reheat (3-25 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.
Note: 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 Events
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.
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