Johnson Controls
5586996-JSG-A-0120 | Start-Up & Operation Guide
50
Low Cooling DAT
Setpoint – 3.0°F
Evaporator
Temperature
Setpoint
Low
Dehumidification
Evaporator Setpoint
Effective Dehumidification
Setpoint – 2%RH
Effective Dehumidification
Se 5%RH
Effective Space Humidity
LD28012
Figure 21: Active EAT Setpoint Based on Humidity
Level
The unit controller uses the current humidity level as
the basis for the active EAT setpoint.
•
When the humidity level first increases above the
active humidity setpoint, the active EAT setpoint is
equal to the low cooling DAT setpoint
• When the humidity level continues to increase,
the active EAT setpoint resets lower to allow for
moisture removal
If at any time during an active dehumidification se-
quence the changeover sensor sees a demand for
heat, or the current OAT drops to 54.0°F or below,
HGRH operation is terminated regardless of the cur-
rent humidity level.
Hot Gas Reheat Valve
The HGRH bleed valve is a normally open valve that
is piped between the HGRH coil and the compressor
system 2 suction line. The HGRH bleed valve prevents
refrigerant and oil from being trapped in the HGRH
coils when HGRH is inactive.
The HGRH valve is open when there is no demand for
HGRH. Once a demand for HGRH exists, the HGRH
bleed valve closes.
After the demand for HGRH is satisfied, the unit con-
troller starts a 5 minute timer. When the 5 minute timer
expires, the HGRH bleed valve opens.
Return Air
Temperature
DAT Setpoint
(Satisfied Mode
Dehumidification)
Effective Cooling
DAT Setpoint
Effective Cooling
Changeover Setpoint: -1.5°F
Effective Cooling
Changeover Setpoint: -0.5°F
Return Air Temperature
LD28011
Figure 20: Active DAT Setpoint when Unit Is In Idle
Mode
Dehumidification Sequence
The unit controller continuously monitors a %RH value.
This value can be either from the space or return air.
When the current humidity level rises above the dehu-
midification setpoint, the unit enables the HGRH system.
Setpoints for the dehumidification sequence can be found
under the
Details > Humidity Control > Setup
menu.
There are two different operating sequences for HGRH.
Unit in Active Cooling Mode
When the unit is in an active cooling mode and there
is a demand for dehumidification, the HGRH valve
is controlled to the active DAT cooling setpoint.
The compressors are controlled to the active
evaporator air temperature (EAT) setpoint.
Typically the active EAT setpoint is lower than the
active DAT cooling setpoint. This allows for addi-
tional humidity to be removed while still providing
sensible cooling.
Unit in Idle Mode
When the unit is not in an active cooling mode and
there is a demand for dehumidification, the HGRH
valve is controlled to an active DAT setpoint based
on a reset schedule utilizing the effective cooling
DAT setpoint and the current RAT. See
The compressors are controlled to the active EAT
setpoint. This allows for humidity to be removed
from the air while not over-cooling the space.
