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Unit Startup
control devices as well as a split condenser coil with one
placed section in the indoor air stream and the other in the
outdoor coil compartment.
During active dehumidification the discharge unit
discharge air will be controlled to the Supply Air Reheat
Setpoint by modulating the amount of reheat produced by
the reheat coil.The Supply Air Reheat Setpoint, Occupied
and Unoccupied Dehumidification Setpoints are
adjustable via the human interface, BAS/Network control,
or GBAS.
Active dehumidification will be terminated when the
humidity in the space is reduced to the active space
humidity setpoint - 5% or when an overriding condition
such as heating or cooling demand or a failure occurs in a
component required for dehumidification.
On VAV units, at startup, satisfying the VAV Occupied
Cooling setpoint, MWU setpoint, and DWU setpoint will
have priority over dehumidification mode. Once heating
modes are satisfied, and the unit is satisfying the SA
Cooling Setpoint, dehumidification mode will be entered if
no more than half the unit mechanical cooling capacity is
requested.
On SZVAV units, dehumidification will be similar to VAV
modulating dehumidification with the exception of a
dynamic Supply Air Reheat Setpoint. Rather than utilizing
a static Supply Air Reheat Setpoint, once the unit enters
dehumidification, the Discharge Air Setpoint will be
calculated based on the Zone temperature vs. Zone
Cooling Setpoint error and will be capped at the user
selected Supply Air Reheat setpoint.
Dehumidification is not allowed during VAV Heating
Modes (Changeover input closed). Once active,
dehumidification control will remain active for a minimum
of three minutes unless a priority unit shutdown request is
received or the High Pressure Control input opens on
either circuit.
On VAV units dehumidification is best performed in full
airflow mode therefore the "VAV Box Open" relay will be
energized during active dehumidification control and the
supply air pressure control algorithm will indirectly open
the VFD output within a few minutes after entering
dehumidification.
Dehumidification control can be enabled separately for
occupied and unoccupied modes of operation via the
Human Interface and is overridden/disabled whether
active or inactive by the following methods:
– Priority unit shutdown conditions (Emergency stop,
Ventilation Override, Network Stop, etc.)
– Compressor circuit manual reset lockouts on either
circuit. Low Refrigerant Charge monitoring is active
during dehumidification mode and will lockout
compressor circuits based on the same criteria
used for cooling mode.
– Outdoor AirTemp is less then 40°F or greater the
100°F .
102
– Humidity Sensor Failure
– For VAV units, (in occupied) dehumidification will
be disabled if space temp is less than the Dehumid
Override Low Zone Setpoint or higher than the
Dehumid Override High Zone Setpoint. If
dehumidification is inactive it will not be allowed
until it space temp rises higher than the Dehumid
Override Low Zone Setpoint + 1.0°F or lower than
the Dehumid Override High Zone Setpoint - 2.0°F .
– For SZVAV units, dehumidification will be disabled
if space humidity levels have fallen below the
Active Occ/Unocc Dehumidification Setpoint -5%
Dehumidification Hysteresis Offset, the zone
temperature has dropped too close to the Zone
Heating Setpoint in any unit mode (ZoneTemp. is
less than ZHSP + 0.5°F), the zone temperature rises
above the Zone Cooling Setpoint +2°F in any unit
mode, Entering EvaporatorTemperature falls too
low, Froststat input becomes active, or
Dehumidification/Reheat becomes disabled.
– For CV and all units in unoccupied, if space temp is
less than the Zone Heating Setpoint (ZHSP) + 0.5° F
if dehumidification is active, or less than ZHSP + 1.0°
F if not dehumidification mode will be disabled. If
zone conditions result in a cooling request for more
than one-half the available cooling capacity of the
unit dehumidification will be disabled and will
transition to cooling control. If dehumidification is
inactive, dehumidification will not be allowed until
the active unit cooling capacity request drops to
half the available cooling capacity or less, unless
the space temp is less than the Zone Cooling
Setpoint.
– In CV units in occupied mode, if the unit is not in
"AUTO" system mode and is set to "HEAT" system
mode via the HI, BAS, or Zone Sensor device,
dehumidification control will be disabled at space
temps above Occupied ZCSP + 1.0° F . If
dehumidification is inactive it will not be allowed to
activate if space temp is greater than the OZCSP .
All units configured for modulating dehumidification will
have a reheat condenser coil purge function to ensure
proper refrigerant distribution in the reheat circuit.This
feature is always enabled and will monitor the amount of
cumulative compressor run time while the reheat
condenser coil pumpout relay is in a certain state. If
compressors accrue an amount of run time equal to the HI-
adjustable purge interval time without the pumpout relay
changing states a purge cycle will be initiated lasting for
three minutes.
During this cycle all compressors but the 2nd compressor
on circuit #1 will be energized if not already, the reheat
valve and cooling valves to will be set to 50%, and the
reheat coil pumpout relay will be toggled to its opposite
state. After the three-minute purge cycle completes the
purge interval timer will be reset and all system
RT-SVX24J-EN
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