RT-SVD03L-EN
67
Dehumidification Operation
SZVAV units will support both traditional staged and
modulating dehumidification configurations. Most
functions will be identical to traditional
dehumidification control.
Modulating Dehumidification - Entering
Dehumidification
At startup a ZT value greater than the active ZCSP, or
below the active ZHSP, will prevent dehumidification
operation. At this point the unit will perform normal
sensible cooling or heating control until the respective
setpoint is satisfied.
After startup, the unit will monitor the Space Humidity
(SH), OA temperature, ZT, and unit status to determine
when to enter and leave dehumidification mode. As
long as the unit is not actively heating or cooling with
more than half the mechanical cooling capacity or the
unit is not disabled, it will enter dehumidification mode
when there is a call for dehumidification.
When dehumidification mode is entered the unit will:
•
Energize the supply fan, if not already ON, and
ramp the fan speed up to 80% airflow.
•
Stage up all compressors with approximately 2
seconds between stages.
•
Command the OA damper to minimum position.
•
The supply air reheat setpoint will be mapped to the
active maximum SZVAV heat setpoint which will be
the maximum value of DASP during active reheat.
•
Begin to control the reheat valve and cooling valve
to maintain the discharge air temperature to meet
the DASP.
Once the unit is actively dehumidifying, DASP will
continue to calculate and will be used as the Supply Air
Reheat setpoint that the cooling and reheat valves will
be modulated to meet. The user adjustable Supply Air
Reheat setpoint will be used as an UL for DASP during
active dehumidification and will be limited to the
normal range of 65-80°F.
Leaving Dehumidification
On a call to leave dehumidification mode the unit will
perform the following:
•
Mechanical cooling will stage back to 50% of the
available capacity then will be released to normal
SZVAV control to meet the load demand.
•
The economizer will be released to normal control.
•
The supply fan will ramp down to minimum speed.
•
The cooling valve will be driven to 100% and the
reheat valve will be driven to 0%.
•
The reheat pumpout relay will be energized if the
reheat circuit is requested or de-energized if the
reheat circuit de-energizes.
Staged (On/Off) Dehumidification w/HGRH
for Non-17 SEER SZVAV
Dehumidification Transitions (Zone Overrides)
At startup, a ZT value greater than the ZCSP, or below
the ZHSP, will prevent dehumidification operation. At
this point the unit will perform normal sensible cooling
or heating control until the respective setpoint is
satisfied. Once the zone is satisfied and there are no
other overrides in effect, the following transition rules
shall be followed:
If (HSP+1°F)
<
ZT < (CSP+1°F) and less than 50%
available mechanical cooling capacity is energized,
then dehumidification is enabled and the following
actions will be performed when transitioning into Cool
+Reheat:
•
Energize the supply fan, if not already ON, and
ramp the fan speed up to 80% airflow for VFD, BC
plenum, and non-17 SEER PWM controlled supply
fan units.
•
Stage up all compressors with approximately 2
seconds between stages
•
Energize the reheat solenoid
•
Command the OA damper to minimum position
If ZT
<
(HSP+0.5?F) or ZT
>
(CSP+1°F), or ZT < (CSP+1°
F) with more than 50% available mechanical cooling
capacity energized, then dehumidification is disabled
and the following actions will be performed when
transitioning out of Cool+Reheat:
•
If ZT
>
(CSP+1°F), stage back to Cool 1 in normal
SZVAV cooling
•
If ZT
<
(HSP+0.5°F), leave reheat, de-energize all
compressors and allow the unit to transition into
heat/cool mode based on ZT.
•
If ZT
>
(CSP-0.5°F) and SH < Dehumidification
Setpoint (DhSp) - 5%, stage back to Cool 1 and wait
3 minutes.
•
If ZT < (CSP- 0.5°F) and SH < DhSp - 5%, leave
reheat, de-energize all compressors and allow the
unit to transition into heat/cool mode based on ZT.
Staged Dehumidification w/ HGRH and
Enhanced Dehumidification for 17 SEER
SZVAV
Enhanced Dehumidification - 17 SEER SZVAV units will
perform Enhanced Dehumidification (EDh) in all cases
in which the unit is equipped with a SH sensor or a
valid SH value is being communicated from a BAS and
the active DhSp is less than the active SH value.
Dehumidification Transitions for 17 SEER Enhanced
Dehumidification - At startup, a ZT value greater than
the active ZCSP, or below the active ZHSP will prevent
dehumidification operation. At this point the unit will
perform normal sensible cooling or heating control
until the respective setpoint is satisfied. Once the zone
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