Sequence of Operation
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RT-SVX34F-EN
stages will be energized/de-energized to meet the Zone
Heating demand. Note that all Electric and Gas Heat
staging sequences will be identical to as on Constant
Volume units.
Modulating Heat Operation with SZVAV
Heating
Units configured with Modulating Gas Heat will utilize true
Single Zone VAV control in the same manner as during
Active Cooling.
Heating Sequence.
Once the unit has met all Auto-
Changeover requirements and the control determines that
there is a space heating demand, the unit will transition
into zone heating. Once the Discharge Air Temperature
falls below the calculated discharge air temperature
setpoint, the unit will initiate the Modulating Heat output
request and control the supply fan at minimum speed. At
this point, the Modulating Heat output will be controlled to
maintain the discharge air temperature requirements and
the supply fan speed will be controlled between 58%-100%
to meet the zone heating requirements.
As the heating load in the zone decreases the fan speed
will decrease down to minimum (58%) and control the
modulating heat output as necessary to meet the
discharge air heating requirements. As the load in the zone
continues to drop the fan speed will be maintained at this
minimum airflow and the modulating heat output will be
controlled accordingly.
Note:
The gas heat staging sequences will be the same on
SZ VAV units as on traditional CV units.
Unoccupied Cooling and
Heating Operation
For SZ VAV units, the unit will control Heating, Cool, and
Dehumidification as during Occupied periods using the
normal heating and cooling Single Zone VAV algorithms.
In Unoccupied periods the unit will utilize setback
setpoints, a 0% Minimum OA Damper position, and Auto
fan mode operation as on normal Constant Volume units.
Dehumidification Operation
Singe Zone VAV units support modulating
dehumidification operation. Most functions will be
identical to dehumidification control on CV and Traditional
VAV units.
Modulating Dehumidification
Entering Dehumidification.
At startup a zone heating
or cooling demand will prevent dehumidification
operation as on a non-Single Zone VAV unit. 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 unit conditions to
determine when to enter and leave dehumidification
mode. As long as the unit is not actively heating or actively
cooling with more than half the unit design mechanical
cooling capacity and it has not been disabled due to the
Override limits described below, dehumidification mode
will be allowed.
When dehumidification mode is entered the unit will:
1.
Energize the Supply Fan, if not already ON, and ramp
the Fan Speed output up to 80% airflow.
2. Stage up all compressors with ~2 seconds between
stages.
3. Command the OA damper to minimum position.
4. The Supply Air Reheat setpoint (R130 located on the
RTOM) will become the maximum discharge air
control setpoint.
5. The reheat and cooling valves will be modulated to
meet the calculated discharge air setpoint.
Leaving Dehumidification.
On a call to leave
dehumidification mode the unit will perform the
following:
1.
Mechanical cooling will stage back to 50% (Cool 1) of
the available capacity then will be released to normal
Single Zone VAV control to meet the space demand.
2. The economizer will be released to normal control.
3. The Supply Fan output will be released to meet the
space load.
4. The cooling valve will be driven to 100% and the reheat
valve will be driven to 0%.
5. The Reheat Pumpout relay will be energized if the
reheat circuit is requested or de-energized if the reheat
circuit de-energizes.
Typical causes to leave dehumidification are:
1.
Space humidity levels have fallen below the Active
Occ/Unocc Dehumidification Setpoint -5%
Dehumidification Hysteresis Offset,
2. The zone temperature has dropped too close to the
Zone Heating Setpoint in any unit mode (Zone Temp.
≤
ZHSP + 0.5°F).
3. The zone temperature rises above the Zone Cooling
Se2°F in any unit mode.
4. Entering Evaporator Temperature falls too low or
Froststat input becomes active.
5. Dehumidification/Reheat becomes disabled.
Dehumidification Overrides.
Sensible cooling or
heating control overrides dehumidification control. Any
heating request will terminate dehumidification control. If
heating is active at the time a call for dehumidification
control is received the heating operation must complete
and an additional 5 minutes from the time heat is
terminated must elapse before dehumidification will be
allowed. Dehumidification will also be disabled if any of
the functional disables that apply to CV or traditional VAV
have gone active.
