Sequence of Operation
RT-SVX25Q-EN 47
Heating on units configured with Staged Heat types will be
identical to traditional Constant Volume units; heating
stages will be energized/de-energized to meet the Zone
Heating demand.
Note: All
heat staging sequences will be identical to
Constan
t Volume unit staging sequences.
Heating Sequence
Once the unit 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 63%-100% to meet
the zone heating requirements.
As the heating load in the zone
decreases the fan speed
wi
ll decrease down to minimum (63%) 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.
Low Ambient Operation
The indoor fan motor (IDM) will operate at 100% fan speed
during this evaporator defrost cycle (EDC) and the
compressor and outdoor fan will return to normal
operation once the defrost cycle has terminated and the
compressor “Off” time delay has been satisfied.
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 moni
tor the un
it 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.
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.
Purge Mode (Comfort and Dehumidification)
Purge cycle operation will operate identically to Purge on
non-Single Zone VAV Dehumidification units; if the Reheat
Circuit operates in one mode (dehumidification or cooling)
for a cumulative 90 minutes the unit will initiate a 3-minute
Purge cycle with all compressors energized.
During an active Purge Cycle the Supply Fan Spee
d will
operate at the appropriate speed based on the active
compressor step. If a dehumidification purge is initiated,
the unit will run at 82%, if performing a cooling purge the
supply fan will track based on the appropriate minimum
speed for the associated number of compressors
energized. After the Purge Cycle is complete, the Supply
Fan will be released to normal control based on the
Cooling/Dehumidification demand.
Dehumidification - Humidistat Operation
A humidistat input located on the options module will be
supported as on non-SZ VAV.
Failure and Override Modes
Certain failure and overriding conditions require special
handling of the Supply Fan Speed on units configured with
Single Zone VAV. See below for a list of these conditions:
• Supply Fan Proving Failure - If a Supply Fan Proving
failu
re is detected, the Sup
ply Fan will be de-energized
after 40s of run time and the Fan Speed output will go
to 0 Vdc (0%).
• Ventilation Override Mode -
If a VOM goes active in
which
the Supply Fan is commanded ON (Purge,
Pressurize, etc.), the Supply Fan will be energized and
the Fan Speed output will ramp to 100%.
• Zone Temperature Sensor Failure - If the Active Zone
Temp
eratur
e input goes out of range, the unit will
discontinue all Heating, Cooling, and Dehumidification
operation.
• Supply Air Temperature Sensor Failure - If the Supply
Ai
r Tem
perature input goes out of range, the unit will
revert back to Full Airflow, Traditional CV control. The
unit will call out a Supply Air Temperature Sensor
Failure Alarm, the RTRM System LED will flash the 2-
blink error code, and the Zone Sensor Heat
(Modulating Heat Only) and Cool LEDs will flash.
• Frostat Failure - If a Frostat failure occurs, all active
Heatin
g, Cooling, and Dehumidification will be de-
energized immediately and the Supply Fan will ramp
up to 100%.
• Heat Failure (High Temp. Limit Trip) - If a unit
con
figured w
ith Gas Heat has a High Temp. Limit trip,
the Supply Fan will be requested to remain ON and the
Fan Speed output will ramp to full speed.
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