Modulating Hot Gas Reheat; Operation; Conditions - Trane Voyager Commercial Installation, Operation And Maintenance Manual

S S e e q q u u e e n n c c e e o o f f O O p p e e r r a a t t i i o o n n
Modulating Hot Gas Reheat Operation
Singe Zone VAV units support modulating hot gas
reheat operation. Most functions will be identical to hot
gas reheat control on CV and Traditional VAV units.
Entering Hot Gas Reheat
At startup a zone heating or cooling demand will
prevent hot gas reheat 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 hot gas reheat
mode. As long as the unit is not actively heating or
actively cooling with more than half the unit design
mechanical cooling capacity for Standard Efficient
units and have a call for stage 3 or below for High
Efficient units (5 stage units), hot gas reheat mode will
be allowed (also the unit has not been disabled due to
the override limits described below).
When hot gas reheat mode is entered the unit will:
• Energize the Supply Fan, if not already ON, and
ramp the Fan Speed output up to 80% airflow.
• Stage up all compressors with ~2 seconds between
stages.
• Command the OA damper to minimum position.
• The Supply Air Reheat setpoint (R130 located on
the RTOM) will become the maximum discharge air
control setpoint.
• The reheat and cooling valves will be modulated to
meet the calculated discharge air setpoint.
Leaving Hot Gas Reheat
On a call to leave hot gas reheat mode the unit will
perform the following:
• 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.
• The economizer will be released to normal control.
• The Supply Fan output will be released to meet the
space load.
• 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.
Typical causes to leave hot gas reheat are:
• 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 (Zone
Temp. ? ZHSP + 0.5°F).
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• The zone temperature rises above the Zone Cooling
Setpoint +2°F in any unit mode.
• Entering Evaporator Temperature falls too low or
Froststat input becomes active.
• Dehumidification/Reheat becomes disabled.
Hot Gas Reheat Overrides
Sensible cooling or heating control overrides hot gas
reheat control. Any heating request will terminate hot
gas reheat control. If heating is active at the time a call
for hot gas reheat control is received the heating
operation must complete and an additional 5 minutes
from the time heat is terminated must elapse before
hot gas reheat will be allowed. Hot gas reheat 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 Hot Gas Reheat units; if the
Reheat Circuit operates in one mode (dehumidification
or cooling) for a cumulative 60 minutes the unit will
initiate a 3-minute Purge cycle with all compressors
energized, the Cooling and Reheat Valves at 50%, and
the Reheat Pumpout relay de-energized.
During an active Purge Cycle the Supply Fan Speed will
operate at the appropriate speed based on the active
compressor step. If a dehumidification purge is
initiated, the unit will run at 80%, 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.
Hot Gas Reheat - Humidistat Operation
A humidistat input located on the Options module will
be supported as on non-SZ VAV.
Other Hot Gas Reheat Related Topics
The following aspects of Single Zone VAV units
configured with Modulating hot gas reheat will operate
identically to non-Single Zone VAV units:
• Outdoor Fan Control.
• Low Pressure/High Pressure Cutout input handling.
• Function Enable/Disable Details.
Failure and Overriding Conditions
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
failure is detected the Supply Fan will be de-
energized after 40s of run time and the Fan Speed
output will go to 0 Vdc (0%).
RT-SVX34R-EN