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(as described below) until the discharge air target setpoint value is calculated below the discharge
air temperature Minimum Setpoint (limited discharge air target setpoint).
As the cooling load in the zone decreases the zone cooling algorithm will reduce the speed of the
fan down to minimum per compressor stage and control the compressors accordingly. As the
compressors begin to de-energize, the supply fan speed will fall back to the Cooling Stage's
associated minimum fan speed, but not below. As the load in the zone continues to drop, cooling
capacity will be reduced in order to maintain the discharge air within the ± ½ discharge air target
deadband.
Constant Volume (CV), Single Zone Variable Air Volume (SZVAV), &
Variable Air Volume (VAV) Units
Occupied Heating - Supply Air Temperature
When a Discharge Temperature Control unit is equipped with "Modulating Heat" , and the system
is in an occupied mode, and the field supplied changeover relay contacts (5K87) have closed, the
supply air temperature will be controlled to the customer specified supply air heating setpoint. It
will remain in the heating status until the changeover relay contacts are opened.
Occupied Cooling - Supply Air Temperature
When a Discharge Temperature Control unit is in the occupied mode, the supply air temperature
will be controlled to the customers specified supply air cooling setpoint by modulating the
economizer and/or staging the mechanical cooling "On and Off" as required. The changeover relay
contacts must be open on units with "Modulating Heat" for the cooling to operate.
Daytime Warm-up (CV & VAV Only)
On Discharge Temperature Control units equipped with heat, if the zone temperature falls below
the daytime warm-up initiate temperature during the occupied mode, the system will switch to full
airflow. During this mode, the VAV box/unocc relay, RTM K3, will be energized (this is to signal the
VAV boxes to go to 100%). After the VAV box max stroke time has elapsed (factory set at 6 minutes),
the VFD output will be set to 100%. The airflow will be at 100% and the heat will be turned on to
control to the occupied heating setpoint. When the zone temperature reaches the daytime warm-
up termination setpoint, the heat will be turned off, the K3 relay will be de-energized, releasing the
VAV boxes, the VFD output will go back to duct static pressure control and the unit will return to
discharge air control. If the occ zone heating setpoint is less than the DWU terminate setpoint, the
heat will turn off when the occ zone heat setpoint is reached, but it will stay in DWU mode and cycle
the heat to maintain setpoint.
Unoccupied Heating - Zone Temperature
When a Discharge Temperature Control unit is equipped with gas, electric, or hydronic heat and is
in the unoccupied mode, the zone temperature will be controlled to within the customers specified
setpoint deadband. During an unoccupied mode for a VAV unit, the VAV box/unocc relay will be in
the unoccupied position and the VFD output will be at 100%. This means that if there is a call for
heat (or cool) and the supply fan comes on, it will be at full airflow and the VAV boxes in the space
will need to be 100% open as signaled by the VAV box/unocc relay.
Supply Air Tempering
On Discharge Temperature Control units equipped with "Modulating Heat" , if the supply air
temperature falls 10°F below the supply air temperature setpoint, the hydronic heat valve will
modulate to maintain the supply air temperature to within the low end of the setpoint deadband.
RT-SVX36G-EN
General Information
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