Discharge Air Tempering; Heating Or Cooling Mode; Auto-Changeover Entering Water Temperature Sampling Function - Trane Tracer UC400 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
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Discharge Air Tempering

If the UC400 Controller is in cooling mode, cascade zone control initiates a discharge air
tempering function when:
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Heating or Cooling Mode

The heating or cooling mode can be determined by the following methods:
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A communicated heating signal permits the UC400 Controller to only heat and a communicated
cooling signal permits the unit to only cool. A communicated auto signal allows the controller to
automatically change from heating to cooling and conversely. In heating or cooling mode, the
controller maintains the zone temperature based on the active heating setpoint and the active
cooling setpoint, respectively. The active heating and cooling setpoints are determined by the
occupancy mode of the controller.
For 2-pipe and 4-pipe changeover units, normal heat/cool operation does not begin until the
ability to conduct the desired heating or cooling operation is verified. This is achieved using the
entering water temperature sampling function that requires a valid entering water temperature is
required. When neither a hard wired nor a communicated entering water temperature value is
present on changeover units, the controller operates in only heating mode and assumes the coil
water is hot. The sampling function is not used. The entering water temperature sampling
function is used only for changeover applications and for information and troubleshooting. It
does not affect the operation of the controller.

Auto-Changeover Entering Water Temperature Sampling Function

The auto-changeover entering water temperature sampling function is used with 2-pipe and 4-
pipe changeover units (including those with face and bypass dampers) and requires a valid
entering water temperature value. If the entering water temperature value is less than 5°F (2.8°C)
above a valid zone temperature value for hydronic heating, and greater than 5°F (2.8°C) below a
valid zone temperature value for hydronic cooling, the sampling function is enabled. When the
sampling function is enabled, the UC400 Controller opens the main hydronic valve to allow the
water temperature to stabilize. After three (3) minutes, the controller again compares the
entering water temperature value to the zone temperature value to determine if the desired
heating or cooling function can be accomplished. If the entering water temperature value
remains out of range to accomplish the desired heating/cooling function, the controller closes the
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S S i i m m p p l l i i f f i i e e d d Z Z o o n n e e C C o o n n t t r r o o l l : : if discharge air temperature failure occurs, then the UC400
Controller switches to simplified zone control algorithm, whereby, the space is controlled to
the unoccupied setpoints by means of the valve modulation and a cycling fan. This is similar
to the unoccupied operating mode. In the unoccupied mode, the controller maintains the
zone temperature by calculating the required heating or cooling capacity (0–100%) according
to the measured zone temperature and the active zone temperature setpoint.
D D i i s s c c h h a a r r g g e e A A i i r r T T e e m m p p e e r r a a t t u u r r e e C C o o n n t t r r o o l l : : is the backup mode that runs only if there is not valid
zone temperature. In this mode, the active space temperature setpoint is used as the
discharge air temperature setpoint.
N N o o t t e e : : This is not a normal operating mode. The source of the invalid zone temperature needs
to be corrected to restore normal operation.
The discharge air temperature falls below the discharge air temperature low limit setpoint
(configured using the Tracer TU service tool).
All cooling capacity is at a minimum. The discharge air tempering function allows the UC400
Controller to provide heating capacity (if available) to raise the discharge air temperature to
the discharge air temperature low limit setpoint.
The cold outdoor air is brought in through the outdoor air damper and when the damper is at
(HIGH) minimum position. This causes the discharge air temperature to fall below the
discharge air temperature low limit setpoint.
By a communicated signal from a BAS or a peer controller.
Automatically, as determined by the UC400 Controller.
BAS-SVX48H-EN