Face-And-Bypass Damper Operation - Trane Tracer AH540 Setup And Operation

CNT-SVX05B-EN
Entering water temperature sampling is disabled when:
Unit configuration is dedicated heating and cooling or two-pipe
•
changeover without purge (three-way valve application).
Entering water temperature is communicated to the controller.
•
For cooling, entering water temperature is less than five degrees
•
colder than the space temperature or greater than 110°F.
For heating, entering water temperature is greater than 5º warmer
•
than the space temperature.

Face-and-bypass damper operation

This sequence applies to both space temperature control and discharge
air temperature control.
The face-and-bypass damper modulates a percentage of air to the face of
the heat coil and around the coil (bypass) to maintain the supply air tem-
perature setpoint. The air passing through the hot water coil is mixed
with the air bypassing the coil to produce a desired discharge air temper-
ature.
The Tracer AH540/541 controller supports face-and-bypass operation for
low outdoor temperature heating modes of operation only. During low out-
door temperatures, when the outdoor air temperature is lower than the
face-and-bypass heat modulation setpoint, the heating valve is fully
opened and the face-and-bypass damper is used for heating to prevent the
coil from freezing. During economizer cooling operation, when outdoor
temperature is less than the face and bypass heat modulation setpoint,
the face-and-bypass damper is full bypass.
The face-and-bypass heat modulation setpoint is the outdoor air tempera-
ture (40°F default, configurable) at which the controller changes over to
face and bypass heating operation. The face-and-bypass heat modulation
setpoint can be changed using the Rover service tool.
When the outdoor air temperature is greater than 3°F above the face-and-
bypass heating modulation setpoint, the hydronic heating valve is modu-
lated to maintain discharge air temperature. The face-and-bypass
damper is positioned for full face air flow. When the outdoor air tempera-
ture is less than the face and bypass heating modulation setpoint, the
controller fully opens the heating valve and uses the heat face-and-bypass
damper to modulate heating capacity to maintain the desired discharge
air temperature.
During diagnostic and fan Off conditions, when the controller shuts down
unit operation, the face-and-bypass damper is in the full bypass position.
During freeze avoidance operation or a Low Temp Detect diagnostic, the
face-and-bypass damper is driven to full face.
Face-and-bypass damper operation
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