Trane Tracer ZN.520 Manual page 67

CNT-SVX04A-EN
Sequence of operation
1 If the controlled space requires heating or cooling, the controller changes
from heating to cooling and cooling to heating based on the space
temperature, active setpoint, and the integrated error between the two.
Once the controller changes modes, it verifies the entering water
temperature.
2 When the entering water temperature is not appropriate for the desired
capacity (either heating or cooling), the controller remains at 0% capacity for
hydronic capacity.
3 When the entering water temperature is appropriate to deliver the desired
heating or cooling, the controller resumes normal operation. If the controller
does not have a valid entering water temperature, the controller assumes
hot water is always present in the main coil.
4 If the entering water temperature is appropriate for heating or cooling, the
controller energizes the appropriate output to control the space temperature
at the heating setpoint or dehumidifying setpoint in heat mode, or at the
cooling setpoint in cool mode.
Table 37 2-pipe main coil operation
Unit Entering water
temperature
2-pipe heating only NA
2-pipe cooling only NA
2-pipe changeover Cold
Hot
For more information about 2-pipe operation, see "Coil changeover" on
page 58.
Four-pipe operation
Four-pipe units normally assume there is chilled water in the primary coil and
hot water in the secondary coil. The controller energizes the cooling valve
output (valve 1) to control the space temperature at the cooling setpoint in cool
mode. The controller energizes the heating valve output (valve 2) to control the
space temperature at the heating setpoint in heat mode. During normal unit
operation, the controller never opens both the heating valve and the cooling
valve at once.
The controller also supports 4-pipe main coil changeover—using the main
hydronic coil for both heating and cooling. For all 4-pipe units, the auxiliary coil
is always assumed hot by the controller. During normal unit operation, the
controller never uses the main coil and auxiliary coil simultaneously.
Main coil Operation
Hot Main coil heating
Cold Main coil cooling
Cold Main coil cooling
Hot Main coil heating
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