Trane UniTrane Installation, Operation And Maintenance Instructions page 94

Start-Up
If both a zone sensor module and communicated request exist, the communicated request has
priority.
Fan Cycling Operation
Tracer ZN520 does not support fan cycling in occupied mode. The fan cycles between high
speed and off in the unoccupied mode only. The controller's cascade control algorithm requires
continuous fan operation in the occupied mode.
Fan Off Delay
When a heating output is controlled off, the Tracer ZN520 controller automatically holds the fan on
for an additional 30 seconds. This 30-second delay gives the fan time to blow off any residual heat
from the heating source, such as a steam coil. When the unit is heating, the fan off delay is normally
applied to control the fan; otherwise, the fan off delay does not apply.
Fan Start on High Speed
On a transition from off to any other fan speed, the Tracer ZN520 controller automatically starts the
fan on high speed and runs the fan at high speed for 0.5 seconds. This provides the ample torque
required to start all fan motors from the off position.
Entering Water Temperature Sampling Function
Only units using the main hydronic coil for both heating and cooling (2-pipe changeover and 4-pipe
changeover units) use the entering water temperature sampling function. Two-pipe changeover
and 4-pipe changeover applications allow the main coil to be used for heating and for cooling;
therefore, these applications require an entering water temperature sensor.
When three-way valves are ordered with a Tracer ZN520 control, the controller is factory-
configured to disable the entering water temperature sampling function, and the entering water
sensor is mounted in the proper location. Disabling entering water temperature sampling
eliminates unnecessary water flow through the main coil when three-way valves are used.
The controller invokes entering water temperature sampling only when the measured entering
water temperature is too cool to heat or too warm to cool. Entering water is cold enough to cool
when it is five degrees below the measured space temperature. Entering water is warm enough to
heat when it is five degrees above the measured space temperature.
When the controller invokes the entering water temperature sampling function, the unit opens the
main hydronic valve for no more than three minutes before considering the measured entering
water temperature. An initial stabilization period is allowed to flush the coil. This period is equal
to 30 seconds plus 1/2 the valve stroke time. Once this temperature stabilization period has expired,
the controller compares the entering water temperature against the effective space temperature
(either hardwired or communicated) to determine whether the entering water can be used for the
desired heating or cooling. If the water temperature is not usable for the desired mode, the
controller continues to compare the entering water temperature against the effective space
temperature for a maximum of three minutes.
The controller automatically disables the entering water temperature sampling and closes the
main hydronic valve when the measured entering water exceeds the high entering water
temperature limit (110°F). When the entering water temperature is warmer than 110°F , the controller
assumes the entering water temperature is hot because it is unlikely the coil would drift to a high
temperature unless the actual loop temperature was very high.
If the entering water temperature is unusable — too cool to heat or too warm to cool — the
controller closes the hydronic valve and waits 60 minutes before initializing another sampling. If
the controller determines the entering water temperature is valid for heating or cooling, it resumes
normal heating/cooling control and effectively disables entering water temperature sampling until
it is required.
94 UNT-SVX07B-EN