Trane VUV Installation, Operation And Maintenance Manual page 77

Table 26. Local fan switch disabled or not present
Communicated Fan
Speed Input
Off
Low
Medium
High
Auto (or not present)
Continuous Fan Operation
During occupied and occupied standby modes, the fan
normally is on. For multiple speed fan applications, the fan
normally operates at the selected or default speed (off, high,
medium, or low). When fan mode is auto, the fan operates at
the default fan speed.
During unoccupied mode, the controller controls the fan off.
While unoccupied, the controller heats and cools to maintain
the unoccupied heating and cooling setpoints. In unoccupied
mode, the fan is controlled on high speed only with heating or
cooling.
The unit fan is always off during occupied, occupied standby,
and unoccupied modes when the unit is off due to a diagnostic
or when the unit is in the off mode due to the local zone sensor
module, a communicated request, or the default fan speed
(off).
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 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.
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
controller, 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
UV-SVX005G-EN
Fan Operation
Off
Low
Medium
High
Auto (fan runs at the default speed)
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 half of the valve stroke time. Once this temperature
stabilization period has expired, the controller compares the
entering water temperature against the effective space
temperature (either hard-wired 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.
Electric Heat Operation
Tracer® controllers support 1-stage electric heat. Also,
Tracer® ZN520 and UC400-B/Symbio™ 400-B support
2-stage electric heat. Tracer® ZN520 and UC400-B/Symbio™
400-B cycle the electric heat to control the discharge air
temperature. The rate of cycling is dependent upon the load in
the space and the temperature of the incoming fresh air from
the economizer (if any). Two-pipe changeover units with
electric heat use the electric heat only when hot water is not
available.
Economizer Damper
Tracer® ZN520 and UC400-B/Symbio™ 400-B
only
With a valid outdoor air temperature (either hard-wired or
communicated), Tracer® ZN520 and UC400-B/Symbio™
400-B use the modulating economizer damper as the highest
priority cooling source. Economizer operation is only possible
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