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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.
Fan Start on High Speed
On a transition from off to any other fan speed, the
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.
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 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
UNT-SVX07E-EN
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
The Tracer ZN520 controller supports one or two-stage
electric heat operation for heating. To control the space
temperature, electric heat is cycled 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.
Damper Options
Manual Fresh Air Damper
Units with the manual fresh air damper option ship with
the damper in the closed position, which is adjustable
from zero to 100 percent in 25 percent increments. To
adjust the position, first remove the air filter to expose the
damper stop screw on the control panel end. Relocate the
stop screw to the appropriate position. Then loosen the
stop screw wingnut and adjust the linkage.
Economizer Damper
With a valid outdoor air temperature (either hard-wired or
communicated), the controller uses the modulating
economizer damper as the highest priority source of
cooling. Economizer operation is only possible through
the use of a modulating damper.
Economizing is possible during the occupied, occupied
standby, unoccupied, and occupied bypass modes.
The controller initiates the economizer function if the
outdoor air temperature is cold enough to be used as free
cooling capacity. If the outdoor air temperature is less than
the economizer enable setpoint (absolute dry bulb), the
controller modulates the outdoor air damper (between the
active minimum damper position and 100 percent) to
control the amount of outdoor air cooling capacity. When
the outdoor air temperature rises 5°F above the
economizer enable point, the controller disables
Startup
109
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Troubleshooting
