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56 UNT-SVX07A-EN
Operation
sequence of
operation
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 mea-
sured 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 consid-
ering the measured entering water
temperature. An initial stabilization period
is allowed to flush the coil. This period is
equal to 30 seconds plus ½ 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 com-
pare 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.
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% in 25% 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 Option
With a valid outdoor air temperature
(either hardwired or communicated),
Tracer ZN520 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%) to control the amount
of outdoor air cooling capacity. When the
outdoor air temperature rises 5°F above
the economizer enable point, the control-
ler disables economizing and moves the
outdoor air damper back to its predeter-
mined minimum position based on the
current occupancy mode or communi-
cated minimum damper position.
Dehumidification
Dehumidification is possible when
mechanical cooling is available, the
heating capacity is located in the reheat
position, and the space relative humidity
setpoint is valid.The controller starts
dehumidifying the space when the space
humidity exceeds the humidity setpoint.
The controller continues to dehumidify
until the sensed humidity falls below the
setpoint minus the relative humidity
offset.The controller uses the cooling and
reheat capacities simultaneously to
dehumidify the space. While dehumidify-
ing, the discharge air temperature is
controlled to maintain the space tem-
perature at the current setpoint.
A typical scenario involves high humidity
and high temperature load of the
space.The controller sets the cooling
capacity to 100% and uses the reheat
capacity to warm the discharge air to
maintain space temperature control.
Dehumidification may be disabled via
Tracer or configuration.