CNT-SVX04A-EN
Sequence of operation
60
The Tracer ZN.520 zone controller offers a
control solution for two-way valve
applications that does not require special
unit consideration, such as those required
by bleed lines. The controller includes an
entering water temperature sampling
function that periodically opens the two-
way valve to allow temporary waterflow,
producing reliable entering water
temperature measurement.
When the controller invokes the entering water temperature sampling function,
the unit opens the main hydronic valve for no more than three minutes while
measuring entering water temperature. An initial stabilization period is allowed
to flush the coil, a period of time 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
enetering 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.
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 not usable for the required capacity
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.
Fan operation
For multiple fan speed applications, the Tracer ZN.520 zone controller offers
additional fan configuration flexibility. Separate default fan speeds for heating
and cooling modes can be configured. The fan runs continuously for requested
speeds (high, medium, or low). When the fan mode switch is in the Auto
position or a hardwired fan mode input does not exist, the fan operates at the
default configured speed. If the fan mode is auto and the default fan speed is
auto, then the fan may switch speeds, depending on the error. If the cool default
is auto, as capacity needed, the fan speed increases as space temperature
increases above the active cooling setpoint. If the heat default is auto, need for
heating capacity needed, the fan speed increases as space temperature
changes below the active heating setpoint.
See Table 33 on page 61 for default fan configuration for heat and cool mode.
During unoccupied mode, the fan cycles between high speed and off with
heating and cooling fan modes. If the requested speed is off, the fan always
remains off.
