Installation and Maintenance Manual

Startup
UV-SVN02C-EN 73
The controller only cycles the fan off with heating and
cooling capacity in the unoccupied mode.
The economizer is used for cooling purposes whenever
the outdoor temperature is below the economizer enable
setpoint and there is a need for cooling. The economizer is
used first to meet the space demand, and other forms of
cooling are used if the economizer cannot meet the
demand alone. See modulating outdoor air damper
operation for additional information.
Cascade cooling control initiates a discharge air tempering
function if the discharge air temperature falls below the
discharge air temperature control low limit, all cooling
capacity is at minimum, and the discharge control loop
determines a need to raise the discharge air temperature.
The controller then provides heating capacity to raise the
discharge air temperature to its low limit.
Discharge Air Tempering (Tracer ZN520)
The discharge air tempering function enables when cold
outdoor air is brought in through the outdoor air damper,
causing the discharge air to fall below the discharge air
temperature control low limit. The controller exits the
discharge air tempering function when heat capacity has
been at 0 percent for five minutes.
Heating Operation (Tracer ZN520)
During heating mode, the Tracer ZN520 controller
attempts to maintain the space temperature at the active
heating setpoint. Based on the occupancy mode of the
controller, the active heating setpoint is one of the
following:
• Occupied heating
• Occupied standb
y heating
• Unoccupied heating
During dehumidification in the heating mode, the
controller adjusts
the heating setpoint up to the cooling
setpoint. This reduces the relative humidity in the space
with a minimum of energy usage.
The controller uses the measured space temperature, the
active heating setpoint, and discharge air temperature,
along with the control algorithm, to determine the
requested heating capacity of the unit (0 percent–
100 percent). The outputs are controlled based on the unit
configuration and the required heating capacity.
Unit diagnostics can affect the Tracer ZN520 controller
operation, causing unit operation to be defined as
abnormal. Refer to “Diagnostics,” p. 88 for more
information about abnormal unit operation.
The heating output is controlled based on the heating
capacity. At 0 percent capacity, the heating output is off
continuously. Between 0 percent and 100 percent
capacity, the heating output is controlled according to
modulating valve logic (modulating valves) or cycled on
(two-position valves). As the load increases, modulating
outputs open further and binary outputs are energized
longer. At 100 percent capacity, the heating valve is fully
open (modulating valves) or on continuously (two-
position valves).
The Tracer ZN520 fan output(s) normally run continuously
during the occupied and occupied standby modes, but
cycle between high and off speeds with heating/cooling
during the unoccupied mode. When in the occupied mode
or occupied standby mode and the fan speed is set at the
high, medium, or low position, the fan runs continuously
at the selected speed. Refer to “Diagnostics,” p. 88 for
more information on abnormal fan operation.
When the unit’s supply fan is set to auto, the controller’s
configuration determines the fan speed when in the
occupied mode or occupied standby mode. The fan runs
continuously at the configured heating fan speed or
cooling fan speed. For all fan speed selections except off,
the fan cycles off during unoccupied mode.
The economizer outdoor air damper is never used as a
source of heating. Instead, the economizer damper (when
present) is only used for ventilation; therefore, the damper
is at the occupied minimum position in the occupied
mode. The damper control is primarily associated with
occupied fan operation.
Fan Mode Operation (Tracer ZN520)
For multiple fan speed applications, the Tracer ZN520
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 (off, high, medium, or low). When the
fan mode switch is in the Auto position or a hard-wired fan
mode input does not exist, the fan operates at the default
configured speed. See Tab le 18 , p . 7 3 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.
During dehumidification, when the fan is on Auto, the fan
speed can switch depending on the error. Fan speed
increases as the space temperature rises above the active
cooling setpoint.
Additional flexibility built into the controller allows you to
enable or disable the local fan switch input. The fan mode
request can be either hard-wired or communicated to the
controller. When both are present, the communicated
request has priority over the hard-wired input. See
Ta b l e 1 8. Fan configuration (Tracer ZN520)
Auto Fan Operation Fan Speed Default
Heating Continuous Off
Low
Medium
High
Cooling Continuous Off
Low
Medium
High
UV-SVN02_-EN.book Page 73 Thursday, January 24, 2013 4:48 PM