CNT-SVX04A-EN
Sequence of operation
56
Refer to
DX operation
on page 70 and
Electric heat operation
on page 70 for a
more detailed description of operation.
Cooling operation
The heating and cooling setpoint high and
low limits are always applied to the
occupied and occupied standby setpoints.
During the cooling mode, the Tracer ZN.520 zone controller attempts to
maintain the space temperature at the active cooling setpoint. Based on the
controller’s occupancy mode, the active cooling setpoint is one of the following:
■
Occupied cooling setpoint
■
Occupied standby cooling setpoint
■
Unoccupied cooling setpoint
The controller uses the measured space temperature, the active cooling
setpoint, and discharge air temperature along with the control algorithm to
determine the requested cooling capacity of the unit (0–100%). The outputs are
controlled based on the unit configuration and the required cooling capacity. To
maintain space temperature control, the Tracer ZN.520 zone controller cooling
outputs (modulating hydronic valve, 2-position hydronic valve, face/bypass
damper, DX, or outdoor air economizer damper) are controlled based on the
cooling capacity output.
The cooling output is controlled based on the cooling capacity. At 0% capacity,
all cooling capacities are off and the damper is at minimum position. Between 0
and 100% capacity, the cooling outputs are controlled according to modulating
valve logic (modulating valves) or cycled on (DX and 2-position valves). As the
load increases, modulating outputs open further and binary outputs are
energized longer. At 100% capacity, the cooling valve or damper is fully open
(modulating valves) or on continuously (DX and 2-position valves).
Face and bypass unit configurations use 2-position, cooling isolation valves to
allow water to flow through the coil. The Tracer ZN.520 zone controller drives
the face and bypass damper to modulate air flow across the water or steam
coil. By modulating the volume of air passing through the face to the coil, the
unit cooling capacity is modulated. For more information about face and
bypass unit operation, see “Face and bypass damper operation” on page 69.
Diagnostics can affect fan operation,
causing occupied and occupied standby
fan operation to be defined as abnormal.
Refer to Table 61 on page 96 for more
information about abnormal fan operation.
The Tracer ZN.520 zone controller operates the supply fan continuously when
the controller is in the occupied, occupied bypass, and occupied standby
modes, for either heating or cooling. 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
