CNT-SVX04A-EN
Sequence of operation
58
The controller uses the measured space temperature and discharge air
temperature, along with the control algorithm, to determine the requested
heating capacity of the unit (0–100%). The outputs are controlled based on the
unit configuration and the required heating capacity.
Diagnostics can affect the Tracer ZN.520
zone controller operation, causing unit
operation to be defined as abnormal. Refer
to Table 60 on page 92 for more
information about abnormal unit
operation.
The heating output is controlled based on the heating capacity. At 0% capacity,
the heating output is off continuously. Between 0 and 100% capacity, the
heating output is controlled according to modulating logic or cycled on
(2-position valves). As the load increases, modulating outputs open further and
binary outputs are energized longer. At 100% capacity, the heating valve is fully
open (modulating valves) or on continuously (2-position valves).
Face and bypass unit configurations can use 2-position, heating isolation valves
to allow water or steam flow through the coil. The Tracer ZN.520 zone controller
drives the face and bypass damper to modulate airflow across the hydronic
coil. By modulating the volume of air passing through the face of the coil, the
unit heating capacity is modulated. For more information about face and
bypass unit operation, see “Face and bypass isolation valve operation” on
page 64.
The Tracer ZN.520 zone controller fan output(s) normally run continuously
during the occupied, occupied bypass, and occupied standby modes, but cycle
between high and off speeds with heating/cooling during the unoccupied
mode. When in the occupied mode, occupied bypass, or occupied standby
mode and the unit supply fan is set to high, medium, or low (when present), the
fan runs continuously at the selected speed. Refer to Table 61 on page 96 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, occupied bypass, 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
during the heating mode; therefore, the damper is at the occupied minimum
position in the occupied mode. For more information about damper operation,
see “Modulating outdoor air damper operation” on page 66.
Coil changeover
Coil changeover refers to using the main hydronic coil for both heating and
cooling. Coil changeover is only used in a hydronic system designed to use the
main water coil (Valve/Coil 1) for both heating and cooling. If the coil is
configured for main coil changeover, the controller requires a valid entering
