sequence of
Operation operation
Tracer ZN520 Sequence of Operation
The Tracer ZN520 operates the fan in the following modes:
1)occupied
2)unoccupied
3)occupied standby
4)occupied bypass
5)Tracer Summit with supply fan control
Occupied
When the controller is in the occupied mode, the unit attempts to maintain the space temperature at the active occupied heating or cooling setpoint, based on the measured space temperature, the discharge air temperature, the active setpoint, and the proportional/integral control algorithm. The modulating control algorithm used when occupied or in occupied standby is described in the following sections. Additional information related to the handling of the controller setpoints can be found in the previous Setpoint operation section.
Unoccupied Mode
When the controller is in the unoccupied mode, the controller attempts to maintain the space temperature at the stored unoccupied heating or cooling setpoint, based on the measured space temperature, the active setpoint and the control algorithm, regardless of the presence of a hardwired or communicated setpoint. Similar to other configuration properties of the controller, the locally stored unoccupied setpoints can be modified using Rover™ service tool.
In unoccupied mode, a simplified zone control algorithm is run. During the cooling mode, when the space tempera- ture is above the cool setpoint, the primary cooling capacity operates at 100%. If more capacity is needed, the supplementary cooling capacity turns on (or opens to 100%). During the heating mode, when the space temperature is below the heat setpoint, the primary heating capacity turns on. All capacity is turned off when the space temperature is between the unoccupied cooling and heating setpoints. Note that primary
heating or cooling capacity is defined by unit type and whether heating or cooling is enabled or disabled. For example, if the economizer is enabled and possible, it will be the primary cooling capacity. If hydronic heating is possible, it will be the primary heating capacity.
Occupied Standby Mode
The controller can be placed into the occupied standby mode when a communicated occupancy request is combined with the local (hardwired) occupancy binary input signal. When the communicated occupancy request is unoccupied, the occupancy binary input (if present) does not affect the controller’s occupancy. When the communicated occupancy request is occupied, the controller uses the local occupancy binary input to switch between the occupied and occupied standby modes.
During occupied standby mode, the controller’s economizer damper position goes to the economizer standby mini- mum position. The economizer standby minimum position can be changed using Rover service tool.
In the occupied standby mode, the controller uses the occupied standby cooling and heating setpoints. Because the occupied standby setpoints typically cover a wider range than the occupied setpoints, the TracerZN520 controller reduces the demand for heating and cooling the space. Also, the outdoor air economizer damper uses the econo- mizer standby minimum position to reduce the heating and cooling demands.
When no occupancy request is commu- nicated, the occupancy binary input switches the controller’s operating mode between occupied and unoccupied. When no communicated occupancy request exists, the unit cannot switch to occupied standby mode.
Occupied Bypass Mode
The controller can be placed in occupied bypass mode by either communicating an occupancy request of Bypass to the controller or by using the timed override On button on the Trane zone sensor.
When the controller is in unoccupied mode, you can press the On button on the zone sensor to place the controller into occupied bypass mode for the duration of the bypass time (typically 120 minutes).
Occupancy Sources
There are four ways to control the controller’s occupancy:
•Communicated request (usually provided by the building automation system or peer device)
•By pressing the zone sensor’s timed override On button
•Occupancy binary input
•Default operation of the controller (occupied mode)
A communicated request from a building automation system or another peer controller can change the controller’s occupancy. However, if communication is lost, the controller reverts to the default operating mode (occupied) after 15 minutes (configurable, specified by the “receive heartbeat time”), if no local hardwired occupancy signal exists.
A communicated request can be pro- vided to control the occupancy of the controller. Typically, the occupancy of the controller is determined by using
Tracer Summit with Supply Fan Control If the unit is communicating with Tracer Summit and the supply fan control programming point is configured for Tracer (the factory configures as local), Tracer Summit will control the fan regardless of the fan mode switch position.
When the fan mode switch is set to Off or when power is restored to the unit, all Tracer ZN520 lockouts (latching diagnos- tics) are manually reset. The last diagnos- tic to occur is retained until the unit power is disconnected. Refer to Trane publica- tion,
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