Trane LO - Manual (Page 65)

UNT-IOM-6 65
Tracer ® ZN.520
The Tracer ® ZN.520 controller operates the supply fan continuously
when the controller is in the occupied 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 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 tempera-
ture 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.
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% for five minutes.
During heating mode, the Tracer ® ZN.520 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 standby 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 (0-100%). The outputs are controlled based on the unit configura-
tion and the required heating capacity.
Discharge Air Tempering
Heating Operation