Packaged Rooftop Air Conditioners - RT-DS-9 - Page 929

Control Sequences of Operation Common to Both VAV and CV Units

Ventilation override (VOM)

Ventilation override allows an external system to assume control of the unit for the purpose of exhaust or pressurization. There are two inputs associated with ventilation override, the initiate input and the select input. When the UCP senses a continuous closed condition on the initiate input at the low voltage terminal board the unit will begin ventilation override depending on the condition of the select input. The default condition of the select input is exhaust (input open). A closed select input will yield pressurization. The component state matrix for ventilation override is as follows:

Coil Freeze Protection FROSTAT™

The FROSTAT system eliminates the need for hot gas bypass and adds a suction line surface temperature sensor to determine if the coil is in a condition of impending frost. If impending frost is detected primary cooling capacity is shed as necessary to prevent icing. All compressors are turned off after they have met their minimum 3 minute on times. The supply fan is forced on until the FROSTAT device no longer senses a frosting condition or for 60 seconds after the last compressor is shut off, whichever is longer.

Occupied/Unoccupied Switching

There are 3 ways to switch Occupied/Unoccupied:

1

NSB Panel

2

Electronic time clock or field-supplied contact closure

3

TRACER

Night Setback Sensors

Trane’s night setback sensors are programmable with a time clock function that provides communication to the rooftop unit through a 2-wire communications link. The desired transition times are programmed at the night setback sensor and communicated to the unit.

Night setback (unoccupied mode) is operated through the time clock provided in the sensors with night setback. When the time clock switches to night setback operation, the outdoor air dampers close and heating/cooling can be enabled or disabled. As the building load changes, the night setback sensor communicates the need for the rooftop heating/cooling (if enabled) function and the evaporator fan. The rooftop unit will cycle through the evening as heating/cooling (if enabled) is required in the space. When the time clock switches from night setback to occupied mode, all heating/cooling functions begin normal operation.

When using the night setback options with a VAV heating/cooling rooftop, airflow must be maintained through the rooftop unit. This can be accomplished  by electrically tying the VAV boxes to the VAV heat relay contacts on the Low voltage terminal board or by using changeover  thermostats. Either of these methods will assure adequate airflow through the unit and satisfactory temperature control of the building.

Timed override Activation—ICS

When this function is initiated by pushing the override button on the ICS sensor, TRACER will switch  the unit to the occupied mode. Unit operation (occupied mode) during timed override is terminated by a signal from TRACER.

Timed override Activation—Non-lCS

When this function is initiated by the push of an override button on the programmable zone sensor, the unit will switch to the occupied mode. Automatic Cancellation of the Timed override Mode occurs after three hours of operation.

Comparative Enthalpy Control of Economizer

The Unitary Economizer Module (UEM) receives inputs from optional return air humidity and temperature sensors and determines whether or not it is feasible to economize. If the outdoor air enthalpy is greater than the return air enthalpy then it is not feasible to economize and the economizer damper will not open past its minimum position.

Fan Failure Switch

The fan failure switch will disable all unit functions and "flash" the Service LED on the zone sensor.

Emergency Stop Input

A binary input is provided on the UCP for installation of field provided switch or contacts for immediate shutdown of all unit functions. The binary input is brought out to Low Voltage Terminal Board One (LTB1).