SEQUENCE OF OPERATIONS



SEQUENCE OF OPERATION 3-50 TON RTU SYSTEM

MODULATING VALVE HOT GAS REHEAT SYSTEM

DISCHARGE AIR CONFIGURATION

A. DISCHARGE AIR OCCUPIED CONTROL:

Control of the Trane Hot Gas Reheat DX rooftop unit is accomplished through a dedicated standalone control module (MP581, UC400, UC600). One controller will be installed on each system. This controller utilizes four temperature sensors. The function of this controller is to maintain a programmed discharge air temperature.

A temperature sensor is mounted after the DX evaporator coil. This temperature sensor will be used by the controller to control the compressors to cycle on the compressor stages to maintain 53 F (adjustable) with PID logic.

A temperature sensor is field mounted in the duct after the hot gas reheat coil at least 15 feet down from the discharge of the unit. Once discharge temperature setpoint 74 (adjustable) is reached an analog signal is sent to the hot gas reheat and condenser modulating valves to modulate free condenser heat into the airstream to maintain the discharge temperature setpoint.

A suction line temperature will be located on the suction line of circuit #1. This temperature sensor will disable both cooling circuits when the temperature goes below 28 degrees (adjustable) and enable them above 38 (adjustable).

An outside air temperature sensor is mounted in the outside air intake section and monitors the outdoor air intake temperature. The controller is programmed to lock the unit cooling out if the outside air intake is 68 degrees (adjustable). The cooling will be reenabled after the outside air temperature rises above 70 degrees (adjustable).

The heat will be enabled when the outside air intake temperature is below 66 degrees (adjustable). When heating is called for, the controller will go into heating mode and the heat will be controlled as required to maintain the discharge air heating setpoint (adjustable).

The actual output signal for the heat control will operate based on the type of heat the unit has.

If the unit has electric or gas staged heat then the following will occur:

The 1st stage will come on when the discharge air temperature is below the heat setpoint (adjustable) and off above the heat setpoint plus a deadband (adjustable). The 2nd stage will be on below the heat setpoint (adjustable) minus 2 degrees and off above the heat setpoint minus 2 degrees plus a deadband (adjustable)

If the unit has modulating heat then the following will occur:

The modulating heat is controlled by an analog signal from the controller based on the heating setpoint. On a multiple stage heater the analog signal controls the first stage. Once the analog signal reaches 100% a binary output closes to energize the next stage of heat and the analog signal modulates the 1st stage beginning at 0%. Once the analog signal reaches 100% again a binary output closes to energize the next stage of heat at 100% and the analog signal modulates the 1st stage beginning at 0% again.

The controller is programmed to maintain discharge air setpoint (for heating and cooling). The controller has all of these functions pre-programmed. However, since it is fully programmable, the setpoints and sequence can be modified if required.

B. FAN PROVING DEVICE:

A contact closure into the controller will indicate to the controller proof of airflow from a differential pressure switch that will allow either cooling or heating to operate.

C. OCCUPIED/UNOCCUPIED CONTROL:

A contact closure into the controller will indicate an occupied mode of operation.,The controller will operate in the unoccupied mode while the contacts are open. The dry contact input will be wired from either a time clock or the building energy management system. The system can also be scheduled locally if equipped with a display, controlled by a Trane Building Automation system, or by a command from a LonTalk or BacNet network depending on controller type.

D. UNOCCUPIED MODE:

When the controller is indexed to the unoccupied mode, the system will be off.

E. SETPOINT SELECTION:

The setpoints for unit operation will be selected based on a priority level that will seek communications with various devices to determine the correct set of setpoints to be used. An MP581 will search 1st for a LonTalk device. If no such network server is found or if the communication to that device has been lost, then the MP581 will try to control to setpoints that are being sent from a Tracer Summit BCU (Building Control Unit). If a Tracer Summit system is not located or has lost its communication to the MP581, then the MP581 will default to its own “Local” setpoints. These setpoints are embedded in the software that is loaded at the factory and can only be accessed from Trane “ROVER” software/hardware or if the MP581 has a display connected to the controller. A UC400 will operate based on local setpoints unless setpoints are overridden by a BacNet control system.

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