Trane RT-PRC010-EN manual Controls, CV Units Only, Sequence Of Operation, Cooling/Economizer

Controls (CV Units)

CV Units Only

Sequence Of Operation

1

Occupied ZoneTemperature Control

Cooling/Economizer

During Occupied cooling mode, the economizer (if provided) and mechanical cooling are used to control zone temperature. If the enthalpy of outside air is appropriate to use “free cooling”, the economizer will be used first to attempt to satisfy the cooling zone temperature setpoint; then the compressors will be staged up as necessary. Minimum on/off timing of compressors prevents rapid cycling.

On units with economizer, a call for cooling will modulate the fresh air dampers open.The rate of economizer modulation is based on deviation of the zone temperature from setpoint, i.e., the further away from setpoint, the faster the fresh air damper will open. First stage of cooling will be allowed to start after the economizer reaches full open.

Note that the economizer is only allowed to function freely if one of the following conditions is met: For dry bulb economizer control, the ambient temperature must be below the dry bulb temperature control setting. For reference enthalpy economizer control, outdoor air enthalpy must be below the enthalpy control setting. For comparative enthalpy economizer control, outdoor air enthalpy must be below the enthalpy of the return air.

At outdoor air temperatures above the enthalpy control setting, mechanical cooling only is used and the outdoor air dampers remain at minimum position.

If the unit does not include an economizer, mechanical cooling only is used to satisfy cooling requirements. Outdoor air dampers may be set manually for a maximum of 25 percent outdoor air, if rooftop is equipped with 0 to 25 percent manual fresh air damper.

A

Heating

Gas Heating -Two-Stage

Upon a call for heating, the UCM closes the first stage heating contacts beginning the firing sequence. First, the heat exchanger combustion blower begins operation. Upon positive proving of combustion airflow, a pre-purge cycle is executed.Then the ignition sequence takes place.

If ignition is not proven, the ignition transformer is de-energized. After a time delay another pre-purge cycle takes place followed by another attempt to ignite. If ignition fails a second time, the cycle repeats on 235 and 350 MBh modules. 500, 850 and 1000 MBh modules, the heating section will be shut down and locked out until manually reset at the unit mounted Human Interface Panel.

As additional heat is required, the UCM will close the second stage heating contacts and depending on heat module size, will open either the second stage of the gas valve, or a second stage gas valve.

During heating operation, an electronic flame safety control provides continuous flame supervision. If combustion should become unstable for any reason, heating will automatically shut down. After one minute, another 60 second pre-purge and ignition cycle begins.

As the heating requirement is satisfied, the UCM will open the second stage heating relay, de-energizing the second stage of heat. When the requirement is fully satisfied, the first stage contacts are opened, de-energizing the first stage of heat.The specific sequence of operation of the gas heat will depend on the size of the heat exchanger.

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