Trane RT-PRC010-EN manual Controls, VAV Units, Heating: HotWater or Steam, Heating: Modulating Gas

Controls (VAV Units)

Heating: HotWater or Steam

On units with hot water or steam heating, the supply air temperature can be controlled to a heating set point during the Occupied mode.The supply air temperature heating set point and deadband are user-defined at the Human Interface Panel.VAV Occupied heating on hot water and steam heat units is enabled by closing a field-supplied switch or contacts connected to an changeover input on the RTM.

Heating: Modulating Gas

Upon a call for heating, the UCM closes the 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 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 air damper opens, increasing the firing rate.

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 reduce the combustion air and the firing rate will lower to maintain the desired outlet temperature. When the requirement is fully satisfied, the heating contacts are opened, de- energizing the heat.The specific sequence of operation of the gas heat will depend on the size of the heat exchanger.

Supply Air Setpoint Reset

Supply air reset can be used to adjust the supply air temperature setpoint on the basis of a zone temperature or on outdoor air temperature. Supply air reset adjustment is available from the Human Interface Panel for supply air heating and supply air cooling control.

A

reset based on outdoor air temperature

Outdoor air cooling reset is sometimes used in applications where the outdoor temperature has a large effect on building load. When the outside air temperature is low and the building cooling load is low, the supply air setpoint can be raised, thereby preventing subcooling of critical zones. This reset can lower usage of mechanical cooling, thus savings in compressor KW, but a increase in supply fan KW may occur.

Outdoor air heating reset is the inverse of cooling, with the same principles applied.

For both outdoor air cooling reset and heating reset, there are three user defined parameters that are adjustable through the Human Interface Panel.

-beginning reset temperature

-ending reset temperature

-maximum amount of temperature reset

B

reset based on zone temperature

Zone reset is applied to the zone(s) in a building that tend to overcool or overheat.The supply air temperature setpoint is adjusted based on the temperature of the critical zone(s).This can have the effect of improving comfort and/or lowering energy usage.The user- defined parameters are the same as for outdoor air reset.

Supply AirTempering

Hot water, steam, and modulating gas units only — When supply air temperature falls below the supply air

temperature deadband low end, the heating valve is modulated open to maintain the set minimum supply air temperature.

3

ZoneTemperature Control

Unoccupied Zone Heating and Cooling

During Unoccupied mode, the unit is operated as a CV unit. Inlet guide vanes and VAV boxes are driven full open.The unit controls zone temperature within the Unoccupied zone cooling and heating (heating units only) deadbands.

Daytime warm-up

This feature is available on all types of heating units. During Occupied mode, if the zone temperature falls to a preset, user-defined zone low limit temperature setpoint the unit is put into Unoccupied mode and Daytime Warm-up is initiated. The system changes over to CV heating (full unit airflow), the VAV boxes are fully opened and full heating capacity is provided until the Daytime Warm-up setpoint is reached.The unit is then returned to normal Occupied mode.

4

Outdoor Air CFM Compensation

The purpose of this feature is to modulate the minimum position of the economizer to compensate for varying unit airflows in VAV units, thereby minimizing the large variation of outdoor air CFM that can occur.The feature allows the user to set (calibrate) the economizer minimum position with inlet guide vanes (or VFD) at 0 percent and at 100 percent. On units with inlet guide vanes (or VFD) and economizer, the minimum position of the economizer is modulated based on inlet guide vane position (or VFD speed).

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