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Sequence of Operation
50 RTU Open
Cooling, heating, and supply fan outputs are not interrupted except where the RTU Open is configured for
Heat Pump operation. When configured for Heat Pump, and in the heating mode, a compressor safety fault
will cause the available stages of electric heating to be enabled in place of mechanical heating.
Normal operation resumes when the compressor safety circuit is de-energized.
Fan Status
Fan Status may be configured on any unused binary input channel. A typical application would be an airflow
switch, current sensing relay, or other device that provides a supply fan running verification.
Enabling this function displays the supply fan’s status on the equipment graphic.
If the controller loses fan status during operation, heating and cooling are disabled, the economizer damper
(if available) is closed, and an alarm for loss of status is indicated.
If the fan status is on when the controller is commanding the fan off, the unit remains in the off state. An
alarm is generated indicating that the fan is running when it should be off.
Filter Status
Filter status may be configured on any unused binary input channel. A typical application is a differential
pressure switch that senses the pressure drop across a filter bank.
When the pressure across the filter bank exceeds the setpoint of the differential pressure switch, the Filter
status is displayed as
Dirty on the controller graphic. An alarm indicates a dirty filter.
Alarms
NOTE Some of the Alarms functions described in this section will only be visible on the Properties page >
Equipment tab > Alarms when the appropriate inputs are configured. Alarms are not initiated when the input
is not configured.
Safety Chain - You may use the RTU Open's safety chain circuit to shut down the unit for a safety condition.
Examples: Low or High Temperature Cutouts (Freezestat / Firestat). See To wire inputs and outputs (page
14)
for additional wiring instructions. This alarm indicates the safety chain circuit (Input 4) is open. Cooling,
heating, and supply fan operation stop after appropriate time guards. Normal operation resumes when the
safety chain circuit is complete.
Fire Shutdown – You may configure the RTU Open to accept a Fire Shutdown contact on Input 5. Examples:
Smoke detectors or fire shutdown relays. This alarm indicates this device (Input 5) has tripped. Cooling,
heating, and supply fan operation immediately stop. Reset fire shutdown contact to resume normal operation.
Compressor Safety – You may configure the RTU Open to monitor the base unit’s compressor safety circuit.
This alarm indicates the base unit's compressor safety circuit is energized. Cooling, heating, and supply fan
outputs are not interrupted except when the RTU Open is configured for Heat Pump. Normal operation
resumes when the compressor safety circuit is de-energized.
If the Heat Pump is in the heating mode, it will automatically replace the compressor stage(s) with the
equivalent number of auxiliary heat stages, as available.
• If it's a Carrier Heat Pump, there is only one auxiliary heat stage output and the staging is done by the
machine itself, if it's two-stage gas or electric.