Failsafe is a mode of operation introduced in the 75F system. It provides the capability to a zone controller to try and maintain the setpoints, although it loses communication from the CCU (supervisor).
It has been implemented in the DAB terminal profile and eventually will be a part of all the other terminal profiles used to control a zone-level VAV box.
| How it works.
The Failsafe mode of operation uses the Discharge Airflow Temperature of the AHU or the RTU which is the Supply Air Temperature for the zone.
And compares the discharge airflow temperature value with the following predefined tuner values (which can be altered for better results in the tuner section of the system)
- airflowHeatingTemperature= 95° F
- airflowCoolingTemperature= 60° F
Determine whether the airflow is trying to cool or heat the space.
It then uses the current temp values from the zone to operate the damper positions accordingly.
Smart Damper Control
Actuator Damper Control
- Analog-out1 connects to the modulating damper actuator.
- Thermistor-1 connects to airflow 10K sensor.
- Thermistor-2 connects to optional 10K sensor for condensation sensor.
- Optionally differential pressure sensor connects to the RTH sensor bus (but is not used in the failsafe mode)
- Optionally HyperSense connects to the RS485 port to provide CO2, IAQ and occupancy sensing (but is not used in failsafe mode)
The SmartNode with all its wired input and output capability provides the required parameters for the operation.
The configuration remains the same as the DAB terminal profile with the reheat capability. for more information refer DAB Terminal Profile
| Operating Modes
By default, the conditioning mode is sent to the SN from the CCU. And when the connection is lost and the SN goes to failsafe mode, it runs a failsafe mode evaluation every 30 seconds.
During the evaluation,
- If the TH1 (Discharge Airflow Temperature) value is > airflowHeatingTemperature then the system is assumed to be in heating mode.
- If the TH1 (Discharge Airflow Temperature) value is < airflowCoolingTemperature then the system is assumed to be in cooling mode.
| Damper and Reheat Operations
|When the zone requires cooling
|When the zone temperature drops below the cooling setpoint
|When the zone temperature goes 2 degrees below the heating setpoint
|When the zone temperature is above the cooling setpoint
|When the zone requires heating
Note: This ensures there is sufficient airflow in the case of a staged electric reheat.