Failsafe for DAB Terminal Profile

| Overview

Failsafe is a mode of operation introduced in the 75F system. With this, the zone controller can maintain the setpoints, even after losing communication with the CCU (supervisor).

The SmartNode/ HelioNode enters the failsafe mode two minutes after losing the communication with the CCU.

| How it works

The Failsafe mode of operation uses the Supply Air Temperature of the AHU or the RTU and compares the value with the following predefined tuner values (which can be altered for better results in the tuner section of the system)

airflowHeatingTemperature= 105° F
airflowCoolingTemperature= 60° F

Determine whether the airflow is trying to cool or heat the space.

It then uses the current temperature values from the zone to operate the damper positions accordingly.

| Wiring

Smart Damper Control

Actuator Damper Control

Connections

Analog-out1 connects to the modulating damper actuator.
Thermistor-1 connects to airflow 10K sensor to measure Discharge Air Temperature (DAT).
Thermistor-2 connects to airflow 10K sensor to measure Supply Air Temperature (SAT).
Optionally the Thermistor- 2 connects to a 10K sensor for condensation sensing in case of an Active Chilled Beam Profile.
A Differential Pressure Sensor ( DPS) connects to the RTH sensor bus, for CFM-based control during Failsafe
Optionally HyperSense connects to the RS485 port to provide CO2 and occupancy sensing (but is not used in failsafe mode)

The SmartNode/HelioNode with all its wired input and output capability provides the required parameters for the operation.

| Configuration

The configuration remains the same as the DAB terminal profile with the reheat capability. for more information refer to

| Operating Modes

By default, the conditioning mode is sent to the SN/HN from the CCU. When the connection is lost and the SN/HN goes to failsafe mode, it runs a failsafe mode evaluation every 30 seconds.

During the evaluation,

If the TH2-Supply Air Temperature (SAT) value is > airflowHeatingTemperature then the system is assumed to be in heating mode.
If the TH2-Supply Air Temperature (SAT) value is < airflowCoolingTemperature then the system is assumed to be in cooling mode.

| Damper and Reheat Operations

Cooling Mode

Conditions	Operations
When the zone requires cooling	Based on the Cooling loop output, the damper modulates from mid-point to the maximum.
When the zone temperature drops below the cooling setpoint	Based on the Cooling loop output, the damper modulates from mid-point to minimum. The reheat is off.
When the zone temperature goes 2 degrees below the heating setpoint	The Damper remains at a minimum from 0% - 50% of the Heating Loop Output, till a maximum Discharge Air Temperature (DAT) is reached, defined by the reheatZoneMaxDischargeTemp tuner (90) After 50% of the Heating Loop Output, the Reheat loop output is enabled, based on the DAT set point and actual DAT for the loop output calculation. When the CFM-based control is enabled, the Heating Loop Output above 50% factored, CFM Loop Output, the result of active CFM set point (Result of minimum CFM reheating & Max CFM reheating), and measured CFM drive the damper position.

Heating Mode

Conditions

Operations

When the zone temperature is above the cooling setpoint

Based on negative Heating loop output the Damper modulates from mid-point to minimum.

When the zone requires heating

Based on positive Heating loop output the Damper modulates from mid-point to maximum.
The reheat loop output shall modulate based on the damper position.
Stage 1 reheat is triggered when electric reheat is used.

Note: This ensures there is sufficient airflow in the case of a staged electric reheat.

For more indepth information on the damper position calculations refer to VAV Terminal Profile- Damper Position Calculation & Operation
Note: The Occupancy, IAQ, CO2, and VOC controls will not be available in Fail Safe, only CFM and all its tuners and configs will work as intended in Failsafe.

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