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 failsafe mode two minutes after losing 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 the airflow 10K sensor to measure Supply Air Temperature (SAT).
Thermistor-2 connects to the airflow 10K sensor to measure Discharge Air Temperature (DAT).
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 TH1-Supply Air Temperature (SAT) value is > airflowHeatingTemperature then the system is assumed to be in heating mode.
If the TH1-Supply Air Temperature (SAT) value is < airflowCoolingTemperature then the system is assumed to be in cooling mode.

| Setpoint for control

When the system is in a single temperature mode, the operation is clear since only one temperature is involved.

And, when the system is in dual temperature mode and 2 separate heating and cooling temperatures are sent/set for the user, then the system operation mode (see above) determines the setpoint that is used.

if the system is in cooling mode, the cooling desired setpoint is considered for all control calculations. Eg. in dual temperature mode with 70/74 as setpoints, the thermistor detects that the supply temperature is below the cooling threshold - hence the control setpoint will be assumed as 74.

| 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 is in deadband	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, and the zone demands heating	The Damper remains at a minimum from 0% to 50% Reheat is set at 30% (Staged or modulating)

Heating Mode

Conditions	Operations
When the zone temperature is above the cooling setpoint and the zone demands cooling	Reheat disabled The damper is set to the minimum position
When the zone demands heating	Based on the positive Heating loop output the Damper modulates from mid-point to maximum. The reheat set to 30% (Staged or modulating)
When the zone is in deadband	Damper is set to minimum postion Rehat is disabled (Staged or modulating)

For more indepth information on the damper position calculations, refer to DAB Terminal Profile- Damper Position Calculation & Operation
Note: The Occupancy, IAQ, CO2, and CFM controls will not be available in Fail Safe.

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