Failsafe for VVT-C Terminal Profile

| Overview

Failsafe is a mode of operation introduced in the Carrier 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 VVT-C 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.

| Wiring

Smart Damper Control

Actuator Damper Control

Connections

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 1-wire 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.

| Configuration

VVT-C IMAGE9.PNG

The configuration remains the same as the VVT-C terminal profile with the reheat capability. for more information refer VVT-C 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

Cooling Mode

Conditions	Operations
When the zone requires cooling	Damper modulates from its mid position to maximum position based on the Cooling loop output.
When the zone temperature drops below the cooling setpoint	Damper modulates from its mid position to minimum position based on the cooling loop output The reheat is off.
When the zone temperature goes 2 degrees below the heating setpoint	Damper is at the minimum damper position. The reheat loop output shall be turned to 30% when modulating reheat is used or Stage 1 reheat is triggered when electric reheat is used.

Heating Mode

Conditions

Operations

When the zone temperature is above the cooling setpoint

Damper modulates from its mid position to minimum position based on the negative Heating loop output.

When the zone requires heating

Damper modulates from its mid position to maximum position based on the positive Heating loop output.
The reheat loop output shall be turned to 30% when modulating reheat is used or
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.

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