| Overview
The Single Stage Equipment (SSE) Fail-safe feature enables a SmartNode configured with the Single Stage Equipment (SSE) profile to continue controlling the connected HVAC equipment if communication with the CCU is interrupted.
During fail-safe operation, the SmartNode uses the last available operating parameters and locally controls the connected heating or cooling equipment based on the measured room temperature. This ensures the conditioned space continues to be maintained until communication with the CCU is restored.
| Supported Equipment
The SSE profile is designed for simple single-stage HVAC equipment, including:
- Split air conditioners
- Unit heaters
- Single-stage heating equipment
- Single-stage cooling equipment
| System Architecture
The following diagram illustrates a typical Single Stage Equipment (SSE) installation using a SmartNode. The SmartNode receives 24 V AC/DC power, monitors the room and supply air temperatures, and controls the connected HVAC equipment through dedicated heating/cooling and fan outputs.
| System Connections
| Ref. | Interface | Connection | Function |
|---|---|---|---|
| 1 | Relay 1 | Heating/Cooling equipment | Controls the single-stage heating or cooling output. |
| 2 | Thermistor-1 (UI3/TH1) | Supply air temperature sensor | Monitors the supply air temperature. |
| 3 | Relay 2 | Fan | Controls the fan output. |
| 4 | OWI Interface | Remote Temperature & Humidity (RTH) Sensor | Measures the room temperature and humidity. |
| 5 | Power Input | 24 VAC | Supplies operating power to the SmartNode. |
| - | Thermistor-2 (UI4/TH2) | External 10K Temperature Sensor (Optional) | Supports an optional external temperature sensor. |
| Fail-safe Operation
During normal operation, the SmartNode receives the desired temperature and equipment configuration from the CCU.
If communication with the CCU is lost, the SmartNode enters Fail-safe mode and continues controlling the connected HVAC equipment using:
- Last received desired temperature
- Configured equipment type
- Current room temperature
- Fan configuration
Normal operation resumes automatically when communication with the CCU is restored.
| Control Logic
The SmartNode controls the connected equipment based on the configured equipment type.
For additional configuration information, refer to the SmartNode - Single Stage Equipment
Heating Operation
When Relay 1 is configured for Heating:
| Condition | Action |
|---|---|
| Room temperature ≤ Desired temperature − 1°F | Heating relay turns ON |
| Fan enabled | Fan relay turns ON |
| Room temperature ≥ Desired temperature + 0.5°F | Heating relay turns OFF |
| Fan enabled | Fan relay turns OFF |
Cooling Operation
When Relay 1 is configured for Cooling:
| Condition | Action |
|---|---|
| Room temperature ≥ Desired temperature + 1°F | Cooling relay turns ON |
| Fan enabled | Fan relay turns ON |
| Room temperature ≤ Desired temperature − 0.5°F | Cooling relay turns OFF |
| Fan enabled | Fan relay turns OFF |
| Temperature Inputs
The SmartNode supports the following temperature inputs during SSE operation.
| Input | Function |
|---|---|
| Remote Temperature & Humidity (RTH) Sensor | Primary room temperature and humidity sensor |
| Thermistor-1 (UI3/TH1) | Supply air temperature monitoring |
| Thermistor-2 (UI4/TH2) | Optional external 10K room temperature sensor |
When the External 10K Temperature Sensor is enabled, the SmartNode uses the sensor connected to TH2 as the room temperature source.
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