Safety Mode / Sensor Fault Tolerance

[lnagel]

Summary

Implement a fail-safe mode that activates when the primary temperature sensor becomes unavailable.

Motivation

In heating control, sensor failures can have serious consequences. If a temperature sensor goes offline, the system might keep heating indefinitely (wasting energy, potential overheating) or shut off completely (risking frozen pipes in winter). A defined "Safety Mode" ensures predictable, safe behavior during sensor outages.

Proposed Behavior

When the primary temperature sensor is unavailable for a configurable period (default: 60 minutes):

1. Log a warning and create a persistent notification
2. Enter Safety Mode with one of these strategies:
   • Low duty cycle: Maintain a fixed low duty cycle (e.g., 20%) to prevent freezing without excessive heating
   • Frost protection: Heat only enough to maintain a minimum safe temperature (e.g., 7°C) if a secondary sensor is available
   • Summer/Off mode: Disable heating entirely (appropriate for non-freezing seasons)
3. Auto-recover when the sensor comes back online, restoring normal PID operation

Configuration Options

• Timeout before entering Safety Mode (minutes)
• Safety Mode strategy selection
• Minimum safe temperature for frost protection
• Optional secondary/fallback sensor

Prior Art

• Versatile Thermostat includes a "Security Mode" with similar functionality
• Commercial HVAC systems always include fail-safe modes