Furnace display shows E19, unit may not start or may shut down when an accessory (humidifier, HRV, etc.) is active.

Safety first: Turn off power before working on accessory wiring.

• 1. Identify connected accessories: Note any humidifiers, HRVs/ERVs, or other devices wired to the Thermia furnace control board.
• 2. Disconnect accessories: Remove accessory wires from the control board one at a time and test furnace operation. If E19 clears when a specific accessory is disconnected, that device or its wiring is likely at fault.
• 3. Inspect wiring: Check for shorts to ground or between terminals on the accessory circuits. Repair or replace damaged wires.
• 4. Verify correct terminals: Ensure accessories are wired only to the designated Thermia accessory terminals and not to thermostat or safety circuits.
• 5. If E19 persists with all accessories disconnected: The accessory control section of the Thermia control board may be defective and require replacement.