Furnace shows E11; the burner does not start for a heating demand.

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

• Check thermostat display: Ensure the room thermostat or controller is powered and calling for heat.
• Inspect wiring: At the furnace terminal strip, verify that the thermostat wires are correctly connected and secure. Look for broken or loose conductors.
• Bypass test (qualified persons only): Temporarily bridge the room thermostat terminals at the furnace to simulate a heat demand. If the furnace starts, the fault is in the thermostat or its wiring.
• Check control settings: In the installer menu, verify that the correct control type (OpenTherm, on/off, etc.) is selected for the installed thermostat.
• Replace faulty control: If wiring is intact but E11 persists, replace the thermostat or have a technician check the control PCB input.