Boiler displays E18; external room thermostat or controller not recognized

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

Steps to check:

• Verify wiring diagram: Refer to the Kospel installation manual and confirm that the room thermostat or controller is wired to the correct terminals (e.g., TA, RT, or BUS terminals).
• Check cable continuity: Inspect the cable for damage and test continuity between the boiler and the thermostat.
• Confirm control type: Ensure the boiler parameter settings match the type of controller used (on/off, OpenTherm, proprietary Kospel controller, etc.).
• Bypass test: Temporarily link the thermostat terminals at the boiler to simulate a call for heat. If the boiler responds, the external controller or its cable is faulty.

When to call a technician: If you are unsure about control wiring or parameter settings, consult a heating professional.