Boiler shows E18; internal temperature too low or frost protection active.

Safety: Ensure the area around the boiler is safe and free from freezing water hazards.

• Check ambient conditions: If the CTC boiler is in a garage or loft, ensure the space is adequately insulated and ventilated to prevent freezing.
• Verify frost settings: In the boiler's installer menu, confirm frost protection parameters are correctly set according to the CTC manual.
• Check pump operation: During frost demand, the pump should run. If not, test and replace the pump if necessary.
• Sensor test (technician): Check the internal frost sensor or NTC reading via diagnostics and compare with actual temperature. Replace the sensor if faulty.