Boiler shows error 20, will not operate, often after overheating or dry firing.

Safety: Allow the boiler to cool completely before attempting any reset. Do not bypass safety devices.

• Check system pressure and air: Ensure the system is correctly pressurised and bleed all radiators and the boiler's automatic air vent to remove trapped air.
• Reset overheat thermostat: Some Immergas models have a manual reset overheat stat on the primary heat exchanger. With power off and boiler cool, press the reset button if present.
• Check pump operation: Confirm that the circulation pump runs during a heat demand. If it is not running or is noisy and hot, it may be seized or air-locked. Vent the pump and replace if necessary.
• Inspect primary heat exchanger: If overheating recurs, the primary heat exchanger may be scaled or sludged. A professional chemical clean or replacement may be required.
• Check NTC sensor: Test the primary flow NTC sensor resistance against Immergas values. Replace if readings are out of range or intermittent.
• Final test: After resolving the cause, reset the boiler and run it at full load while monitoring temperature rise to ensure no further overheat trips.