Heater locks out with F9, often associated with overheating or poor circulation.

Safety: Turn off power and allow the heater to cool completely. Surfaces may be very hot.

• Reset overheat stat: Locate the manual reset overheat thermostat on the primary flow or heat exchanger. Press the reset button firmly until a click is felt (if fitted).
• Check system pressure (if sealed system): Ensure the pressure gauge is within the recommended range per the Ideal manual. Top up via filling loop if trained to do so.
• Bleed air: Airlocks can cause overheating. Bleed radiators or cylinder coil (if applicable) and any automatic air vents.
• Check pump operation: Listen for the circulation pump running when there is a heat demand. If silent or excessively noisy, the pump may be seized or failing and needs professional attention.
• Persistent F9: Repeated trips indicate a circulation or scaling problem; arrange a service to inspect the heat exchanger and system water quality.