Boiler locks out with E09, often accompanied by rapid temperature rise on display

Safety first: Turn off power and allow the boiler to cool before working on valves or filters.

• Check isolation valves: Ensure the flow and return isolation valves directly under the ACV boiler are fully open.
• Clean system filter: If a magnetic or mesh filter is installed on the return to the boiler, isolate it, place a tray underneath, and clean out sludge and debris according to the manufacturer's instructions.
• Bleed air: Air locks can prevent flow. Bleed radiators and any high-point vents.
• Professional checks: A technician can test the flow sensor or flow switch on the boiler's hydraulic block and replace it if it is not changing state with flow.