F.23 displayed, boiler cycles off quickly, flow temperature rises much faster than return

Safety: Isolate power and allow the system to cool before opening any components.

Steps:

• Ensure all radiator valves and any motorized zone valves are open and operating.
• Bleed radiators and the boiler's air vent to remove trapped air.
• Check system pressure and top up if low.
• Listen to the pump: if noisy, grinding, or very hot, it may be partially blocked or failing.
• Clean the system filter (if fitted on the return line) to remove sludge and debris.

Advanced: If F.23 persists, the plate heat exchanger or main heat exchanger may be restricted and require chemical cleaning or replacement, and the pump may need replacement. These tasks should be done by a qualified technician.