Boiler locks out with overheat indication, flow temperature very high, boiler very hot to touch on flow pipe

Safety: Turn off power and allow the boiler to cool fully before touching any pipework. Do not remove any safety devices or bypass thermostats.

• Check system valves: Ensure all radiator and isolation valves on the flow and return near the boiler are open. A closed valve can cause rapid overheating.
• Bleed air: Bleed air from radiators and any automatic air vents near the boiler. Re-pressurise the system to the recommended cold pressure (typically 1.0–1.5 bar) using the filling loop.
• Check circulation pump: Listen for the pump running when there is a heat demand. Carefully touch the pump body (once cool) to feel for vibration. If silent and cold, the pump may be seized or not powered.
• Reset overheat stat: Locate the manual reset overheat thermostat on the boiler (often a red button on the front of the primary heat exchanger). Once cooled, press to reset. If it trips again, do not continue to reset—find and fix the cause.
• Call a technician: If overheating persists, a professional should check pump operation, system flow, and replace the overheat thermostat or NTC sensor if faulty.