Boiler attempts to start but shuts down with overheat alarm, flow temperature very high

Safety first: Switch off power and allow the boiler to cool before touching any pipework. Surfaces may be extremely hot.

• Check system pressure: On the Nibe display, confirm that system pressure is within the recommended range (typically 1.0–1.5 bar when cold). If low, top up via the filling loop according to the manual.
• Open valves: Ensure all isolation valves on the boiler flow and return are fully open. Check any nearby zone valves are not manually closed.
• Bleed radiators: Use a radiator key to bleed air from the highest radiators until water flows steadily. Re-check and adjust system pressure afterwards.
• Check circulation pump operation: With power on and a heat demand present, place a hand on the Nibe circulation pump body; you should feel vibration and slight warmth. If the pump is silent and cold, it may be seized or failed and needs replacement.
• Reset overheat: Some Nibe boilers have a manual overheat thermostat under the front cover. Only if you are competent and it is safe, press the reset button once the boiler has cooled. If the fault recurs, call a technician to investigate pump and sensor issues.