Boiler displays F2 and locks out shortly after starting or during operation

Safety: Switch off power and isolate gas. Wait until the boiler is completely cool before touching any components.

Check the overheat thermostat and circulation:

• Locate the manual reset overheat thermostat (STB) on the primary heat exchanger or flow pipe. Some Protherm models have a reset button on the thermostat body.
• Press the reset button firmly if present. If it clicks, it had tripped. Do not repeatedly reset without finding the cause of overheating.
• Inspect all radiator valves and isolation valves near the boiler to ensure they are open and allowing water circulation.
• Bleed air from radiators and the boiler’s automatic air vent. Air locks can cause local overheating.
• Check the system pressure on the gauge (typically 1.0–1.5 bar when cold). If low, top up via the filling loop to the recommended pressure and bleed air again.
• With power restored, listen and feel for the circulation pump operation. If the pump is silent, excessively hot, or not vibrating, it may be seized or electrically faulty and should be replaced.
• If the boiler overheats quickly again, the primary heat exchanger may be scaled or blocked. Power‑flushing or replacing the heat exchanger is a job for a qualified technician.

Final step: After resolving the cause, reset the boiler according to the user manual and monitor a full heating cycle to ensure normal temperatures and no recurrence of F2.