Boiler displays F8 and will not operate, often associated with low or no system pressure

Safety: Allow the boiler to cool. Isolate electrical power before removing covers. Do not over‑pressurize the system.

User‑level checks:

• Check the pressure gauge on the boiler. If below the recommended cold pressure (usually 1.0–1.5 bar), top up using the filling loop as described in the user manual.
• After topping up, bleed air from radiators and re‑check pressure, adjusting as needed.

Technical checks:

• If pressure is correct but F8 persists, locate the water pressure sensor or switch on the hydraulic block.
• Inspect the sensor and its connection for leaks or corrosion. Tighten or reseal as necessary.
• Check the small sensing tube (if fitted) for blockages with sludge. Clean or replace the tube.
• Test the sensor electrically according to Protherm specifications. Replace the water pressure sensor if readings are incorrect.
• Inspect the system for visible leaks at radiators, valves, and boiler components. Repair leaks and re‑pressurize.

After repair: Restore power, reset the boiler, and confirm that the pressure reading is stable and F8 has cleared.