Display shows 11, furnace does not start, may be associated with low system pressure.

Safety: Allow the system to cool before topping up the pressure. Hot water can cause scalding.

Steps:

• 1. Check pressure gauge: On the display or mechanical gauge, verify that the pressure is at least the minimum specified (often 0.8–1.0 bar cold). If below this, the boiler will not start.
• 2. Top up system: Open the filling loop valve slowly and watch the gauge rise to approximately 1.2–1.5 bar. Close the valve completely afterwards to avoid overfilling.
• 3. Bleed radiators: Bleed air from the radiators and re-check the pressure, topping up again if needed.
• 4. Inspect for leaks: Look around radiators, valves, and under the boiler for signs of water. Fix any leaks promptly.
• 5. Sensor check (technician): If the pressure is correct but code 11 persists, the pressure sensor or switch may be blocked or defective. The technician will clean or replace it and check the sensing tube.