Furnace displays F38, high system pressure warning, safety valve may discharge water

Safety first: System may be hot and under high pressure. Allow to cool before relieving pressure.

• Check pressure: Read the pressure gauge. If above the recommended maximum (often 2.5–3 bar), reduce pressure.
• Relieve pressure: Bleed a radiator or use a drain point to reduce system pressure to normal. Collect water safely.
• Check filling loop: Ensure both filling valves are fully closed and the hose is disconnected if external.
• Expansion vessel test (technician): A professional should check the expansion vessel pre-charge and recharge or replace it if the diaphragm has failed.