Furnace shows error 402, pressure rises rapidly during heating and may vent water from safety valve.

Safety: Hot water under pressure can cause burns. Allow the system to cool before working on it.

• Check cold pressure: When the system is cold, verify that the pressure is not above the recommended level. If over 2 bar, bleed water from a radiator to reduce pressure.
• Observe pressure rise: During heating, if pressure climbs above 3 bar and the relief valve discharges, the expansion vessel may have lost its air charge.
• Check expansion vessel: Locate the red expansion vessel inside or near the furnace. With the system depressurized, measure the air charge at the Schrader valve using a tire gauge. It should match the specification (often around 0.8–1.0 bar). If low or water comes out, the vessel is faulty.
• Do not cap the relief valve: If the pressure relief valve is dripping constantly, it may need replacement. Never block or plug this valve.