Boiler pressure rises rapidly when heating is on and may discharge water from the pressure relief valve.

Safety first: Turn off heating and allow the system to cool. Do not cap or block the pressure relief discharge pipe.

• Check cold pressure: When cold, ensure the system pressure is not above the recommended level. If overfilled, bleed water from a radiator to reduce pressure.
• Test expansion vessel (technician preferred): The air charge in the vessel should be checked with a gauge and recharged if low. This requires isolating and partially draining the system.
• Inspect PRV: If the PRV has discharged, debris may prevent it from reseating. It may need replacement.
• Professional service: Expansion vessel replacement or recharging is best done by a heating engineer familiar with Rinnai boilers.