D4 alarm, high system water pressure, safety valve may discharge water

Safety: Hot water may discharge from safety valves. Use caution and protective gear.

• Check pressure reading: Compare the Nibe display with the mechanical gauge if present. If both show high pressure (e.g., >2.5–3 bar), action is needed.
• Verify filling loop is closed: Ensure both valves on the filling loop are fully closed and not passing.
• Relieve pressure: When the system is cool, bleed a small amount of water from a radiator or drain point until pressure returns to normal.
• Check expansion vessel: If pressure rises sharply when heating, the expansion vessel may have lost its air charge. A technician should test and recharge or replace it.