D3 alarm, low system water pressure, unit stops to protect system

Safety: Turn off power before topping up. Do not over‑pressurize the system.

• Check pressure gauge: Read the system pressure on the Nibe display or mechanical gauge. If below the recommended level (often ~1 bar cold), the system needs topping up.
• Top up via filling loop: Locate the filling loop (usually a braided hose with two valves). Slowly open the valves and watch the pressure rise to the target value, then close both valves tightly.
• Inspect for leaks: Walk around radiators, valves, and visible pipework looking for drips or stains. Repair leaks before repeated topping up.
• Reset alarm: After restoring pressure, reset the D3 alarm via the control panel.

If pressure drops again quickly, or if the expansion vessel is waterlogged, contact a heating technician.