Low system pressure warning, boiler may not fire or may shut down frequently

Safety: Allow the system to cool before adding water. Do not overfill beyond the recommended pressure.

• Check pressure gauge: On the front of the Viessmann boiler, read the system pressure. If below ~1.0 bar (cold), the system needs topping up.
• Use filling loop: Locate the filling loop (flexible hose with two valves) beneath the boiler. Open both valves slowly until the pressure rises to 1.2–1.5 bar, then close both valves tightly.
• Check for leaks: Walk around the property and inspect radiators, valves, and visible pipework for signs of water. Repair any leaks before repeated refilling.
• Bleed radiators: After filling, bleed air from radiators and recheck pressure, topping up again if necessary.
• Persistent pressure loss: If pressure drops repeatedly, the expansion vessel or a hidden leak may be at fault. Contact a technician to test the expansion vessel pre-charge and inspect the system.