F13 error, unit indicates low water pressure or lack of water in system

Safety: Allow the unit to cool before adding water. Overfilling can cause relief valve discharge.

• Check system pressure: Read the pressure gauge on the front or underside of the unit. For most Viessmann systems, cold pressure should be around 1.0–1.5 bar.
• Top up system: Open the filling loop or integrated fill valve slowly and watch the gauge rise. Close the valve at the correct pressure.
• Bleed air: Bleed radiators or air vents in the system to remove trapped air, then recheck pressure and top up if needed.
• Inspect for leaks: Look for water around the boiler, relief valve discharge pipe, and visible pipework. Fix leaks before refilling.
• Check pressure sensor: If pressure is correct but F13 persists, the pressure sensor or its tube may be blocked or faulty. Carefully remove and clean the sensing tube, or replace the sensor as required.