Radiators are not heating properly even though the boiler is running.

Safety: Radiators and pipes can be hot; use gloves when bleeding.

• Bleed radiators: Starting from the top floor, bleed all radiators until water flows steadily.
• Check valves: Ensure thermostatic and lockshield valves are open, then balance by slightly closing the hottest radiators.
• Clean system: If some radiators remain cold at the bottom, the system likely has sludge. Arrange for a power-flush and fit a magnetic filter on the return to the Viessmann boiler.
• Pump assessment: Verify the pump speed is adequate; upgrade or replace if circulation remains poor.