Furnace is running but some radiators remain cold or only partially warm

Safety: Radiators and pipes can be hot. Use gloves and allow the system to cool if necessary.

• Bleed radiators: Starting with the highest radiators, use a bleed key to release trapped air until water flows steadily. Protect walls and floors from drips.
• Check TRVs: Ensure thermostatic radiator valves are fully open on cold radiators. Gently tap the valve body if the pin is stuck, or remove the head to free the pin.
• Adjust pump speed: On adjustable pumps, increase the speed setting to improve circulation, following the pump and Viessmann guidelines.
• System flush (technician): If multiple radiators are cold and water is dirty, a professional power flush and installation of a magnetic filter may be required.