The furnace runs, but radiators remain cold or only lukewarm, and the burner cycles frequently.

Safety: Allow the system to cool before bleeding radiators or opening filters.

• Bleed radiators: Starting with the highest radiators, use a bleed key to release trapped air until water flows steadily. Monitor system pressure and top up as needed.
• Check pump speed: Locate the circulation pump and verify its speed setting. Increase it by one step if the system is large or distant from the Viadrus furnace.
• Clean filter: Isolate and clean any Y‑strainer or magnetic filter on the return line. Remove sludge and reinstall.
• Mixing valve setting: If a 3‑way or 4‑way mixing valve is installed, ensure it is not set too low. Adjust gradually while monitoring radiator temperature.