Furnace runs but radiators remain cold or only lukewarm

Safety first: Allow radiators to cool slightly before bleeding to avoid scalding.

• Bleed radiators: Starting with the highest radiators, bleed air until water flows steadily. Recheck system pressure and top up if needed.
• Check pump speed: Set the circulation pump to an appropriate speed for the system size as per the installation manual.
• Clean filters: Isolate and clean any magnetic or mesh filters in the return line.
• Balance system: Adjust lockshield valves on radiators so that all rooms heat evenly. A heating engineer can perform a full balancing procedure.