Furnace runs but some radiators stay cold or only lukewarm

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

• Bleed radiators: Starting with the highest floor, use a radiator key to release air until water flows steadily. Retighten the bleed valve.
• Check system pressure: After bleeding, top up the system via the filling loop to the recommended pressure.
• Clean filters: If a magnetic or mesh filter is installed, isolate and clean it to remove sludge.
• Balance radiators: Slightly close lockshield valves on radiators that heat quickly to force more flow to colder radiators. Adjust gradually and monitor results.
• Power flush (technician): If sludge is severe, a professional power flush and inhibitor treatment may be required.