Boiler runs but some radiators remain cold or only heat at the top.

Safety: Turn off the boiler and allow radiators to cool before bleeding.

• Bleed radiators: Use a radiator key to bleed air from the top vent of each radiator until water flows steadily. Protect floors with a cloth.
• Check valves: Ensure both the thermostatic valve and lockshield valve on each radiator are open.
• Flush affected radiators (technician): If radiators are hot at the top but cold at the bottom, they may be sludged. Remove and flush them outside or perform a system power flush.
• Rebalance system: Adjust lockshield valves to ensure even heating across all radiators.