Boiler runs but some radiators stay cold or only partially warm

Safety first: Radiators may be hot; use caution when bleeding.

Steps:

• Bleed cold radiators: Use a radiator key to release air until water flows steadily.
• Check valves: Ensure thermostatic radiator valves (TRVs) and lockshield valves are open on cold radiators.
• Balance system: Partially close lockshield valves on radiators that heat quickly to force more flow through colder ones.

If problems persist, a heating engineer may need to flush the system and check pump sizing.