Boiler cycles on and off, but some radiators remain cold while others are hot.

Safety: Radiators and pipes can be hot. Use a cloth when opening bleed valves.

• Bleed cold radiators: Use a radiator key to open the air vent on cold radiators until water flows steadily, then close it.
• Check radiator valves: Ensure both the supply and return valves on each radiator are fully open.
• Zone valves: If an entire heating zone is cold, check the corresponding zone valve for power and movement. Manually open it to test.
• Sludge issues: If certain heating loops never heat, a professional power flush or the installation of a magnetic dirt separator on the Crown boiler's return pipe may be needed.