Boiler cycles on and off but some radiators stay 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.
• Check radiator valves: Ensure both supply and return valves on each radiator are open.
• Zone valves: If an entire zone is cold, check the corresponding zone valve for power and movement. Manually open it to test.
• Sludge issues: If certain loops never heat, a professional power flush or installation of a magnetic dirt separator on the Crown boiler return may be needed.