Boiler cycles on and off rapidly, radiators only warm at the bottom, some rooms never reach set temperature.

How to Fix / Troubleshooting

Safety first: Radiators can be hot; use gloves when adjusting valves.

Homeowner checks:

Bleed radiators to remove air.
Check that thermostatic radiator valves (TRVs) are open in cold rooms.
Partially close lockshield valves on radiators that heat very quickly to encourage flow to colder rooms.

Engineer actions:

Balance the heating system properly using temperature measurements across each radiator.
Powerflush the system if sludge is present and fit a magnetic filter.
Check pump setting and adjust to suit the system size.

Proper balancing and cleaning improve comfort and reduce boiler cycling and wear.