Radiator Cold at the Top:There may be air trapped inside the radiator. If o this will mean that the radiator needs to be bled, so you should open the bleed valve with a bleed key. The valve is on the top corner of the radiator. Finchley Boiler Repairs craftsmen fault find.Radiator Cold in Middle and Bottom:The symptoms may indicate that some corrosion has formed and settled in the radiator. This is restrictive to the free flow of water through the radiator. In this instance it is necessary to remove the radiator and fully flush through. Add some corrosion inhibitor to the system when the process is completed.Some Radiators Cold:Visually check the zone valve thermostat/timer is correctly set, reset it if required. If all is set correctly, isolate the power and check the wiring connections in accordance with the original operating manual. The zone valve itself could be faulty, or the system may need to be balanced.Single Radiator Cold:This could be due to something simple like the manual valve on the radiator is turned off or maybe the TRVs are incorrectly set or even faulty. If the valve is set correctly but the radiator still does not work, then, it may need to be replaced.Radiators Cold with Boiler on:Possible defective circulating pump. A potential issue with the pump timer or thermostat that is incorrectly set or even faulty. If set correctly, then, turn off the power and inspect electrical connections. It may be an incorrectly set lock-shield valve. Firstly, open the manual valve fully, then remove the plastic cap from the lock-shield valve and use an adjustable spanner to open the valve so the radiator can warm. You can always call a professional plumbing service like the ones in Finchley Boiler Repair who will rectify heating problems.Corrosive deposits could be blocking the inlet and outlet pipes. Flush out or replace it if necessary and consider adding corrosion inhibitor to the system.