If your radiators appear to be colder at the bottom than at the top, then there will be air in the system. To release the air you will need to ‘bleed’ the radiators. If this is not done then your system is not working efficiently and will put an extra strain on your boiler and is effectively wasting you money.
To bleed your radiators, first switch the heating off before. Slot the key into the bleed valve and turn the key anticlockwise for around a quarter of a rotation. You should then hear a hissing sound as the air escapes. Have an old towel or kitchen towel available as once all air as escaped then water will start to drip. Once water starts to drip then turn the key back to its original position.
Carry out the same process for all radiators that appear to be colder at the bottom. Once they are all done then turn your heating back on. Your radiators should then have even heat throughout them.