DIYbanter - Air in radiators

A friend complains that he is constantly bleeding his CH radiators.
Can anyone identify the problem and the remedy

Either air is getting into the system, or hydrogen is escaping from the
system. Hydrogen would be a byproduct of the radiators corroding.

In either case, there is a serious problem.

The system should be checked for leaks. Assuming a header tank based system,
it should be checked that no pumping over is occuring and that water never
dumps out of the overflow.

The constant replenishment of water that this would cause can mean entrained
air enters the system (hence radiators need bleeding of air), or that it
dilutes away the inhibitor until it starts corroding from within (hence
radiators need bleeding of hydrogen).

Once any leaks have been stopped and the header tank made not to pump
over/overflow, then you can flush the system and refill with inhibitor.

Christian.