Why do my radiators need to be bleed at least once a fortnight?
I had this issue too. Bled the system once myself, twice with professional plumbers who kept telling me that i didn't do it correctly. Cost me an arm and a leg. The worst part was when i asked a "plumber with 25 years of experience" where the air is coming from he couldn't explain it. Some people were saying it's because i have an old type of expansion tank with the air, others were saying that the water from the city comes with some air.

What solved it is installing "Spirovent Air Eliminator". Not a sound this winter and i could finally sleep without hearing a waterfall.

P.S. Got a few more opinions on this forum