into cylinder in 10mm pipes, this goes off to a airjec, seperate from existing vent, banging pipes full of air,vent rads all ok , 10 mins later, immediately drawing in air, banging again, I know it`s bad pipework config, it wasn`t doing it 12 months ago, but then again it has been drained recently. I`m a plumber and so-so heating engineer. Thanks for help.