I have got a vented system. I have just drained, removed and flushed out my radiators. Now refitted and bled the system. I switched on I get hot water in the heating pipes as far as the radiators. Further bleeding lets hot water into the rads. but when left to their own devices the rads do not stay hot. Is there a possibility of an airlock on the return side, if so how do I shift it? To complicate matters further I live in a bungalow with 6 different central heating loops from the one pump.