Sime Friendly rads getting hot when running hot water?

Hi when i turn on the hot water for a bath etc the rads get very hot ??? to fill a bath takes about half a hour to get a hot bath if i turn on the hot tap to fast the water run cold ?? the boiler is about 6 to 7 years old thanking about replaceing with a better type .which is the best make to have ????
thanks kev