Hw problem

If I turn just the Hw on I get a loud banging noise from boiler and pipes and it seems the system is working backwards! It seems to be sucking in air and the overflow tank is filling with hot water. If I have Hw and Ch on it all works fine! though in the summer having the ch heat on is not very logical! Also as a note when I put the Ch heating on I can hear the pipes and boiler filling with water! Any ideas or help!? Thanks!

Either very bad design, or there's a blockage in a pipe which is creating a pressure difference where there otherwise wouldn't be, at the base of the feed / vent pipes.

Thanks, it was a blockage I guess somewhere I let the system run for an hour both as hw and ch then turned off the ch and all is now fine!