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!