Hi All, I happened to mention that I was considering replacing my ageing boiler and vented HW cylinder with a Veissmann Vitodens 200-W System boiler and the HW cylinder with the above unvented indirect one, my main reasoning is that has a self sustaining air gap and doesn't need an expansion tank. A number of people have advised against using this HW cylinder but no one has given me a reason why not. Can anyone on here enlighten me as to why this is an ill considered move? Many Thanks, Phil