If you have the space, then it will work. The best way is to pipe the hot water from the first one to the inlet of the second. This has several advantages and does not require that you try to equalize the flow patterns so that both are used equally, (in the real world, this is impossible so you would never get full efficiency from both heaters and in some cases would not ever improve your presnt situation).: I currently have a 90lt storage hot water system which does not meet our hot water needs. Rather than throw the system out(its only 4yrs old) I thought it would be cheaper to add another cheap 90lt system and run them together. Is this possible?