The name of the system or "hot water cylinder" your referring to is called "elephant" These systems were installed many years ago for various reasons including being able to run a radiator on the low pressure side, disadvantage is the temperature drop you will get when running the tap (shower) for a longer period of time (especially these days when everything is medium to mains pressure), temperature fluctuation plus the heat transfer time is very noticeable if your heat source cant keep up heating the large body of water on demand. It's critical to get the right cylinder with the correct heat exchanger as modern models have the heat exchanger placed for the ideal reverse function. For the correct application though, this system is brilliant. By the way im not sure where it got its name "elephant" from (I can only guess) so if anyone can shed some light on that, it would be greatly appreciated. 

seems to be an interesting project best of luck to you