I am thinking of adding storage to my eko 25 system I am not sure of all the subtltys of my install as a plumber was hired to do it. we do have a 3 way mixing valve so I am thinking we are running in pralell with out fuel oil boiler. both boilers also heat our DHW. my basement is not very big so I would have to use a vertical tank to add storage. how difficult would this be to do and what kind of $ would it take to do so.