Tom's well-taken points raise another issue. Regarding the cost of expansion tanks for pressurized storage-- the Europeans apparently often use a "semi-open" system where the expansion tank is located relatively high in the building, above any of the served loads, and that expansion tank has a small opening at the top. The weight of the water from there down to the boiler at the lowest part of the system supplies a certain amount of pressure. Since the elevated expansion tank can be a relatively ordinary vessel (no need to contain actual pressure there, or have bladder, air, etc.) it can be fairly low-tech and inexpensive (compared to what we normally think of as "expansion tanks." So, if building a building with an intent to run pressurized storage (if that is what one chose to do), perhaps try to build in (space-wise and structurally) a place for such an elevated expansion tank.