To get a heater that would produce the same volume of hot water (6 gpm at 90° rise), you would need two Rinnai RC80HPi's. My cost on each heater is approx 1000$ plus labor and materials, at least another 1500, if not 2000$. One larger Rinnai, a RC98HPi, could be installed, but that only produces 4gpm, and total installed would be 3000-3200. Each of the above heaters is 96% efficient tho. And those quotes above are relatively low.
Plus, some people really like their old heaters.. Theyre a dying breed... but when you see 15" of flame inside the heater dancing around, its pretty sweet.. (damn, forgot to take a pic with the burner on.. derp.)
if it aint broke, tweak it!!
currently on FiOS (kick aZZ!)