The other upside of a larger captive air tank is that your pump will cycle less often, greatly increasing its useful life and saving you another hunk of change on your light bill. The electricity draw at startup (most pumps are capacitor assisted at startup) is substantial compared to sustained operation. The fewer times it starts, and/or the longer the run time per start, the more efficient.

We usually recommend a tempering tank (can be a simple inexpensive steel tank) for tankless heater installations, especially if you can locate it in a warm location, like a mechanical room. But you are correct in that a large pressure tank would serve that purpose as well.

All that said, the big tanks are quite pricey; however, I would say without hesitation, a good long term investment.

Jim

__________________
2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)