I can prove that a boiler tank with a Kaa-2 (which only holds about 24 gals) will improve efficiency. There is no doubt.
But most folks burn coal for reasons of economics. What I cannot prove is that such improvements in efficiency would ever save the owner any money.
My Kaa-2 system would save around 5% in overall efficiency with such a unit. If one is considering the Ergomax at around $1200+ installed, I would advise caution. $1200 buys me (in CNY) almost 5 tons. The return on investment would be longer than the anticipated lifespan of the Ergomax. An older electric water heater or galvanized well tank from Tractor Supply fro $200 would pay back much faster.
Remember, you can heat with electricity at about 100% efficiency. But you don't. You heat with coal at about 80% efficiency because it costs 75% less.