Indirect Tank Sizing
Indirect water heaters are sized by the total hot water use required. If you have a single major draw, such as a hot tub, it may be more beneficial to size it separately from other hot water appliances. Once you determine the hot water gallons needed, compare the tank's first hour rating and boiler size.
This is for residential applications only.
When you do all the math, compare it to the first hour rating of the chosen indirect.
The choice would be an MS2-030 in this example. This should work on any indirect water heater tank with an internal coil.