Gas-fired water heaters, volume III, storage water heaters with input ratings above 75,000 Btu per hour, circulating and instantaneous (ANSI Z21.10.3:19/CSA 4.3:19)

Standard that applies to storage water heaters, instantaneous water heaters, and circulating water heaters including booster water heaters.  The current version of this standard was published in the year 2019 and it is available for purchase along with a digital preview on the ANSI webstore page.  The ANSI webstore also has previous versions of the standard available for purchase published in the years 2017, 2015, 2014, 2013, 2011, and 2004.