The volume test instructions I’ve seen measure the hot water output until there’s a 30-degree drop from a peak temperature of 120 degrees. We may not like it or agree with it, but that’s the standard (90 degrees minimum) that American Water Heater Co. and Bradford White use to define “usable” hot water and determine whether the unit meets performance specs.
The results of our test suggest that the amount of time I’d actually be showering in 90-degree water is zero, even if I was the last of three consecutive users. I didn’t note the temperatures at different volume levels, but it seems to me that the temperature remained above 115 until the gallon count was in the 25-30 range. It started dropping more quickly as we ran the last five gallons, but was still a tick above 100 as we reached 35 gallons.
Thus, in actual use, we’d be mixing in cold water for all of the first two morning uses and nearly all of the third, with just-warm-enough hot water still coming through at the end. That’s the theory.
Of course, I don’t know whether the actual peak temperature of 135 made a difference in the total “usable hot water” output. That’s the question I’m wondering about.