When I first entered the trades in 1972, residential tank-style water heaters shipped with the aquastat set to approximately 140° F. Dishwashers had no need to incorporate a sanitizing cycle. Around 1977, water heater manufacturers were required to lower the aquastat temperature setting to approximately 120°, and now, dishwashers needed to incorporate a sanitizing cycle where rinse water in their reservoir was raised to 140° or higher.
We found out — the hard way — that replacing a 40-gallon tank water heater with another same-sized model suddenly resulted in customer complaints due to running out of hot water. The lower water storage temperature resulted in a larger volume of hot water utilized for the final mixed water temperature in the bathing module. Given that the aquastat is easily adjusted to raise the stored water temperature, many consumers did that on their own. With the good intention of lowering the water temperature to reduce the thermal scald injuries and deaths, the end result over the ensuing years was there had not been a reduction in thermal scalding cases. Instead of 40-gallon water heaters, we upgraded to 50-gallon models to provide similar performance without raising the storage temperature. From our perspective, our resetting the aquastat to provide hotter water was a serious liability issue. Ignorance was bliss, but not for long because other dangers lurked.