It seems to be "common knowledge" that to help prevent Legionella, hot water tanks/heaters should be set at 140F. This temperature is enough to cause terrible burns and scalding so devices like thermostatic mixing valves are also commonly used.
However, in Texas where I am from, people set their water heaters to 120F for scalding safety. I've never seen a thermostatic mixing valve installed in a house and haven't known anyone to set a water heater to what I would consider a very dangerous 140F.
I do notice that people that seem to be from colder climates make the 140F recommendation, so does it mainly come from areas with "boiler" systems? Larger hot water tanks that would hold the water for longer (50 gallon is the max common size in TX)? Is this a newer suggestion that I just haven't seen implemented? Is it a commercial thing?
I'm just trying to find out where the discrepancy is between the people that say it's a hard requirement and the other people that would never consider heating water that hot in a residential system.