I had a Nyeltherm Heat Pump water heater installed last week and I'm in the process of adjusting the aquastat that controls it to the ideal temperature. My plumber recommended that I set the aquastat so that water at the tap is 125F at it's hottest. He mentioned that there are two concerns when fine tuning in the temp. 1. It shouldn't be set too high or scalding can occur. 2. It shouldn't be set too low or the legionella bacteria may develop. Whenever I've heard Legionnaires mentioned in the past it was always with regards to AC units. I did a little reading on the subject today and found some interesting stuff. Many sources refer to the temperature of your hot water but do not say whether it's in the tank or at the tap. I'm assuming when they do not do this they actually mean at the tap. I'm also assuming that the water is bound to lose a few degrees as it traverses your plumbing between the tank and the fixture. I remember seeing tables for heat loss values per foot of copper pipe, uninsulated vs insulated and all that. Most of the sources that focus on preventing Legionnaires state that the hot water should be set to 140F while most of the sources that focus on or discuss hot water safety recommend keeping the water at no higher than 120F I thought it was interesting how the two were at odds. I also saw one source that recommended keeping the water temp at roughly 120F for normal every day use but also raising the temp up to 140F and running the water at the fixtures for 1/2 hour. To me that seems like a an awfully energy wasteful way of preventing Legionnaires. It would almost make more sense to raise the temp in the tank and exercise caution at the fixture. I know this isn't a boiler topic but there didn't seem to be a fittign category to post this under.