Legionella and occasionally used tanked heaters
In Australia tanks must be set to bring the water to at least 60 C (140 F) and so it seems that thermostatic mixing valves at the output of the heater are promoted there to lower the temperature in the distribution lines to 50 C (122 F) or a little less. You might consider such a mixing valve on the output of the tank.


  Legionella bacteria can be present in water and become a health
  concern when present in high concentrations. Water should be heated
  and stored above 60 C for at least 35 minutes to ensure Legionella
  bacteria are killed. http://www.yourhome.gov.au/energy/hot-water-service


I wonder if Australia goes overboard on regulations? For 20 years in Dallas before the Legionella regulations were out we kept our 40 gal NG fired tanks at ~120 F, and nobody got Legionella (that I know of).