What is a safe temperature to set a hot water heater?

Answer

There are two issues related to setting a safe hot water temperature - one is preventing hot water burns, especially in infants and the elderly. The other is limiting the possibility of Legionella (legionaire's disease). Unfortunately, water heater temperatures which quickly kill Legionella are too hot for the skin - they will scald users unless the hot water system contains valves which will mix the very hot water with cooler water before it reaches the bathtub, shower, or sink.

Since the number of scald injuries far exceeds the occurrence of Legionella, the usual recommendation is to set a water heater temperature which delivers water between 120 and 125 degress F (about 50 degrees C) at the tap where it is used.

This is also a safe and effective temperature for cleaning clothes and washing dishes (unless commercial level sanitization is required, but most dishwashers with this function will heat the water internally to the required temperature).

For further information see:

http://www.med.umich.edu/1libr/pa/pa_hotwatr_hhg.htm
http://www.cdc.gov/nasd/docs/d000701-d000800/d000702/d000702.html
http://www1.va.gov/vhapublications/ViewPublication.asp?pub_ID=219
http://www.iphe.org.uk/databyte/legionella.pdf