What is the recommended storage temperature for building hot water tanks?

I work for a building management company which involves controlling domestic hot water tanks, I would like to know what the official minimum storage temperatures are and the length of time they have to be at that temperature. I am under the impression that it is a minimum temperature of 60oC for at least 1 hour in any 24 hours (therefore the temperature can be dropped for the remaining 23 hours) can you tell me if this is correct. 

The temperature should remain stable at about 140oF at the tank. It would not be wise to only hold this temperature for only one hour and then reduce it.

Please obtain a copy of the American Society of Heating, Air Conditioning and Refrigeration Engineers (ASHRAE) guide to “Minimizing the Risk of Legionella in Building Water Systems”. This document has a section regarding hot water storage temperature. You can also download a copy from the Baltimore Aircoil web site (www.baltimoreaircoil.com).

The “official” minimum storage temperatures and durations may vary from country to country and even state to state depending on local guidelines. As a general source you may want to request guidelines from the following organizations:

Your suggestion of maintaining a water temp greater than 60oC in the tank for 1 hour may kill Legionella in the tank; however, regrowth of Legionella is likely to occur in the hot water distribution system where temperatures could drop to less than 50oC depending on use. Ideally for Legionella disinfection you would want to maintain a high temperature throughout the distribution system (hence flushing).

Whatever heating regime you try, monitoring of distal sites and the hot water tank is necessary to document the effectiveness of disinfection specific to your system and temperature set points.