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Domestic Hot Water:

How Hot Should We Go?


Plumbing engineers have two main safety concerns: scalding and legionella. Legionella, a bacteria known to cause the pneumonia-type illness, Legionnaires' disease, thrives in warm water ranging from ~68°F and ~122°F. It takes 32 minutes for the bacteria to die when stored at 140°F...however, at that temperature, water will cause a third-degree burn in just five seconds. So how do we mitigate legionella AND prevent people from being scalded? It's a fine line to walk.

​As engineers, we understand that several variables need to be considered when designing domestic hot water systems, including what temperature to store the water, what temperature to see at the fixtures, and what temperature to size the recirculation system for. After reading all the standards, guidelines, white papers, and attending seminars at ASPE conventions, it's our opinion that the optimal system incorporates the following:

  • Storing water at 150°F, which will kill the bacteria in only two minutes. This way, even in peak demand times, we can assume that all the new water coming into the heater from the city supply will have at least two minutes to kill the bacteria.

  • Utilizing a master mixing valve to mix the water to 130°F and distribute to our system at this temperature.

  • A circulation system sized to allow for a 5°F drop, returning water to the heater at a minimum of 125°F.

  • A point-of-use mixing valve at each fixture. It should be ASSE 1070 rated, and installed as tightly as possible to the fixture outlet to limit the actual fixture outlet temperature to below 120°F and prevent scalding. With these design temperatures, the entire domestic hot water system is kept above 120°F, with some safety factor.  

  • Consideration for occupants. Buildings with compromised occupants, such as healthcare or senior living facilities, should be conservative in their plumbing system design parameters and treated differently than other buildings. For example, an office with only bathroom lavatories and breakroom sinks.

​Sure, one could argue that we can keep storing at 140°F and distributing at 120°F, as that meets the latest ASHRAE Guideline. However, that guideline states those temperatures are the absolute minimum, and we are not the kind of engineers who only give our clients the minimum…especially when it deals with something as serious as a deadly disease.

​While we feel the strategy above is one of the best, it is by no means the only one. There are many other system design factors and legionella mitigation technologies out there. If you’re interested in learning more about these options, and how to change domestic hot water systems to ensure the health and safety of a building’s occupants, we invite you to reach out to us. Let’s discuss it! 

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