Recently, a seemingly straightforward question about hot water temperatures on ASPE Connect evolved into a deeper discussion on thermostatic mixing valves (TMVs) and Legionella prevention. Philip Gee, an Assistant Mechanical Engineer at Burns & McDonnell, noticed a possible contradiction in the current edition of Unified Facilities Criteria (UFC) 3-420-01: Plumbing Systems, which must be followed for U.S. Department of Defense projects. UFC Section 4-6.5 calls for the distribution of hot water at 131°F through the use of a master thermostatic mixing valve to limit the potential for Legionella growth, and the same section also calls for a storage temperature not less than 140°F. Philip wondered, “Since most thermostatic mixing valves require a 15–20°F delta from the inlet hot water temperature to the outlet water temperature, should the water heater storage temperature be raised beyond the 140°F setpoint?”
How TMVs Work
Based on how TMVs work, the quick answer is yes. According to Kevin Freidt, LEED AP, Director of Product Management with Caleffi North America, the valve modulates to proportionally mix the two inlets and needs room to work. If the hot inlet temperature is not much different than the mixed outlet temperature setpoint, the valve will be operating near full open to the hot side and near full closed to the cold inlet, resulting in very little cold water entering the mixing chamber. All control valves work better when they operate somewhere in mid-stroke and not near either end of the stroke. Without a good delta T, the valve may hunt, causing the mixed temperature to swing.
However, most digital mixing valves can provide good control without the large delta T, and electronic control can overcome the limitations of thermostatic control.
Point of Use vs. Master Mixing Valves
Victoria Johnson, PE, a Plumbing Engineer with Bass, Nixon & Kennedy Engineers, then asked if there is a code or standard that dictates going through an ASSE 1017 master mixing valve because she prefers to run 140°F water through the piping system and use ASSE 1070 mixing valves at each point of use.
Uniform Plumbing Code (UPC) Section 504.5 states that “a water heater installation or a hot water storage vessel installation shall be provided with over-temperature protection by means of an approved, listed device installed in accordance with the terms of its listing and the manufacturer’s installation instructions.” The International Plumbing Code (IPC) requires the use of ASSE 1070 devices on public handwash sinks and ASSE 1016 shower and bathtub faucets.
Thus, while not necessarily a code requirement, having anti-scald (ASSE 1070) valves at all points of use allows higher recirculation temperatures and protects users from scalding.
What About Legionella?
The experts appear to agree that 140°F is an appropriate temperature in the water heater to prevent Legionella growth. Higher temperatures are hard on piping system components and can increase scale throughout the system.
However, the use of mixing valves in Legionella control is a little more controversial. For instance, adding a master mixing valve reintroduces cold water, which may contain Legionella. Also, there is some concern about the increased risk for Legionella growth from point-of-use mixing valves, which is addressed in ASHRAE 188.
To control Legionella growth, regularly scheduled high-temperature thermal disinfection (~160°F) via an electronic master mixing valve could be considered. Other best practices also help with an overall approach to safe water management, such as constant recirculation, minimizing dead legs, and proper balancing.
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