It’s a sign that a topic is going mainstream when it becomes cited in the model codes. Unfortunately, Legionella prevention has broken that barrier during this cycle due to increased hospitalizations and deaths. It has taken years to be mentioned in the model codes, but there finally appears to be some movement to address the issue.

Since the early days, the Uniform Plumbing Code (UPC) and International Plumbing Code (IPC) have required the disinfection of new potable water systems, or as required by the authority having jurisdiction (AHJ). If the AHJ does not have a method prescribed, the method is to:

1. Flush the entire system with potable water.
2. Fill the system with chlorinated water, typically as chlorine gas or sodium hypochlorite (bleach). If the chlorine concentration is greater than 50 ppm, let the system stand for 24 hours. Else if the concentration is greater than 200 ppm, wait 3 hours.
3. Flush the entire system again with potable water until the residual chlorine is removed.
4. Perform a bacteriological evaluation and repeat the disinfection until satisfied.

In addition, the IPC alternatively allows use of the procedures found in AWWA C651, Disinfecting Water Mains, or AWWA C652, Disinfection of Water Storage Facilities, again using chlorination.

While not specified as a method of disinfection in the model codes, elevated water temperature is also widely used. ASSE 1084, Performance Requirements for Water Heaters with Temperature Limiting Capacity, and ASSE 1085, Performance Requirements for Water Heaters for Emergency Equipment, are both incorporated into the 2021 IPC and have tentatively been incorporated into the 2021 UPC. Water heaters conforming to these standards give another way to combat Legionella. By designing systems with these devices at points-of-use, there is less opportunity for dormant, warm water at the ideal growth temperature range to be present within the hot water distribution system. These water heaters can precisely control the output temperature so tightly that an ASSE 1070 (Water Temperature Limiting Device) or ASSE 1071 (Temperature Actuated Mixing Valve for Plumbed Emergency Equipment) is not required downstream. 

Alternatively, a water heater conforming to ASSE 1082, Performance Requirements for Water Heaters with Integral Temperature Control Devices for Hot Water Distribution Systems, can be installed at the system inlet and can maintain an elevated temperature to the same tolerances as an ASSE 1017 (Temperature Actuated Mixing Valve for Hot Water Distribution Systems). Note that an ASSE 1069 (Automatic Temperature Control Mixing Valve) is used for gang showers and an ASSE 1016 (Automatic Compensating Valve for Individual Showers and Tub/Shower Combinations) is used for individual showers and tub/shower combinations.

One of the other public comment proposals for the UPC aims to add Appendix N (Impact of Water Temperature on the Potential for Scalding and Legionella Growth), which gives guidance of various water temperature ranges and its relationship to both scalding and Legionella growth. This will be useful in designing hot water distribution systems because one can quickly understand and reference that it is inappropriate to set the maximum system temperature at the water heater to 120° F (49° C). The resulting downstream heat losses could potentially allow for ideal growth of Legionella at 95° – 115° F (35° – 46° C). Instead, design your system to have a time and temperature dwell that disinfects incoming and recirculation loop water, and minimize stagnant water by eliminating or reducing the number of dead legs. 

Finally, the proposal incorporates ANSI/ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, as an acceptable reference in Table 1701.2. The standard lays out the risk management requirements for designing, building, repairing or expanding water systems in order to minimize the effects of Legionella. This will be the first time ASHRAE 188 is referenced in a model code and it symbolizes a noteworthy step.

As Legionella prevention gets more attention and becomes more mainstream, ASSE International will be looking closely at the next code cycle for the year 2024 while continuing to develop product and professional qualifications stand-ards to help installers, designers and the public.