Plumbing Engineer June 2021/39
logical to expect higher infection rates due to higher ambi- ent water temperatures - is that the South and Southwest typically maintain much higher levels of disinfectant resid- ual in the municipal water supply because of the higher water temperatures. Conversely, hot water in the same tepid cold temperature range of 77 F to 85 F (25 C to 29 C) represents a sig- nificantly higher Legionella risk. Hot water will frequently have levels of heterotrophic bacteria 100 and sometimes 1,000 times higher than in cold water; it will typically have low to no levels of disinfectant residual. The higher the water heater's temperature and the longer the water is circulated throughout the building, the lower the disinfectant residual. In many cases, it has been com- pletely dissipated. Accordingly, 85 F (29 C) water in a cold water drop leg will almost always be at little to no risk for Legionella growth. Conversely, 85 F (29 C) water in a hot water drop leg provides an excellent habitat for Legionella growth. In most cases, when Legionella is found in a cold-water sam- ple, it is because the cold water was contaminated when passing through the fixture already contaminated with the bacteria from the hot water drop leg. This can occur from small leaks across the seals in POU mixing valves or single-handle valves where the hot and cold water are separated by seals and mixed before discharging through a common pipe to a faucet, shower or tub spout. This is the reason why the Centers for Disease Control and Prevention document, titled "Sampling Procedure and Potential Sampling Sites Protocol" for collecting envi- ronmental samples for Legionella (https://bit.ly/32YFiaJ), states that, "in most situations, it's appropriate to sample only the hot water." Essentially what this is saying is you can typically ignore premise plumbing cold water as a Legionella risk potential. And this is sound advice. Impact on Construction and Conservation Recognizing the difference in Legionella risk in hot ver- sus cold premise plumbing systems is critically important for plumbing system designers and building operators, and efforts to conserve water and energy. Water temperatures greater than or equal 65 F (18 C) are needed for comfort- able handwashing. Parts of the United States with these water tempera- tures could supply handwashing sinks with cold water only, greatly reducing construction costs, energy costs and dramatically reducing the risk for Legionella and all other waterborne pathogens. If municipal supply is slightly colder or somewhat warmer temperatures are desired, the water can be heated up to 80 F (27 C) with little increase in Legionella risk if properly designed. Since the tempera- ture rise is small and close to the building temperature, the energy costs will be small. Structures where there are few, if any, showers, such as schools, office buildings and manufacturing facilities, would dramatically be impacted by this. If only a few shower rooms exist in school, office or manufacturing facilities, they could be supplied with POU water heaters, each one sized for the showers it serves. POU water heaters are dramatically more energy-effi- cient than circulated hot water systems because they elimi- nate the largest energy loss in a central hot water system - the heat loss in the circulated mains and returns. In POU hot water systems, the water remains on the cold-water side of the heater until it is used, with no disinfectant decay due to high water storage or circulation temperatures. When there is a demand for hot water, the water is heated and because of the short time and no stored volume, much of the disinfectant residual is maintained. After the hot water event, the heater turns off and cools down until the next event. It's easy to see the dramatic impact this design has on both Legionella risk and energy efficiency. ASHRAE 188 purposefully requires a water management plan only for "centralized potable water heater systems" because POU water heaters present little to no Legionella risk. O Tim Keane is a consulting engineer with Legionella Risk Management. He was an original member of the ASHRAE 188 committee through publication of ASHRAE 188 and ASHRAE Guideline 12 2020. He specializes in identifying and cost-effectively resolving building water system design, operation and maintenance issues related to Legionella and other waterborne pathogens. Gary Klein, president of Gary Klein & Associates, is intimately involved in energy efficiency and renewable energy, with an emphasis on the water-energy-carbon connection.
Legionella
Water temperature and age have a significant impact on the Legionella growth risk in building water systems. Photo: iStock.com/xrender
The articles that follow in this series will provide additional graphs, charts and examples of the concepts introduced here. We will also describe how to design the next generation of buildings to improve health and safety of the premise plumbing systems (which includes pathogen growth prevention and management) while simultaneously reducing water and energy con- sumption, often at a lower first cost.
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