Let us be clear: Legionella bacteria live in our water systems and enter our buildings through the water supply. Typically, if Legionella is present in the municipal water system, the levels are too low to measure. However, given the right conditions, Legionella can multiply to disease-causing levels in the premise plumbing system. 

The task we face is how to design premise plumbing systems to minimize the factors contributing to the growth of Legionella while also minimizing construction and maintenance costs and maximizing water and energy conservation over the life of the system. 

The most significant problem today in addressing these tasks is the common misconception that achieving them all is impossible. They are absolutely achievable. More importantly, if we are to reach our net-zero carbon goals, achieving these tasks on new construction ASAP is an absolute requirement! 

Often buildings looking for LEED certification will forgo the water component because the Legionella risk is too high. This is a real issue. Trying to achieve low water use on buildings designed for much higher water use is a significant Legionella risk.  

In 2016 we, along with Marc Edwards, jointly presented a program at the International Emerging Technology Symposium titled, “Water Quality, Water Savings and the Water-Energy Nexxus: Three Issues, One Solution.” 

We discussed the numerous variables impacting Legionella growth in premise plumbing systems: temperature, water age, disinfectant residual, total organic carbon, microbial population, biofilm, water velocity and plumbing materials. The impact of some variables on Legionella growth, such as piping material (CPVC, PEX, copper, etc.), is difficult to quantify. 

Two of these variables, temperature and water age, have a huge impact on the Legionella growth risk in building water systems. These two variables also directly impact water conservation and energy efficiency. You can describe water age in several ways, but the simplest is the volume of water in the premise plumbing system, including all tanks and piping divided by the daily water consumption. 

Temperature is well-documented as key to Legionella control. Temperature alone can be used for Legionella control or temperature can be used in combination with other controls such as disinfectants. Several factors can impact the efficacy of water temperature in controlling Legionella: the presence and amount of biofilm, disinfectant residual, ambient temperature and contact time. 

The architect and the plumbing design engineer directly impact the stored water volume and, consequently, water age. The architect does it by determining the location of the wet rooms, the number of plumbing fixture appliances and the associated flow rates and flush volumes. The plumbing design engineer does it by selecting pipe sizes and piping layout. Both are guided by the applicable codes and standards.

IAPMO 2021 UPC, Appendix N

One of the most important Legionella standards for building water system design operation and maintenance is the IAPMO 2021 Uniform Plumbing Code Appendix N (Figure 1). It simply and clearly names various temperature ranges from cold to disinfecting hot and identifies the temperatures in each range. For each temperature range, it presents both the scald and Legionella-related risks. 

Many believe that addressing the Legionella issue results in increased scalding risk. Appendix N shows this is not the case. Effective Legionella control can be implemented while effectively controlling scalding risk.  

The Legionella control temperatures in Appendix N agree with the temperatures listed in ASHRAE Guideline 12 2020 and most other Legionella guidance documents. This IAPMO UPC Appendix was developed by a diverse committee of plumbing system design experts, master plumbers, Legionella control experts and academic researchers. 

An important caveat in this appendix is Note 2, which states these temperatures were determined in a lab setting in the absence of a realistic microbial community. All published research on Legionella control temperatures uses these same criteria to limit the number of uncontrolled variables. Also, this note states that Legionella can survive for longer periods of time at temperatures higher and lower than the growth ranges indicated. 

One item missing from Note 2 that is well understood is these temperatures were also determined in the absence of any disinfectant residual.

Importantly, this standard addresses a pervasive misconception that Legionella control means more costs in energy and water and chemicals. This is not the case. Good plumbing system design will save water, energy and construction costs while dramatically reducingLegionella risk as well as operational and maintenance costs. Understanding the role of temperature in plumbing system design, operation and maintenance is critical to effective, as well as cost-effective, control of Legionella.

The recommended temperature range listed in this UPC document for the best Legionellacontrol with low scald risk is the tempered hot range of 120 F to 130 F (49 C to 54 C). It is typically found in sinks and showers at good hotels. 

This tempered hot range was the basis of the Department of Veterans Affairs VHA Directive 2009-009 titled, “Domestic Hot Water Temperature Limits for Legionella Prevention and Scald Control.” This 2009 VA Directive (https://bit.ly/3nw92Fo) was successfully implemented at many VA hospitals with no Legionella or scalding issues. 

These temperature ranges were removed from VA guidance after the outbreak at the VA hospital in Pittsburgh. However, the VA Pittsburgh hospital was never in compliance with the recommended temperatures in the 2009 VA Directive. 

The tepid temperature range of 85 F to 110 F (29 C to 43 C) clearly stands out in this appendix as the range with the highest Legionella risk. It is well-documented in numerous guidelines and standards as the highest risk range for Legionella growth and should be avoided when possible. 

Some plumbing codes and health-care guidance recommend maintaining hot water at above 120 F (49 C) in the circulating system and reducing down to 110 F at the fixture with point-of-use (POU) mixing valves. However, there are risks associated with such valves. ASHRAE Guideline 12 identifies Legionella growth potential risk related to POU mixing valves in multiple sections of the document. 

The Illinois Veterans Home in Quincy, Ill., after its 2015 outbreak, set its water heaters to above 150 F (66 C), maintained greater than 130 F (54 C) in their supply and return lines, and installed POU mixing valves at all sinks, lowering the temperature to less than 110 F (43 C). 

Under this protocol, even with extensive daily flushing of each fixture, the veterans' home proceeded to experience additional outbreaks in 2016, 2017 and 2018. This should not be surprising as the water temperature at all sinks, from the POU mixing valves to the faucets, was maintained constantly in the tepid, high Legionella-risk temperature range.

Throughout the United States, municipal supply water is treated with disinfectant, typically chlorine or chloramine. Municipal supply water in the tepid cold temperature range of 77 F to 85 F (25 C to 29 C) is commonly found in many parts of the South and Southwest. This water has a relatively low risk for Legionella growth because the cold-water supply will typically include very low total bacteria counts and significant levels of disinfectant residual. 

Likely one of the main factors resulting in noticeably higher per capita Legionnaires’ disease rates in the Northeast in comparison to the South and Southwest — where it would be logical to expect higher infection rates due to higher ambient water temperatures — is that the South and Southwest typically maintain much higher levels of disinfectant residual 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 significantly 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 completely 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 sample, 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 environmental 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 versus 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 comfortable handwashing. 

Parts of the United States with these water temperatures 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 temperature 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-efficient than circulated hot water systems because they eliminate 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.