Gary and I have worked together on these issues for many years because we understand that Legionella risk management and water/energy conservation are objectives integral to each other. 

After more than 20 years of work in the Legionella field, leading outbreak investigations and resolutions as well as performing building audits, I’ve found the two single most critical variables for controlling Legionella risk are temperature and water age (time-to-tap). If the temperature at the fixture is greater than or equal to 120 F and time-to-tap is less than or equal to one minute, then the likelihood of any Legionella issues is very low. And if the temperature at the fixture is less than or equal to 130 F, then the likelihood of scalding is very low.

A significant issue today is that many misunderstand data presented on Legionella and temperature. Below is a detailed review of temperature recommendations and their impact. 

140 F in Storage-Type Water Heaters 

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Guideline 12 2020 recommends that the outlet temperature for water heaters with any storage capacity be greater than or equal to 140 F; this includes heaters with separate storage tanks as well as semi-instantaneous heaters. The reason for this temperature recommendation for Legionella control is stratification. 

Even small semi-instantaneous heaters can have stratification; the heater outlet water temperature could always be 130 F or higher while the bottom of the heater could be consistently 85 F to 110 F. This is right in the tepid, high Legionella growth temperature range (see Figure 1 in Plumbing Engineer, June 2021, page 37). During high-demand periods, that bottom, bacteria-laden water could quickly pass through the heater and get out into the hot water distribution system piping. 

Importantly, this greater-than-or-equal-to-140-F heater outlet requirement has negligible impact on energy efficiency. The heater has a relatively small surface area when compared to building hot water supply and return piping, and it has relatively high levels of insulation. Maintaining heaters at a higher temperature, then reducing that temperature in a master mixing valve, provides an additional beneficial result of storing more BTUs in the heater for peak demand periods. 

Even if the facility has a secondary disinfection program in place for Legionella control, if the heater has any storage volume, it should be maintained at 140 F or more to comply with ASHRAE, Occupational Safety and Health Administration (OSHA) and all other guidance documents. 

One of the most important statements in ASHRAE Guideline 12 is: “The most effective control for most diseases, including Legionellosis, is prevention of transmission at as many points as possible in the disease's chain of transmission. The rationale for this is that if one preventive measure fails, others will be in place and act as fail-safe mechanisms. With this philosophy in mind, it may be desirable to design interventions to prevent transmission of Legionellosis at as many points as possible in the disease's chain of
transmission.” 

Outbreaks can occur for multiple reasons. When investigating outbreaks, the first step is to determine if the standards of care were implemented. Maintaining water heaters at 140 F or greater is a well-known and well-established standard of care.

120 F to 130 F in Building Hot Water Supply/Return

As discussed in the first article in this series: “The recommended temperature range (in 2021 UPC Appendix N) with the best Legionella control 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.” 

I just completed an audit of a children’s hospital and found water in all patient restrooms and clinician sinks to be within this 120 F to 130 F range; showers were slightly below this range. Unfortunately, many believe the public faucet temperature limit of 110 F is related to scald risk. The public handwashing sink plumbing code requirement has nothing to do with scald risk; it is only intended as an energy-saving code. It is based on the requirements of ASHRAE Standard 90.1 dating back to 1989. 

Not maintaining a temperature range of 120 F to 130 F in both the hot water supply and throughout the return has, in fact, been the source of many outbreaks. Some believe that because studies show killing Legionella bacteria requires hours at 130 F, then the hot water supply and return need to be maintained above 130 F at all times for Legionella control.

This is wrong and, in fact, counterproductive! Maintaining hot water supply and return significantly above 130 F increases disinfectant destruction, corrosion rates and corrosion byproducts of zinc, iron and copper in solution; these metals further increase disinfectant destruction. Since Legionella doesn’t grow above 116 F, then maintaining the hot water supply and return at 120 F or higher 24/7 will control it. In short: 

1. Maintaining the hot water supply and return between 

120 F to 130 F is good.

2. Maintaining the hot water supply and return 

significantly greater than 130 F is not good.

As stated in the first article of this series, the Quincy Veterans Home implemented the VA Directive 1061 protocols after its 2015 outbreak. This 1061 directive includes many statements that are not correct, including, “Water temperatures at [124 F] ([51.1 C]) or higher are necessary to inhibit Legionella growth in hot water systems,” and “It is not possible to maintain water temperatures at the outlet that kill Legionella bacteria and simultaneously eliminate the possibility of scald injury in persons partially or fully insensitive to hot water temperature or having delayed or impaired response capabilities.”

Compare these statements with ASHRAE Guideline 12 2020, which correctly states that “Legionella growth slows, and they begin to die at water temperatures between 113 F and 120 F.” And the 2021 Uniform Plumbing Code Appendix N states that 120 F to 130 F provides excellent Legionella control with a low scald risk. 

After implementing strategies that fully complied with VA Directive 1061, at a very high installation cost and continuing high maintenance cost, the Quincy Veterans Home had three additional outbreaks — one each in 2016, 2017 and 2018. What our team found during the investigation was that the hot water supply temperature above 130 F destroyed all secondary disinfectant added to the system. Without any residual disinfectant, the thermal control program alone was not effective in preventing future outbreaks. 

Impact of Varying Temperature Schemes on Dead Legs, Drop Legs and Fixtures 

If the hot water return pump is constantly running, and the supply and return lines are consistently (i.e., 24 hours a day) maintained above 120 F, then Legionella will not grow in those circulating lines. Circulating the hot water supply and return between 120 F to 130 F does not protect the drop legs because even with excellent insulation, the temperature in those noncirculating lines quickly drops below 110 F. Even if the water is circulating at above 150 F, as was done at the Quincy Veterans Home, temperature alone is often ineffective in Legionella control in the drop legs. 

If the ambient temperature of the space surrounding the hot water drop legs is 70 F, and the temperature of the circulation loop is 120 F to 130 F, there will be a 50 F to 60 F gradient from the start of each drop leg at the loop to the end of the drop leg at each fixture (80 F at Quincy). At some distance away from the circulating loop, the temperature will be 110 F. And at some further distance away from the circulating line and closer to the fixture, the temperature will be
85 F, right in the high-growth range for Legionella. 

This temperature gradient and the high Legionella growth range exists, whether or not the drop legs are insulated. Here is the dilemma: Uninsulated drop legs act like wicks sucking heat out of the loop, which increases energy costs; the quicker the drop legs cool down to below 50 F, the better for Legionella management.

For energy reasons, ASHRAE 90.1 2020 has a provision requiring pipe insulation on at least the first 10 feet of each drop leg. The 2021 International Energy Conservation Code and the 2021 UPC include provisions requiring that the entire hot water distribution system be insulated from the water heater to the angle stops or valves near each fixture. 

Contrast this with the Dutch plumbing code, which prohibits insulation on hot water drop legs, knowing that the quicker the drop leg cools down below 85 F, the lower the Legionella risk. 

Pipe insulation on the drop legs will slow down but not prevent the temperature from dropping. This has the contrary effect of maintaining the temperature on each drop leg in the high growth range for a longer period.

Research into this pipe cool-down effect was published in 2005 by Dr. Carl Hiller, P.E., president of Applied Energy Technology (“Hot Water Distribution System Research — Phase 1”). The full report can be found at https://bit.ly/3yCWJuT. 

Drop legs are often 1/2-inch or 3/4-inch nominal pipe. While Hiller’s research was conducted for another purpose, the results are relevant to this discussion. Figure 1 shows the cool-down times for copper pipe. The starting temperature was 135 F and the ambient air temperature surrounding the pipe was 65 F to 70 F, an average 67.5 F. Both the 1/2-inch and 3/4-inch uninsulated pipe cooled down to 105 F in less than 25 minutes, the upper end of the high Legionella growth zone. 

Pipe insulation with a wall thickness of 3/4-inch doubled the cool-down time of 1/2-inch pipe to 40 minutes and almost tripled the cool-down time for 3/4-inch pipe. It will take longer to cool down to 85 F, the lower end of the high Legionella growth zone and still longer to reach ambient temperature. It will take roughly an hour for the uninsulated piping to get below 85 F and two to three hours for the insulated piping. 

While keeping the drop legs hot has the beneficial effect of keeping the water hot for users of the fixtures on each drop leg when the events are clustered close together, it also has the detrimental effect of maintaining the temperature of the water in the drop legs within the high growth range for Legionella for much longer periods of time. 

But what happens if the drop legs never cool down? Hiller’s research examined the cool-down effects on piping that did not have a constant source of heat at the beginning of a drop leg. ASHRAE Guideline 12 2020 recommends that if using temperature as the sole method of Legionella control, temperatures need to be continuously maintained above 120 F in all parts of the system. 

In large buildings, the water heater is generally located in a mechanical room far from the fixtures. To bring the source of hot water closer to the fixtures, there is often a circulation loop or an electrically heat-traced pipe, either of which provides a constant source of heat at the beginning of the drop legs. This temperature gradient will continue to cause heat to leak into the drop legs due to conduction and convection, effectively keeping a section of pipe on each drop leg within the high growth range for Legionella 24/7.

Starting with higher temperatures entering the hot water supply piping increases the temperature gradient and increases the time within the high growth zone. Higher distribution temperatures do not reduce the risks; they cause the risky conditions to last longer.

Future articles in this series will explore more ways to simultaneously minimize the energy losses in hot water distribution systems and the risks for Legionella growth. We also will examine the effects of several key components on Legionella growth.