Plumbing Engineer October 2021/55
By Tim Keane and Gary Klein
ary 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 tem- perature 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 strati- fication; 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 effi- ciency. 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 tem- perature 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 stor- age 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 diseas- es, including Legionellosis, is prevention of transmission at as many points as possible in the disease's chain of transmis- sion. 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 deter- mine 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.
Cost-Effective Control, Part 3
Temperature alone is often ineffective in Legionella control in the drop legs, whether or not they are insulated.Previous Page