Lower risk of Legionella from instantaneous steam (tank-less) heaters?

 I am a drinking water engineer at a Department of Health. The Health Department by statute is a member of the state Plumbing Board. Recently, the Plumbing Code was updated and due to the Health Department’s efforts to reduce the risk of Legionella, all water tanks must now be maintained 140 degrees F. To prevent scalding, mixing valves are required, so that at the tap, consumers don’t receive water hotter than 120 degrees F. at any tap/shower.

We have been approached by the manufacturers and lawyers of tankless heaters for a waiver. Their argument is that tankless heaters don’t have a reservoir where Legionella could amplify, their product is simply a flash heater of the cold water line. As we don’t treat cold water per se for Legionella, I think their argument holds merit, but I deferred to our nurse epidemiologist. She provided me with the ASHRAE standards which don’t exactly address this situation, but do quote 140 degrees for health care facilities hot water storage and quoted the Legionella growth temperature range of 77-108 degrees F.

Would you be able to comment on tankless heaters and the efficacy of holding them to a standard promulgated specifically to reduce Legionella risk?

We once thought, as you suggested, that elimination of the large volume hot water heater would significantly reduce  Legionella colonization in the hot water system. In a survey of 15 hospitals in western Pennsylvania, in 2 hospitals with instantaneous steam heating systems (tankless heaters) no Legionella was isolated from these hospitals. We then replaced the 1000+ gallon hot water tanks in one of our VA hospitals with instantaneous heaters in an effort to decrease Legionella colonization. These heaters flash heated the cold water to 190o and then blended the water back down to the pre-set temperature (which was 140oF). Disappointingly, this installation had virtually no effect on the downstream colonization of fixtures. We now understand that the entire network of pipes is coated with a slime layer (biofilm) within which  Legionella resided. The bulk of the  Legionella colonization of the water system was downstream of the hot water tanks! This is why active disinfection throughout the system is required to control Legionella bacteria in warm water systems. So, if the temperature of the blended water as it exits from the tankless heater is 140oF, then that should satisfy the Health Department’s objective.