These two articles will prove timely for plumbers over the next several years if not decades:
First, this one in Plumbing & Mechanical gives a great summary of the issues (read the entire article):
Many of the hot topics in plumbing center on one word: Legionella. Legionella is the shortened word used to identify the bacteria that causes Legionnaires’ disease. Unfortunately for the plumbing profession, Legionella can exist in the potable water supply. If it does exist, it can multiply to the point of sickening building occupants with Legionnaires’ disease.
You may hear mention of ASHRAE 188, “Legionellosis: Risk Management for Building Water Systems.” This is a risk-management standard that is designed to reduce possible exposure to the bacteria. There are many sections that reference plumbing systems in this standard.
While ASHRAE 188 is not being referenced in the plumbing codes, some of the concepts within the standard are being analyzed. The issues involving plumbing include: temperature of the hot water, stagnant sections of piping or dead legs, slow-moving water, location of water heaters, and rarely used fixtures.
What should jump out at you should be two things — water not moving fast enough and lukewarm water. Without going into tremendously boring detail, Legionella multiplies, or grows very well, on a biofilm at water temperatures ranging from 85° to 110° F. Biofilm is that slimy stuff on the inside of a water pipe. It is also what is on your teeth in the morning before you brush.
If you move the water quickly enough, it is hard for a biofilm to grow. If you raise the temperature of the hot water above 131°, you start to kill off Legionella bacteria. The higher the water temperature, the faster you kill the bacteria.
This second article from Backflow Prevention Journal gives more specifics about the unintended consequences of water temperature adjustments (read entire article):
The minimum or maximum temperature of water is something that has become topic of discussion in many code hearings and out in the industry itself. If water is too hot, then burns or scalding can be a health and safety issue. More recently the spotlight has been turned on the issue of Legionella. Legionella is an issue in water systems and depending on the water temperature Legionella can multiply and grow within the system.
Temperature affects the survival of Legionella as follows:
- Above 70 °C (158 °F) – Legionella dies almost instantly
- At 60 °C (140 °F) – 90% die in 2 minutes
- At 50 °C (122 °F) – 90% die in 80–124 minutes, depending on legionella strain
- 48 to 50 °C (118 to 122 °F) – can survive but do not multiply
- 32 to 42 °C (90 to 108 °F) – ideal growth range
- 25 to 45 °C (77 to 113 °F) – growth range
- Below 20 °C (68 °F) – can survive, even below freezing, but are dormant
I hope you will take a quick look at these two articles to prepare for the growing prevalence of Legionella control concerns, which will place plumbers even more clearly at the front line of public health protection.
[Constructive comments worthy of licensed professionals are welcomed below.]