How to Avoid Legionella Outbreaks in Domestic Hot Water Systems

By Chad Edmondson


Domestic hot-water systems are frequently implicated as the source of legionellosis (Legionnaires’ disease) outbreaks. Very small quantities of Legionella, the bacteria that causes the disease, is commonly found in the ground, in water, and even in tap water. Legionella is not likely to become a health risk unless it begins to multiply and colonize.  If not properly designed or maintained, a domestic hot water system (as well as other open water systems) can facilitate such growth.  

Two environmental factors will favorably impact the growth of Legionella in a domestic hot water system: low water temperature and stagnation.  Here’s how to avoid both:

Maintain Water Temperature at 140°F.This includes supply water and any hot water storage devices.  Legionella can survive in a temperature range of 68 to 122°F and grow in a temperature range of 77 to 108°F.  However, the bacteria are known to survive outside these ranges if it can find a biofilm in which to hide.  That’s why it is important to maintain hot water systems at 140°F degrees, including any hot water storage vessels.  A high/low flow mixing valve can be utilized to supply 120°F to the fixtures.  While it may be tempting to use temperature setback as a means to save energy, it is better to conserve energy elsewhere as this increases the vulnerability of any hot water system.

Avoid “Dead Legs” in Piping Design.  Stagnation in hot water systems leads to the accumulation of scale and biofilm buildup.  Such environments provide protection and nutrients that enable Legionella to survive and multiply.  Therefore it is important to avoid any areas of stagnation in the hot water piping.

Use Recirculation Pumps Continuously.  Designing a hot water system with recirculation pumps not only saves water by maintaining hot water at the point of use, it minimizes scale and stagnation.  This is another area where health safety trumps energy conservation. 

Make sure backflow preventers are operating properly.  A malfunctioning backflow preventer can cross-contaminate domestic hot water with water i.e. process water.

Perform routine maintenance measures on hot water tanks.  This includes draining the tanks periodically to remove scale and sediment as well as regular chlorination measures.