Four steps to take to Control Legionella Bacteria Growth in Potable Water Systems per ASHRAE Guideline 12-2020

In May 2020, the American Society of Heating, Refrigerating and Air-Conditioning Engineers(ASHRAE) released the long-anticipated update to their ASHRAE Guideline 12, Managing the Risk of Legionellosis Associated with Building Water Systems. Guideline 12 was originally released in 2000 to provide detailed environmental and operational advice to minimize the risk of Legionella bacteria growth in building water systems. ASHRAE’s Guideline 12 is a prescriptive document for the implementation of a Water Management Program based on the ANSI/ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems

Source: Jiawei cui from Pexels

Below are four steps outlined in the guideline that should be implemented in your building water system to reduce the risk of transmission of Legionellosis:

1. Keep the System Clean and Free of Sediment

Legionella bacteria can grow in various building water systems including but not limited to:

  • Potable Water Systems
  • Ornamental Water Fountains
  • Whirlpools Spas/Hot Tubs
  • Cooling Towers and Evaporative Condensers
  • Misters
  • Ice Machines
The accumulation of sediment can provide the perfect environment for biofilm growth and therefore Legionella bacteria growth. Routine cleaning and maintenance are crucial for building water systems to reduce conditions that support Legionella bacteria proliferation.

2. Control Hot-Water and Cold-Water Temperatures

Water temperature plays an important factor in the growth of Legionella bacteria. Studies have shown that Legionella can grow at temperatures between 77°F and 113°F. As temperatures decrease, growth slows, and Legionella bacteria become dormant but do not die. Growth of the bacteria slows at temperatures above 113°F. As the temperature increases the time for Legionella bacteria to die shortens and the bacteria will die rapidly at temperatures at or above 158°F.

Source: ASHRAE Guideline 12-2020 Figure 1 Temperature effects on survival and growth of Legionella in laboratory conditions.

Hot water storage tanks should be maintained to consistently deliver water at or above 140°F. Because these temperatures may pose a scalding risk, mixing valves should be installed and maintained per manufacturer recommendations.

3. Minimize Water Age

Water age is the time water resides in the water system. The probability of Legionella bacteria proliferation increases with water age. Stagnant water or low-flow conditions can cause loss of disinfectant residuals and temperatures favorable to Legionella growth.

Routine flushing should be implemented as a control measure to replace aging water and help purge sediment and deposits from the building water system.

4. Maintain a disinfectant residual

It is important to maintain a disinfectant residual throughout the building water system. Water received from a water utility has gone through some method of disinfection utilizing disinfectants such as chlorine or monochloramine. However, as water travels through the distribution piping, the disinfectant levels begin to dissipate. Furthermore, buildings with complex plumbing may see disinfecting residuals further dissipate as water travels to the points of use within the building.

Supplemental disinfection for a building water system can be considered as an additional control measure to maintain a disinfectant residual throughout a building water system.

Following just one of the outlined control measures does not ensure the control of Legionella bacteria growth which is why implementing a Water Management Program per ASHRAE Standard 188 and ensuring all listed control measures are completed is one of the most effective ways of reducing the risk of Legionella bacteria growth in your facility.

Barclay Water Management can support your facility with many of the control measures mentioned above as well as with the implementation of Water Management Program. Contact us today for more information.


Learn more about ASHRAE Guideline 12 and it’s 2020 update in our presentation.

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ASHRAE Guideline 12-2020 can be purchased from the ASHRAE Bookstore:

A read-only version of the guideline can be accessed through the following link:

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