4 Ways to Minimize Legionella Bacteria Growth in Hot and Cold-Water Tanks

Written by: Hilary Nardone, Lead Environmental Group Manager

Due to their design and the temperatures of the water they store, drinking water tanks often harbor waterborne pathogens, including Legionella bacteria. Under certain conditions, bacteria can amplify in storage tanks causing bacteria to be distributed throughout your building water systems.

Here are four ways to minimize Legionella growth in potable water tanks:

1.Maintain temperatures that do not promote bacterial growth

Legionella bacteria proliferate best when the water temperature is between 77-120°F. As temperature rises above the optimal range, growth slows, then Legionella bacteria begin to die. The higher the temperature, the more quickly the bacteria will be killed: water temperatures 120-140°F will slowly kill Legionella bacteria, while temperatures above 160°F will rapidly kill the bacteria.1

*ASHRAE Guideline 12-2008R, Third Public Review (November 2018)

OSHA states that water heaters maintained below 140°F may foster Legionella bacteria growth.Similarly, ASHRAE Guideline 12-2000: Minimizing the Risk of Legionellosis Associated with Building Water Systems recommends that, where practical in health care facilities, nursing homes, and other high-risk situations, hot water should be stored above 140°F.New York State Department of Health (NYSDOH) recommends hot water be stored above 140°F when a facility has the necessary mixing valves or anti-scald valves to reduce the final water temperature to that required by local codes.3

Conversely, as the water temperature decreases below the optimal growth range, growth rate slows. To minimize growth of bacteria in cold water storage tanks, ASHRAE Guideline 12-2000 recommends that water be stored at less than 68°F.NYSDOH recommends that potable cold water be stored at less than 68°F.3

2.Ensure Each Tank Has a Circulation System to Minimize Stratification and Stagnation

Tank circulation systems prevent both water stratification and water stagnation. Water that sits stagnant in storage tanks can increase Legionella bacteria growth. Disinfectant levels decrease as water sits stagnant, making the water more susceptible to Legionella bacteria growth. Additionally, stagnant water can drop in temperature, again increasing the water’s vulnerability to Legionella bacteria growth.

Water stratification can occur when water sits stagnant. Because heat rises, as water sits in a tank, the hot water tends to rise to the top of the tank, while colder water sits on the bottom. This creates varying water temperatures within the tank, increasing the water’s susceptibility to Legionella bacteria growth. A tank that circulates water within itself helps to prevent stratification and keep water temperature consistent. In addition to circulation, ASHRAE Guideline 12-2000 recommends that to minimize stratification, hot water heaters and tanks should drain at the lowest point possible and the heating element should be located as close as possible to the bottom of the tank to facilitate mixing.1

3.Regularly Clean and Disinfect Water Tanks

Regularly cleaning and disinfecting potable water storage tanks can help to minimize the growth and amplification of Legionella bacteria. If tanks are not cleaned regularly, sediment can build up on the bottom. A study by Qin et. al. found a positive association between opportunistic pathogens (including Legionella pneumophilia) and nutrients found in water storage tank sediment.4

The Association of Water Technologies (AWT) states that Legionella bacteria can grow in many parts of building water systems including hot- and cold-water storage tanks, water heaters, and expansion tanks. The organization recommends flushing and cleaning hot water tanks as part as an overall risk assessment and management plan of the entire building water systems.5

Additionally, ASHRAE Guideline 12-2000 recommends that hot or cold systems that incorporate an elevated hold tank should be inspected and cleaned annually to minimize Legionella bacteria growth.1

In New York City, all domestic water tanks must be drained and cleaned at least once per year in accordance with Chapter 6 of the NYC Plumbing Code.6

NYSDOH also recommends that hot water storage tanks be drained, cleaned and disinfected according to manufacturer’s recommendations or at least annually.3

4.Test for Legionella bacteria

The ideal way to objectively validate any Legionella bacteria growth minimization effort is to regularly test for Legionella bacteria. The Centers for Disease Control and Prevention recommends that each water holding tank be sampled.AWT also recommends that storage tanks should be included in a facility’s overall Legionella bacteria sampling plan.5

NYSDOH advises, as part of their routine Legionella bacteria sampling plans in healthcare facilities, that one water sample for the return piping of the circulating heating system(s) and one water sample of the outlet of the heating system(s) be taken quarterly.3


New York City Facilities: Remember that in addition to draining and cleaning your tanks at least annually, you must also inspect each tank including taking a coliform sample (per NYC Health Code Article 141).8

Remember: Barclay is here to help!

Barclay is permitted in building water tank cleanings by the City of New York Department of Health and Mental Hygiene!

We also offer an array of Legionella bacteria and coliform sampling services for domestic water tanks, including track and trending results.



1.ASHRAE Guideline 12-2000.Minimizing the Risk of Legionellosis Associated with Building Water Systems.Published 2000.

2.OSHA.Control and Prevention.Legionellosis (Legionnaires’ Disease and Pontiac Fever). https://www.osha.gov/SLTC/legionnairesdisease/control_prevention.html#hotwater.

3.New York State Department of Health.Environmental Health Information Related to Legionellosis in Healthcare Facilities. https://www.health.ny.gov/environmental/water/drinking/legionella/docs/eh_info_healthcare_facilities.pdf. Published April 2019.

4.Qin K, Struewing I, Domingo JS, Lytle D, Lu J.Opportunistic pathogens and microbial communities and their associations with sediment physical parameters in drinking water storage tank sediments.Pathogens. 2017(6)54.doi: 10,3390/pathogens6040054.

5.Association of Water Technologies.Legionella 2019: A Position Statement and Guidance Document. Published February 2019.

6.New York City Plumbing Code.Chapter 6: Water Supply and Distribution. Published 2008.


7.CDC.Sampling Procedure and Potential Sampling Sites. https://www.cdc.gov/legionella/downloads/cdc-sampling-procedure.pdf. Published June 2015.

8.New York City Department of Health and Mental Hygiene.Health Code Article 141. https://www1.nyc.gov/assets/doh/downloads/pdf/about/healthcode/health-code-article141.pdf.

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