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Stagnation Increases Health Risks

COVID-19 “stay-at-home” orders have reduced water usage in facilities across the globe as buildings are shutdown or operated with low occupancy. The resulting stagnation in building water system equipment, piping, and fixtures can increase the risk for Legionella and other biofilm associated bacteria to grow, contribute to unsafe levels of lead and copper in potable water systems, and cause other serious problems.


This document was developed to provide building owners and operators with an overview of strategies that can be taken now and when buildings reopen to help address the Legionella risks associated with stagnation. However, due to the complexity of building water systems, the impact of operational and maintenance practices, and the universal nature of Legionella bacteria, no plan, program, or maintenance measures can guarantee the absence of Legionella or transmission of Legionnaires’ disease.

 Core Risk Management Strategies

Building water systems can present significant Legionella risks even when fully occupied. Stagnant water increases these risks by causing conditions that favor the growth of biofilms, which are microscopic colonies of surface-attached bacteria directly linked to the growth of Legionella bacteria, and other problems including corrosion and biofouling.


Without flow, the chlorine disinfectant added to the water supplied to a building degrades as it stagnates in potable water systems, causing residuals to drop to ineffective levels. Similarly, the lack of flow in cooling tower systems, decorative fountains, and hot tubs prevents biocide or disinfectant additions from being effective. To compound the problem, stagnation may allow water temperatures to stabilize into the ideal range for Legionella growth (77 to 108°F).


The U.S. Centers for Disease Control (CDC) recommends that building owners follow a recognized process such as ASHRAE 188 to assess Legionella risks and implement a site specific Water Management Program (WMP). Where an active Legionella WMP is in place, the Plan and the Program Team should be referred to for guidance. 


The WMP development process tailors maintenance and monitoring protocols to the Legionella risks associated with the building water systems and occupants. For example, procedures appropriate for a commercial office building would not be adequate for a facility with high risk occupants (assisted living, hospital, etc.). Likewise, the startup procedures for a building with an established flushing program during shutdown might be a lot different than the requirements where the building water systems were stagnant. Professional assistance is recommended to evaluate these factors so that appropriate measures can be taken.

With the rapid onset of COVID-19, many impacted buildings will not have a WMP or Program Team in place to provide direction. Regardless, it is important for building owners to understand the basic strategies for addressing water system stagnation so they can be used as appropriate. A site specific legionella WMP should als be developed and implemented when feasible.

Strategies for Potable Water Systems and Equipment

Potable Water Systems

  • Flush hot and cold water through all points of use (showers, sink faucets) weekly for at least two minutes after final temperatures (hot and cold) have stabilized. Flushing may need to occur in segments due to facility size and water pressure.  
  • The flush time required to replace the water inside building piping and equipment with fresh water will vary based on system design and operational factors. The flushing program (frequency and duration) should be verified by objective data such as water quality parameters, system volume and flow rates, etc.
  • Before returning the potable water systems to service, you should consult a qualified professional for site specific guidance. Factors to consider include the risk profile of site occupants, duration of shutdown, maintenance measures taken during shutdown, etc. 

Verify Potable Water Disinfectant Residuals

  • Chlorine residuals should be monitored at the point of entry to verify sufficient disinfectant levels are provided to the building.  Contact your Public Water Supplier (PWS) if the incoming chlorine residuals do not meet minimum standards.
  • Chlorine residuals should also be monitored at locations throughout the building to ensure flushing minimizes degradation of disinfectant residuals to the points of use. 
  • Free chlorine residuals should be monitored if the building is supplied with chlorinated water, and total chlorine residuals monitored if supplied with chloramine treated water. Consult with your PWS or Annual Water Quality Report if unsure.

Domestic Water Heaters

  • Maintain water heaters at their designated temperature set point (ideally at or above 120°F), and continue to operate installed hot water recirculation pumps (where equipped).
  • Alternate redundant water heaters frequently to reduce water stagnation. Facilities should ensure all hot water return pumps are operational and set to circulate 24/7 in order to assist with maintaining water flow. 
  • Flush hot water heaters and storage tanks from their lowest points at least once weekly to remove sediment and scale.

Hot and Cold Domestic Water Storage Tanks

  • If the daily building water usage does not exceed the volume of storage tanks, consider adding supplemental chlorine disinfectant and/or implementing flushing procedures to increase water throughput.
  • For a long term shutdown, drain, clean, and disinfect domestic water storage tanks. 
  • Before returning drained storage tanks to service, establish a recommissioning procedure.

Showerheads and Hoses

  • Consider the following additional measures for showerheads, hoses, and wand assemblies before returning to service:
  • Remove the showerhead, shower wand and hose assembly, and manually disinfect with bleach before reinstalling.
  • Where disinfection is not practical, consider replacing the showerhead, shower wand, and hose assembly.

Electronic and Manual Faucets

  • Consider cleaning and disinfecting the aerators and easily accessible components before returning to service.

Ice Machines

  • When low occupancy is expected for an extended period of time, shut off ice machines and disconnect the water supply lines.
  • Disinfect the inlet water supply line and install new filters prior to start up. Consider replacing piping from the wall to the machine.
  • Ice machines should be cleaned, descaled, and disinfected before placing back in service.
  • Upon start up, the first load of ice should be dumped and discarded before resuming ice making for human consumption.

Eye Wash Stations and Emergency Showers

  • Eye wash stations should be flushed weekly.
  • Safety showers should be flushed monthly.

Strategies for Cooling Tower Systems 

  • Where possible, keep HVAC and process cooling tower systems operational. Do not discontinue water treatment and other maintenance measures. Leaving cooling tower systems filled with stagnant water can result in severe corrosion, biofouling problems, and contribute to the transmission of Legionnaires’ disease.
  • Specific water treatment protocols may be required when cooling tower systems operate under low load conditions. This includes circulating biocide treated water through any off-line or load cycled cooling towers, chillers, heat exchangers, and condenser water piping at least twice weekly. 
  • If a cooling tower system cannot be operated during an extended shutdown, it is recommended that all associated cooling towers, chillers, heat exchangers, and piping be completely drained. Where draining is not possible, consult your water treatment service provider to discuss alternate measures and specific cleaning and disinfection requirements prior to startup.
  • Offline cooling tower systems and auxiliary equipment such as sand filters should be cleaned and disinfected using an established protocol prior to returning to service. If cooling towers cannot be drained and physically cleaned prior to startup, online cleaning methods should be considered.

Strategies for Decorative Fountains

  • Drain decorative fountains completely unless approved treatment and monitoring protocols are maintained.
  • All indoor and outdoor decorative fountains and man-made aesthetic fountains, water walls, cascades, etc., not in operation for 3 or more days must be drained, and all components cleaned and disinfected before start up. 

Strategies for Swimming Pools, Hot Tubs, and Whirlpool Spas

  • All swimming pools, hot tubs, and whirlpool spas must be operated and maintained in accordance with manufacturer’s recommendations, state & local licensure and health permitting processes, and the CDC Model Aquatic Health Code. 
  • If one of these devices will be out of service for greater than three days, it should be completely drained unless approved treatment and monitoring protocols are maintained. 
  • All systems should be properly cleaned and disinfected prior to placing back into use.

 Strategies for Misters, Atomizers, and Humidifiers

  • All misters, atomizers, and cold-water humidifiers must be cleaned, disinfected, and have their filters changed (where installed) before being placed back into service.


  1. ASHRAE Standards Committee. (2018) ANSI/ASHRAE Standard 188-2018. Legionellosis: Risk Management for Building Water Systems. Atlanta, GA.
  2. ASHRAE Standards Committee. (2008) ASHRAE Guideline 12-2000. Minimizing the Risk of Legionellosis Associated with Building Water Systems. Atlanta, GA.
  3. CDC. (2020) Guidance for Building Water Systems. CDC.
  4. AWWA. (2014) ANSI/AWWA C651-14. AWWA Standard Disinfecting Water Mains. American Water Works Association.
  5. ESPRI, AH Environmental Consultants, Bartrand, T., Masters, S., Hargy, T., Mccuin, R., Clancy, J., Theiss, R., Pommerenk, P., Mcnamara, S. and Hiltebrand, D. (2020) Building Water Quality and Coronavirus: Flushing Guidance for Periods of Low or No Use.
  6. ESGLI, (2020) European Society of Clinical Microbiology and Infectious Disease Study Group for Legionella Infections. ESGLI Guidance for Managing Legionella in Building Water Systems during the COVID-19 Pandemic.
  7. IAPMO. (2018) Uniformed Plumbing Code (UPC), 28th Edition. Ontario, CA USA.
  8. Purdue University, Proctor, C., Rhoads, W., Keane, T., Salehi, M., Hamilton, K., Pieper, K., Cwiertny, D., Prevost, M. and Whelton, A. (2020) Considerations for Large Building Water Quality after Extended Stagnation.

For additional information and support addressing water stagnation issues in building water systems, supplemental potable water disinfection systems, or any other water management concerns, please contact your local Chem-Aqua Representative or the Chem-Aqua Water Risk Management Services Group by calling 866-209-3373 or emailing


Disclaimer: Due to the universal nature of Legionella bacteria, and its ability to reappear and colonize building water systems, no plan, program, or maintenance measures can guarantee the absence of Legionella or other waterborne pathogens, or eliminate the potential for associated disease. No warranty is made as to the completeness or effectiveness of the risk reduction practices as taught by various published guidelines or governing authorities. Recommendations also do not guarantee that water quality within the assessed systems or procedures will meet governmental or non-governmental standards. Recommendations for Legionella risk reduction may go above and beyond standards or practices intended for optimizing performance or meeting other requirements. 

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Building Water Systems and Devices Impacted By Stagnation Risks

  • Potable Water Systems and Equipment
  • Showers and Hoses
  • Faucets
  • Ice Machines
  • Eye Wash Stations
  • Emergency Showers
  • Cooling Towers
  • Decorative Fountains
  • Swimming Pools, Hot Tubs and Spas
  • Misters, Atomizers, and Humidifiers

Core Strategies for Managing
Biofilms and Legionella Risks

  • Minimize Stagnation By Routine Flushing and/or Circulation
  • Keep Systems Clean and Free From Sediment
  • Keep Hot Systems Hot and Cold Systems Cold
  • Monitor and Maintain Disinfectant Residuals
  • Develop and Implement a Legionella Water Management Plan