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How to maintain a HVAC system for Legionella Control

Jan 4, 2018 9:50:00 AM

CES HVAC Maintenance Legionella Control.jpgLegionnaires' disease bacteria (LDB) cannot survive without water, and a properly operated, well-maintained HVAC system is unlikely to be a source of problems unless water contaminated with the bacteria enters the system.  

Air conditioning units without humidifiers have not been identified as sources of LDB.  For a Legionnaires' disease outbreak to be linked directly with the HVAC system, LDB-contaminated water must enter the system, be aerosolized, and be delivered to building occupants.

CDC Legionella Toolkit 

To effectively control contamination, be aware of the internal and external conditions that may promote growth and distribute LDB:

External sources may emit contaminated aerosolized water that is drawn into a system's fresh-air intake. Consider the following:

  1. Fresh-air intake airways, typically concrete plenums located at grade level, supplying fresh air to air handlers in the basement or lower levels of buildings can collect organic material (such as leaves and dirt).
  2. Aerosols from spray irrigation.
  3. Open windows.
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Internal sources may provide contaminated aerosolized water that is then disseminated by the air-distribution system. Consider the following:

  1. HVAC system humidifiers are potential sources of aerosol exposure if contaminated with LDB.
  2. Direct evaporative air coolers with sprays or misters used as humidifiers include sumps, which may stagnate when not in use.
  3. Indirect evaporative air cooling systems using water coils may develop a leak that may inject cooling tower water directly into the supply air stream.
  4. Air-to-air heat exchangers may develop leaks, which may allow the wet air stream to mix with supply air and cause problems if the wet air stream is contaminated with LDB
  5. Wet evaporative coolers, slinger air coolers, and rotary air coolers with improperly operated and maintained systems that use warm, stagnant sump water may be potential sources of LDB.
  6. Residential humidifiers, such as free-standing or portable units, often contain sumps that are frequently contaminated with LDB.
  7. Computer room air conditioners may contain a humidifier sump filled with contaminated water.

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  1. Locate cooling towers to minimize exposure to people. Place the towers as far away as reasonably possible (preferably at least 100′) from operable windows, outdoor air intakes, parking lots, roads, driveways, sidewalks, and outdoor areas frequented by people. Furthermore, make ground level cooling towers inaccessible to visitors and passers-by (e.g., with a fence or wall).

The only way to estimate Legionella concentrations is to test for Legionella, so that should be done once every month or two during the operating season.  If Legionella is not found in the tower, Facility Managers should not relax the water treatment or preventive maintenance.  If Legionella is found, particularly at levels greater than 10 colony forming units per milliliter (cfu/ml), then the treatment regimen should be adjusted appropriately.

Additional Resources:

CDC Toolkit for Legionella Water Management

Legionella Risk Management Healthcare

The information above has been adapted from the following reference:

OSHA HVAC Maintenance Legionella Prevention


Comprehensive Energy Services

Written by Comprehensive Energy Services



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