Section II:D. Heating,
Ventilating, and Air Conditioning (HVAC) Systems
 Legionnaires' 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.
Control Strategy:
 What
to consider in the system design |
Operate and maintain all HVAC equipment as originally designed, and maintain it so that it can perform as designed. Test all HVAC equipment periodically to ensure that it is performing as designed. Consider the following issues when designing HVAC systems; most apply to all types of microbial contamination:
- Minimize use of water reservoirs, sumps, and pans.
- Provide a way to drain water sumps when not in use, such as, an electric solenoid valve on the sump drain.
- Provide a "bleed" for water sumps so that dissolved solids do not form sediments in the sump.
- Slope collection pans and drain sumps from the bottom so that all the water can drain out and allow the pan to dry.
- Locate HVAC fresh-air intakes so that they do not draw the mist from a cooling tower, evaporative condenser, or fluid cooler into the system. When evaluating this path, consider:
- Prevailing wind direction and velocity.
- Building effects such as low-pressure zones on leeward sides of buildings and on roof.
- Architectural screen walls.
- Distance from tower to intake.
- Design indirect evaporative cooling systems with the knowledge that the failure of the heat exchanger will allow wet systems to mix with the air-distribution systems.
- Do not use raw steam from the central heating boiler to humidify air because it contains corrosion inhibitors and anti-scaling chemicals.
- Atomizing humidifiers should have contaminant-free water.
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How
to maintain a HVAC system |
 Inspect the entire air distribution system (including return and exhaust systems) for visual evidence of water accumulation.
- Properly drain all sumps and permanently drain
inactive sumps to prevent accumulation of sediments.
- If an HVAC sump is used during the hours when a building is occupied, drain the sump during unoccupied hours.
- Maintenance failures can produce stagnant water that can become an ideal environment for
LDB growth if sufficiently warm (such as heated by sunlight).
To effectively control contamination, be aware of the 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:
- 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).
- Aerosols from spray irrigation.
- Open windows.
- Internal sources may provide contaminated aerosolized water that is then disseminated by the air-distribution system. Consider the following:
- HVAC system humidifiers are potential sources of aerosol exposure if contaminated
with LDB. See Section
II:B. Humidifiers and Misters.
- Direct evaporative air coolers with sprays or misters used as humidifiers include sumps, which may stagnate when not in use.
- Indirect evaporative air cooling systems using water coils may develop a leak that may inject cooling tower water directly into the supply air stream.
- 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.
- 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.
- Residential humidifiers, such as free-standing or portable units, often contain sumps that are frequently contaminated with
LDB.
- Computer room air conditioners may contain a humidifier sump filled with contaminated water.
- Improperly drained condenser pans may produce tepid conditions that can encourage microbial and fungal growth.
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How
to treat a contaminated system |
In the event
LDB is detected, follow these steps for treatment:
- Eliminate all water leaks and remove any standing water found in the system to ensure future drainage.
- Replace or eliminate any water-damaged insulation in the system.
- Operate the HVAC system using 100 percent outside air for eight hours before returning the building to normal operation.
- When the building is returned to normal operation, keep outside-air supply rates as high as possible for one month. At a minimum, the outdoor air requirements of ASHRAE Ventilation Standard 62-2001
should be met.
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