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Growing Pains: Dead Legs Lead to Live Legionella
A Cost-prevention Plumbing Feature Can Create Unintended Difficulties
by David S. Jessup, M.S., P.E., C.E.M., Senior Engineer

Construction of new facilities typically incorporates a measure of foresight, for example, anticipation of future expansion of building utilities to support internal growth of the company. However, some design features are now being discovered to inadvertently promote internal growth of another kind. Installation of domestic water and condenser water piping in preparation of future expansion has been found to encourage the colonization of bacteria within the building’s piping systems. Unfortunately, these
so-called “dead legs” in the piping systems can create significant problems down the road for the building owner and occupants, because the water in these areas does not tend to circulate, and warm, stagnating water creates a particularly encouraging environment for many kinds of bacteria, including Legionella
Prior to breaking ground on a new building, as institutions or corporations plan for expansion or a development firm invests in a new facility, they consider the schematic design for the necessary utilities, including the building’s heating, ventilating and air-conditioning (HVAC), electrical, and plumbing systems. These systems are often laid out in such a way that is intended to ease future expansion. The planned or renovated space may, for example, only require three chiller units in order to accommodate the building’s HVAC system, but a fourth may some day prove necessary. To accommodate future needs, extra condenser water piping may be installed and capped off, so that the additional chiller and/or cooling tower equipment may later be easily incorporated. Unfortunately, these so-called “dead legs” in the piping systems can create significant problems down the road for the building owner and occupants, because the water in these areas does not tend to circulate, and warm, stagnating water creates a particularly encouraging environment for many kinds of bacteria, including Legionella.
Legionella are not an exotic strain of bacteria; they normally inhabit freshwater bodies, including ponds, rivers, and even moist soils. That means that these bacteria already populate the ultimate water sources for most buildings—which is typically either a municipal reservoir or a dedicated well. While the water source normally contains relatively small, innocuous amounts of the bacteria, when they reach an appropriate environment, the bacteria can form larger colonies that warrant attention. They can potentially colonize any body of warm water, including domestic hot water sources and heating, ventilating and air-conditioning (HVAC) support systems.
The most common source of exposure to Legionella bacteria is through aerosolized water vapor, e.g., from outdoor air intakes located too closely downwind from a cooling tower. This presents special concern in healthcare facilities containing patients with compromised immune systems; however, others may be exposed, such as facilities personnel performing normal maintenance in close contact with the cooling tower and HVAC systems. Depending upon the circumstances, an outbreak within a building can impact not only workforce productivity, but have a significant impact on public relations.

Based on several recent cases, design engineers and building owners in particular should take notice: small features intended to prevent headaches could be increasing their exposure risk. It is now becoming apparent that HVAC and domestic plumbing dead legs have some significant trade-offs in terms of the cost-savings they’re intended to provide versus the potential risk they represent. As an illustration of the potential risk, the U.S. Centers for Disease Control and Prevention (CDC)[1] currently reports that between 8,000 and 18,000 cases of Legionella are reported in hospitals annually, although they estimate the actual cases to be far greater in number because many cases are not properly diagnosed or reported. To reduce liability given the potential for Legionella outbreaks, designers should include a careful review of the piping design relative to piping size (hence, fluid velocities), intended cooling tower operation, and dead legs associated with future installation.

The quantity of Legionella bacteria (in parts per billion) necessary to make a person become ill varies depending upon age and relative strength of the individual’s immune system. There are a number of exposure pathways. For example, aerosolized water can impact occupants of an affected space via decorative features such as water fountains, a shower or steam room, or through the HVAC system. Any plumbing system containing dead legs can feed one of or all of these exposure pathways. For example, a dead leg of a domestic water plumbing riser that is intended to feed a future expansion project does not receive a flow of water, creating a stagnant environment that can lend itself to the amplification of bacteria and ’seed’ the main portion of the connected system that is in use.

While there are treatment options for prevention of, and if necessary, remediation and decontamination after a Legionella event, dead legs in piping systems defy most of these treatments because of their resistance to water circulation. Water treatment chemicals often do not come in adequate contact with the dead leg/stagnant areas.
In practice, the anticipated benefits of this “problem-solving” feature should be weighed against the costs of a potential Legionella contamination outbreak and subsequent remediation efforts. The cost of such an event could also include shut-down time, lost productivity, and potential negative publicity. For new construction, careful consideration should be given to valve placement in the plumbing systems of the new building. Existing structures that already contain dead legs should be assessed in order to develop engineering solutions that would eliminate the stagnant areas and address these concerns.

About the Author:
EH&E staff have extensive experience resolving Legionella outbreaks and providing
site-appropriate engineering solutions and recommendations to prevent future outbreaks in a variety of large buildings and complex settings. For further information and consultation, please contact
David S. Jessup, M.S., P.E., C.E.M., at Mr. Jessup is a registered professional engineer with extensive experience providing assistance with Legionella concerns.

[1] U.S. Centers for Disease Control and Prevention (CDC)  accessed February 7, 2008


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