Turning Up the Heat on Legionella

When Legionnaires' Disease first appeared in 1976, researchers pointed to a faulty air-conditioning system as the source of the bacteria. But over the years, the finger of guilt has turned away from HVAC—and toward plumbing. "The overwhelming majority of LD cases have been traced to potable hot water systems," said David Yates, a plumbing contractor and owner of F.W. Behler, a contracting company in York, Pa. "There is a growing belief that the original outbreak in Philadelphia was also a result of the potable water being contaminated."

It turns out that hot water systems are the perfect breeding ground for Legionella. While not a major health threat to everyone, Legionella can be deadly for the elderly, people with compromised immune systems and people suffering from chronic diseases. This makes hospitals and nursing homes especially vulnerable. In 1999, several patients in Maryland health-care facilities caught LD. A 2000 report by Maryland's Dept. of Health and Mental Hygiene attributed the cases to potable hot water systems, and cited several problems.

According to the report, the bacteria thrive in temperatures ranging from 77ºF to 108°F and don't begin to die rapidly until the water reaches 131ºF.

Yates said that organizations such as the National Safe Kids Campaign have pushed for water temperatures to be set at 120°F to prevent scalding. But this would only prolong the length of time it took for water to scald and wouldn't kill Legionella. He explained that water needs to be at least 140°F to make sure Legionella died off.

Water heaters are another trouble spot. They can develop layers of water at different temperatures. "Kind of like a stack of hot cakes," Yates said.

Once Legionella has entered a potable water system, it's notoriously hard to get rid of. The bacteria can hide from high-temperature system flushes, even ones as long as 30 minutes, by hunkering down in the biofilms that line most pipes. Chlorine shock treatments don't work well either. "Chlorine is ineffective against Legionella at levels safe for human contact," Yates said. "It does not penetrate biofilms very well and at high percentages will combine with any organic matter to create chloramines, which are carcinogenic."

So what can engineers do? The answer starts with prevention. A few simple additions to the potable water system will encourage owners to keep the water temperatures high enough to kill legionella. Yates recommended installing a thermostatic American Society of Sanitary Engineering (ASSE) 1017 listed device at the point of source outlet to eliminate stacking in the tank, temperature fluctuations or a thermostat overrun. He also suggested installing an ASSE 1016 scald guard device at all points of use. Yates said distribution systems should be outfitted with constant circulation, which could be granted an exemption from energy conservation if it were installed using a low wattage pump and adequately insulated distribution lines.

In buildings where a radiant heat system is called for, the system should always be closed, never a combined or "open" one that connects the potable water system to the radiant heat pipes, Yates said. Stagnant water and high pH levels in radiant pipes amplify an already Legionella-friendly environment. If both systems are connected, there is almost no way to keep the bacteria out of the drinking water. "Open systems offer but one benefit—they're cheaper," said Yates.

Educating decision-makers and the public about the dangers of open systems and Legionella in general is the only way to ensure safety, Yates said. You can find out more about Legionella and plumbing systems, including an analysis of treatment options, by reading the report on the Maryland cases at www.dhmh.state.md.us/html/legionella.htm.