BY ROBERT P. MADER Of CONTRACTOR’s staff
SACRAMENTO, CALIF. — What’s cooking in some water systems? Legionella bacteria, said Dave Yates, who emphasized his point by wearing a chef’s outfit. Yates, owner of F.W. Behler, York, Pa., and CONTRACTOR’s monthly plumbing columnist, told contractors how to minimize Legionella bacteria May 8 during the Radiant Panel Association convention here.
Ubiquitous Legionella is hard to kill, Yates said, because there are 40 divisions of the bacteria and multiple subgroups. One particular subgroup spreads "community acquired" pneumonia, or an infection in an otherwise healthy person who was not exposed to anyone else with Legionnaire’s Disease.
The incidence of Legionnaires’ disease and death has remained pretty constant in the United States from year to year, he said.
Legionella multiplies in potable water systems with a pH between 5.0 and 8.5 and a temperature of 55°-133°F. The bug loves to eat biofilm, sediment and rust.
Curiously, at temperatures under 55°F, Legionella bacteria is eaten by common amoeba, but at temperatures of more than 55°F, the bacteria counterattacks and eats the amoeba from the inside out, Yates said.
Danfoss conducted experiments with superheated flushes of 180°F water, he noted, but discovered that the bacteria hides in biofilms on the pipe walls and then repopulates after the water temperature returns to normal. The only solution is to keep water temperatures higher than 133°F all the time, Yates said.
The bacteria particularly likes to eat iron, he said, and stagnation gives it an opportunity to emerge from the biofilm. Five studies have connected the increased presence of Legionella with water heaters, especially in electric water heaters that have lower temperatures at the bottom of the tank.
"It makes you wonder what’s going on in indirect tanks," Yates noted.
So is it OK to use water heaters for radiant heating? Sure, with some conditions, he said. The floor doesn’t care where the Btu are coming from. Yates, however, reiterated points that he made in his CONTRACTOR column that domestic hot water should be stored at 140°F (March, pg. 24), and that any system should use ASSE Standard 1016 certified temperature- and pressure-balanced valves. A domestic hot water system should also ideally use a re-circulating system with a return temperature of 128°F at the base of the water heater.
Chlorine will not kill Legionella, he said. In fact, most drinking water systems contain 10,000 times less chlorine than0 required to kill Legionella, plus excess chlorine will attack metal components. Ultraviolet light, ionization and hotter storage temperatures will keep the bacteria at bay.
Using an open hydronic/DHW system is a good breeding ground for Legionella, said Yates, who noted that hospital and retirement home cases of Legionnaires’ disease have all been traced to the potable water supply.
"If you cross-connect to a hydronic system, you create a perfect amplifier for Legionella," he said.
Any radiant floor will sit idle at times, with the floor temperature reaching room temperature. Any device that can create a mist, such as a showerhead or faucet aerator, will spread the bacteria.
"When the floor kicks on, if you’re using it as part of a domestic hot water system, it will spray 5 gal. of floor water out of a shower head," Yates said.
A dedicated hydronic water heater is a good solution, he noted. The system should include a T&P relief valve, air eliminator, circulator, 30-lb. relief valve for the hydronic loop, backflow preventer and expansion tank. Another option would be to use a flat-0plate heat exchanger and run the domestic hot water side as constant circulation.
Such a system isn’t expensive, Yates said. He tallied up all the extra components, and they’d cost $255 at R.E. Michel catalog prices, he said.