The recent outbreak of legionnaires’ disease in New York City has brought renewed attention to the inherent risks from legionella bacteria in HVAC and plumbing systems. There are several things that plumbing engineers and designers can do to minimize the risk of legionella colonizing and growing in plumbing systems.
Store water in water heaters or storage tanks at a minimum temperature of 140-deg. F.
The favorable temperature range for the growth of legionella bacteria is 77-108 degree-F. At 140-deg. F, the legionella bacteria do not survive. Of course, there is a high risk for scalding from 140-deg. F hot water. Install an ASSE 1017 thermostatic mixing valve at the outlet of the water heater or storage tank to lower the hot water temperature to a safe range prior to circulation throughout the building.
Recirculate water continuously in the hot water system.
Stagnant, tepid water is a breeding ground for legionella bacteria. Keeping the water flowing in hot water systems reduces the risks of legionella growth.
When a plumbing fixture is more than 50 feet from the hot water source, the plumbing codes require a hot water recirculation system. A hot water recirculation system includes return piping from the farthest fixture back to the water heater and a recirculation pump to provide continuous flow through this loop. The continuous hot water flow minimizes the growth of legionella and provides hot water to the plumbing fixtures in a matter of seconds. This saves water usage by eliminating the water flow during a long wait time for hot water and also improves user satisfaction.
On the other hand, recirculating hot water in the plumbing system increases energy usage. There is more energy loss from the hot water in the piping, and the recirculation pump also uses a small amount of energy. The water heater has to provide additional heating to compensate for the temperature drop in the hot water circulation system. To reduce these energy losses in a hot water recirculation system, some energy codes and designers install a timer on the recirculation pump to shut off the recirculation pump during times of low water usage. Installing aquastats, to shut off the recirculation pump when the return water temperature from the loop is above a preset value, is another method that is often used to reduce energy losses in recirculation systems.
Shutting off the recirculation pump during periods of low usage or when the return temperature is favorable, reduces energy use but increases the risks of legionella growth. This practice should be avoided.
Recirculate hot water at a minimum return temperature of 124-deg. F.
There are differing opinions about the correct water temperature for the hot water recirculation system. The higher the temperature the lower the risk for legionella; but the greater the risk for scalding.
I recommend setting the ASSE 1017 master thermostatic mixing valve for the water heater system at 129-deg. F; then designing the recirculation system for a 5-deg. F temperature drop. This ensures that the hot water in the recirculation system piping will be at or above 124-deg. F at all times. Because hot water can be delivered to some plumbing fixtures at up to 129-deg. F with this arrangement, which is too hot for the faucet outlet temperature, point-of-use thermostatic mixing valves, shower valves, and other temperature limiting devices must be used at or near the plumbing fixtures to reduce the hot water temperature to 120-deg. F or lower.
Avoid dead legs in the plumbing system.
Again, stagnant water promotes the growth of legionella. For this reason, dead legs should be minimized where possible. Eliminating all occurrences of dead legs in a plumbing system is not possible, but with attention to detail, many of the dead legs and stagnant areas can be avoided.
Require disinfection of the plumbing system during project commissioning.
During the construction process, legionella bacteria can be present in piping systems that remain stagnant, often for many months. Flushing and disinfecting the system prior to start-up is imperative. Make sure these requirements are in the project specifications.
For additional guidelines on minimizing legionella bacteria in plumbing systems, ASHRAE Guideline 12 is a good resource.