Stagnant Water, Beware!
– Part 3
by Max Rohr, Education and Training Manager, Caleffi North America
Safely reopening commercial DHW systems post-quarantine, part three of a four-part series
Continuing with our Legionella bacteria thermal disinfection series, we are ready for the last line of defense. In the past issues we have discussed point-of-distribution mixing valves and thermal balancing valves. These devices are great at maintaining proper DHW temperatures throughout the system. However, they are not scald protection devices. A point-of-use mixing valve is key to allow for thermal disinfection of the recirculation lines, without the risk of scalding at a fixture, even when the system is in disinfection mode.
Taking a step back, the goal in any thermal disinfection process is to heat water to the point that it will begin to kill the Legionella bacteria present in the recirculation lines. However, if you are circulating 160 water throughout a building, and someone turns on a faucet, they could be scalded by hot water in less than a second. Point-of-distribution mixing valves and balancing valves alone do not protect from scalding. To protect occupants from water that is dangerously hot at the fixtures, ASSE 1070 approved point-of-use mixing valves are needed.
The most critical locations for ASSE 1070 approved valves are hot water fixtures where an occupant may not be able to quickly get out of the way of a stream of scalding water. Schools, hospitals and nursing homes should always have point-of-use protection. However, there is no guarantee that a child wouldn’t try to wash their hands in any sink at any time. Having point of use mixing valves on any hot water fixture protects occupants from the risk of scalding.
How does a point-of-use mixing valve work? These valves are designed to quickly shut the hot port completely, in the event the cold supply pressure drops. If there is no cold to mix with the hot, a potential scalding risk has been created. Since the occupant’s hand is only a few inches away from these point of use mixing valves, there is no time to waste in shutting down the hot port. ASSE 1070 valves are built to shut down immediately.
When designing or troubleshooting a domestic hot water system, it is important to know that thermostatic mixing valves need a specific range of hot and cold water temperatures at the inlet of the valve. Always check with your valve manufacturer’s technical brochures for specific temperature and pressure windows. As an example, let’s say a home in Phoenix, Arizona has hot and cold water lines running through the attic. On a very hot summer day, the temperature in the attic could be warm enough to heat both the hot and cold lines above 100F. In that case, the mixed temperature at the faucet could be warmer than desired, until you displaced the warm water in the lines. Well-insulated hot water supply mains are an excellent way to avoid wasting energy and to reduce wait times for hot water at fixtures. In the Arizona example above, it could also be helpful to insulate the cold water lines.
What type of point of use mixing valve is best? Depending on the flow rates required and the piping configurations, different valve styles are helpful. If you are protecting a single sink, an “H-pattern” Caleffi SinkMixer is a great option. The 3/8 in. compression connections and included mounting bracket make it easy for contractors to install, with no cold tee required. If you are looking for a higher flow rate for a tub or multiple hot water fixtures, the Caleffi 5213 series with a Cv of 2 is a good fit.
Together, electronic mixing valves, thermal balancing valves, and point-of-use mixing valves are the trifecta needed to manage legionella bacteria effectively.
In this 4-part series, how to safely reopen commercial DHW systems, we’re exploring leading edge equipment and system design for providing clean and safe domestic hot water. For the articles in this series, as well as other resources, visit SouthernPHC.com/legionella.