64/Plumbing Engineer October 2021

By Rick Cota and Eric Costa

H

ot water has been around since man first harnessed the power of fire, but when at elevated tempera- tures, it is dangerous and even deadly. Today's water heaters offer a diverse range of technologies that perform leaps and bounds ahead of their predecessors, but what temperature should they deliver? Some facilities may simply set the heater to a usable temperature. While this may seem like an acceptable practice, there is an issue that needs to be addressed: Legionella , one of the biggest concerns for hot water sys- tems in commercial buildings. Legionella is a bacterium that enters the human body by inhalation and aspiration of water droplets. It is respon- sible for Legionnaire's Disease, a bacterial infection of the lower respiratory system often misdiagnosed as pneu- monia; 1 in 10 people who contract Legionnaire's Disease will not survive. Figure 1 shows Legionella growth behav- ior. These organisms live and multiply in water between 68 F and 122 F and thrive in water from 95 F to 115 F. At 121 F, Legionella bacterium stops multiplying but is still present in water. There are many ways to combat Legionella in a domes- tic hot water system. UV ray treatment is a method used to treat bacterial removal; however, additional heat is the most common method. Avoid Scalding Situations Since the first clinical study was done in 1977 by the Allegheny County Health Department, more and more commercial water heater systems are heating to tem- peratures above 140 F to kill the bacteria. The plumbing design industry generally follows the ASHRAE 2000-12 guideline, which suggests thermal disinfection by sending hot water (165 F to 175 F) through the domestic hot water system. These higher temperatures, while necessary, pose a scald danger for end-users. This is where a master mixing valve becomes extremely valuable. It is regularly used to reduce the temperature to a more comfortable range (between 124 F and 130 F) then circulated throughout the building. A portion of the return water from the domestic hot water line is sent back to the hot water source to be reheated, which helps keep Legionella growth in check. Master mixing valves provide a buffer between the high-temperature heat source and end-point fixtures. Handwashing faucets in public restrooms will typically have hot water supplied from a small point-of-use mixing valve. The presence of these valves will tempt designers and contractors to skip the installation of a master mixing valve. While this does work initially, the building is at a higher risk of scalding end-users since these small valves are often forgotten and not well maintained. Over time, the valves will fail and potentially allow "full hot" water through them. If that is 140 F or above, the risk of scald is significant. By installing a master mixing valve, full hot is tempered down to the fixtures, keeping a set temperature safe for end-users. For reference, Figure 2 is a water burn chart so that the danger can be fully appreciated.

The Case for the Master Mixing Valve

Installing such a valve on a domestic hot water system is simply good practice and adds an extra layer of safety to any system.

Photo: iStock.com/monkeybusinessimages