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  • Writer's pictureCaleb Kline

When Hot Enough Water Is Too Hot

Updated: Aug 12, 2022

Balancing Pathogen Growth and Scalding Risk in Domestic Hot Water

Managing the temperature of a building’s domestic hot water system balances controlling pathogen growth with scalding risk. Legionnaire’s disease, a serious type of pneumonia, is caused when Legionella bacteria are aspirated or inhaled. Since the bacteria’s growth range is 68°F to 122°F, it can grow in domestic hot water systems and infect people when they inhale droplets in the shower. Typical Legionella control measures includes a combination of chlorination and high temperatures.

The first design strategy to control Legionella in domestic plumbing is to avoid stagnant water. The chlorine added to drinking water at water treatment plants losses its effectiveness over time. Water that has been sitting in a hot water tank or pipe can have insufficient levels of chlorine to control bacteria growth. Dead legs, dead-end pipe runs without flow through them, should be avoided since they provide a breeding ground for bacteria. When a fixture is removed or relocated, it is important to demolish any abandoned pipe back to the main line.

The second strategy in Legionella control is to heat the water very hot. Temperatures above 130°F will kill Legionella and other bacteria. Standard commercial plumbing design is to store the domestic hot water at 140°F. This prevents bacteria growth in the tank and kills new bacteria that enter. As the high temperature hot water is used or recirculated, the distribution pipes are also brought up near this temperature. Figure 1 shows the effect of water temperature on Legionella growth and death.

Graph shows Legionella growing between 68 and 122 degrees. Legionella dia at 140 but at this temperature first degree burns occur in 2 seconds and second degree burns in 5 seconds
Figure 1: Hot water temperatures needed to control Legionella can lead to scalding

Figure 1: Hot water temperatures needed to control Legionella can lead to scalding.

The right side of Figure 1 shows the time to receive first and second degree burns from hot water of various temperatures. At 140°F, a person would receive first degree burns in 2 seconds and second degree burns in 5 seconds. Young children and the elderly, with thinner skin and slower reflexes, face a higher scalding risk. For this reason, hot water is typically limited to 120°F or less at public fixtures with sometimes higher temperatures at commercial kitchen sinks and mop sinks. The International plumbing code requires that water temperatures to accessible fixtures and lavatories be limited to 110°F. When surveying buildings, we commonly observe water heaters set to 140°F and then measure maximum water temperatures at fixtures between 130°F and 140°F. These temperatures present a serious scalding risk.

The solution to this problem is to use mixing valves that blend the hot and cold water together to achieve a set temperature. A master mixing valve can be placed at the water heater to blend 140°F water from the tank with cold water. If applicable, the valve also blends in recirculated hot water. The water is then distributed at around 120°F. The American Society of Sanitary Engineers (ASSE) writes the standards to which these valves are manufactured and tested. Master mixing valves should be labeled ASSE 1017.

An even more robust solution is to install ASSE approved mixing valves at each fixture or bank of fixtures. These small valves would be installed under the counters for sinks and lavatories (ASSE 1070) or integrated into shower valves (ASSE 1016). Fixture mixing valves keep the distribution piping temperature near 140°F to kill Legionella in the pipes. An example mixing valve installation at a lavatory is shown in Figure 2.

Figure 2: Mixing valve installed at a lavatory with hot and cold water entering and tempered water and cold water exiting to the fixture.


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Hot Water too hot
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