Last month, I explained that, as an expert witness on plumbing-related lawsuits, I discovered a disregard for safety in many cases, resulting in serious scalding incidents. The system setup I described was a heat exchanger used for domestic hot water (DHW) applications, yet it had no storage tank or code-required and approved temperature controls.

This month, we look at DHW temperature regulation and how it was determined that 120 F was the preferred temperature for bathing and washing fixtures.

To provide safe DHW, the different temperatures of hot water leaving the heat exchanger must be mixed to allow delivery of a more stable temperature to the hot water distribution system and buffer these temperature spikes. 

Many water heater manufacturers have packaged water heaters: heat exchanger, tank or small buffer tank; a thermostatic mixing valve; and all the controls and accessories listed and tested for a water heater application.

• Storage tank with aquastat and master mixing valve (ASSE 1017). One method is to use a hot water storage tank that circulates water between the heat exchanger and the tank with a temperature-sensing element (aquastat) mounted in the storage tank. This allows the temperature sensor to call for heat, then shut it down when the tank temperature is satisfied. 

When using a hot water storage tank, ASHRAE recommends storing hot water at a minimum of 140 F. This temperature is well above the maximum Legionella bacteria growth temperature range. Laboratory tests have shown that at 140 F, Legionella bacteria die within 32 minutes. 

Storage-type water heaters with recirculation will hold the hot water at or above 140 F for more than 32 minutes during periods of nonuse. However, 131 F is the minimum temperature at which Legionella bacteria begin to die. At 131 F, it can take several hours for all the bacteria in a laboratory sample to die. Legionella bacteria can reside deep within a biofilm and often live inside an amoeba, where it is insulated from the water temperature. 

When storing hot water at higher temperatures to control Legionella bacteria growth, the potential for scald injuries exists if a temperature-actuated mixing valve is not used. It will lower the distribution temperature to a safe temperature while ensuring that, at all points in the circulated distribution system, the hot water temperature is a couple of degrees above the Legionella growth temperature of 122 F. 

Many engineers store domestic hot water around 140 F to 150 F and use an ASSE 1017 temperature-actuated mixing valve to provide a stable hot water temperature in the distribution system. 

• Point-of-use controls. Providing a constant distribution temperature allows the successful use of code-required, point-of-use, temperature-limiting controls that meet the standards for shower valves (ASSE 1016 valves) and inline temperature-limiting valves (ASSE 1070 valves). 

• ASSE 1016 shower valves. These valves meet the performance and material requirements of three standards-writing organizations that, in recent years, harmonized their standards to allow manufacturers and the code bodies to have one standard to test the devices to, verifying conformance to a temperature control standard. 

The standards organizations are the American Society of Sanitary Engineering (ASSE), the American Society of Mechanical Engineers (ASME) and the Canadian Standards Association (CSA). They have jointly produced a standard titled ASSE 1016-2017/ASME A112.1016-2017/CSA B125.16:17, Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations. The standard was reaffirmed in 2021. 

The model plumbing codes have required ASSE 1016 shower valves for more than 30 years; they are either pressure-balancing, thermostatic or a combination of pressure-balancing and thermostatic. They must have a maximum temperature limit-stop adjustment to limit the shower valves’ rotation toward the hot setting or limit the maximum temperature flowing from the fixture. 

The installer uses the manual adjustment upon initial installation and the owner must seasonally readjust because the incoming cold water changes seasonally. 

• Inline temperature-limiting valves (ASSE 1070). ASSE Standard 1070, Performance Requirements for Water Temperature Limiting Devices, first published in 2004, refers to a point-of-use or inline temperature-actuated mixing valve with a maximum temperature-limit feature. The device is one that mixes hot and cold water locally and restricts or limits the maximum temperature of the hot water supplying a fixture or fixture fitting(s). 

It can be used as a final tempering device at metering faucets using tempered water, or further tempering can be achieved by adding cold water downstream of the device when using it for a whirlpool tub, a bidet or a series of lavatories in a restroom. 

The standard originally provided for an adjustable and maximum limit feature; however, later revisions by new committee members removed the adjustable feature from the latest version of the standard. 

ASSE 1070 valves can be used on handwashing sinks, shampoo bowls, bathtubs, whirlpool bathtubs, bidets, shampoo sinks, pedicure bowls, or on a group of lavatories or other fixtures requiring tempered water. They also can be used on older two-handle installations that do not have maximum pressure or temperature-compensating controls as a maximum temperature limit. However, this device will not prevent thermal shock from pressure imbalances between the hot and cold water systems. l 

Ron George, CPD, ASSE 12080 Legionella Management Professional, is president of Plumb-Tech Design & Consulting Services. Visit www.plumb-techllc.com for more information or call 734-755-1908.

Why 120 F?

The model codes limit the maximum allowable temperature flowing from various bathing and washing fixtures — such as showers, bathtubs, whirlpool bathtubs and combination tub-showers — to 120 F. That temperature allows a moderate amount of time for a bather to summon help or get out of harm’s way before a serious, irreversible scald injury can occur in adult males. 

Unfortunately, the data used to determine the maximum allowable temperatures did not include times and temperatures for women, children, the elderly and persons with disabilities who often require more time or lower temperatures to avoid serious scald injuries before they can get out of harm’s way. 

The codes require a maximum temperature limit-stop adjustment to 120 F on a shower valve or tub-shower valve that must conform to the shower and tub-shower valve standards listed in the codes. The maximum temperature limit-stop on a shower or tub-shower valve allows an adjustment to a maximum temperature of hot water flowing from the fixture, given a constant supply temperature. 

Instantaneous water heaters and heat exchangers are notorious for providing wildly fluctuating hot water outlet temperatures. So, temperature-actuated mixing valves are recommended downstream of heat exchangers and instantaneous water heaters to meet the temperature limits established by the codes. 

Many packaged water heaters employing a heat exchanger use a mixing valve as part of the water heater package to stabilize the system temperatures. In addition, there are standards for inline temperature-limiting valves conforming to ASSE 1070. The valves can be located near a fixture to mix cold water with hot water and provide hot water that will be limited to a maximum of 120 F or a lower setting. 

I have served on or participated in about every code and standard committee dealing with water heaters and temperature controls in the model plumbing codes dealing with the production and distribution of domestic hot water systems. Over the last 30 or more years, there have been many discussions in these committees over what the maximum safe hot water temperature to require in the codes. 

These committees, working groups, task groups and ad-hoc committees all referred to the research on burn studies done by Drs. Moritz & Henriques at Harvard Medical College. 

In the beginning, when they started their burn studies, they tested for first-degree burns on adult males in the military shortly after World War I. When it came to studying second- and third-degree burns, surprisingly, they could not get any volunteers, so they decided to use young pigs with skin thicknesses measured to be equivalent to that of adult males. 

Women, children and the elderly have thinner skin than healthy adult males. Further research has shown that with thinner skin, women, children and the elderly will receive second-degree blistering burns and third-degree deep tissue burns in less time or at slightly lower temperatures than those published in the burn studies of adult males and pigs (porcine).