Plumbing design professionals and contractors are faced with many challenges when designing, installing or maintaining domestic hot water systems. Two of the more important challenges of a domestic hot water system are providing hot water for bathing and washing that will not cause scald injuries and hot water that is at a temperature high enough to prevent Legionella bacteria growth. I call it the hot water system balancing act. Scalding and Legionella account for a significant percentage of the litigation cases associated with plumbing systems.

Many plumbing industry groups have addressed the scalding issue and it is documented in the plumbing codes that the maximum hot water temperature to prevent scalding is 120 ° F. The minimum temperature to prevent Legionella bacteria growth at any point in the domestic hot water supply or return piping system should be 124 ° F according to ASHRAE. The 124 ° F temperature comes from the new ASHRAE Guideline 12, which is nearing completion for publication.

In Figure 2, the two temperatures seem conflict with each other, but they can actually work together. The plumbing system can be designed to store and distribute hot water at higher temperatures and deliver the hot water from the showers and bathtub/shower fixtures at safe temperatures of 120 ° F or less by simply adjusting the limit stops on the tub/shower valves to limit the hot water to 120 ° F or less.

Many design professionals, contractors, maintenance personnel, tenants and building owners may not be aware of the temperature limit stop feature on all code compliant shower valves. The manufacturers publish information on how to set the limit stop for shower valves. If the shower valve is older without limit stops, it should be replaced or a thermostatic mixing valve conforming to ASSE 1070 should be installed on the hot water supply branch to temper the water to a maximum of 120 ° F or an ASSE 1062 device could be used to prevent scalding. Code-compliant shower valves conform to ASSE 1016 or CSA B125.1, which were recently harmonized with ASME in the standard titled: ASSE 1016/ASME A112.1016/CSA B125.16, Performance requirements for automatic compensating valves for individual showers and tub/shower combinations. The temperature flowing to the shower valves can be as high as 140 ° F and the shower valves should have the maximum temperature limit stops adjusted to limit the temperature leaving the shower valve to a maximum of 120 ° F. In addition, the valves must be seasonally adjusted to account for the changes in the incoming cold water temperature that can affect the mixed water temperature.

Maximum hot water temperature to prevent scalding

I have served on many industry committees dealing with hot water system code requirements, hot water system design standards and product standards related to domestic hot water systems devices for temperature control and scald prevention. There has been consensus in all of these committees that the maximum safe hot water delivery temperature for a shower or bathtub is 120 ° F. (See Figure 1 - Hot Water Scald Burns – Time vs. Temperature Relationship for Second and Third Degree Burns for Adults and Children)

There were discussions in a plumbing code ad-hoc committee on temperature limits for the hot water system where everyone agreed the maximum safe temperature was 120 ° F. The ASPE Hot Water Committee dealt with a proposed standard for temperature limits in hot water systems also and agreed the maximum safe hot water temperature to prevent scalding is 120 ° F. Several ASSE working groups that I have served on about hot water temperature controls have all have discussed the reaction time of bathers, taking into consideration that children, the elderly and people with disabilities usually take longer to get out of harm’s way if the water suddenly gets hot. The ASSE and they agreed that 120 ° F is the maximum safe hot water temperature that a valve should deliver.

At 120 ° F it takes about 80 seconds to develop a second degree burn in a child and it takes about 8 minutes to develop a second degree burn in an adult. As shown in Figure 1, the 120 ° F temperature limit gives bathers or users an adequate amount of time to get out of harm’s way before an irreversible scald burn injury can occur. Each of these committees looked back to the data that was the result of burn studies done by Dr. Moritz and Dr. Henrique at Harvard Medical College in the 1940s. The burn studies were done using baby pigs that had skin thicknesses similar to that of adult males. The studies exposed the pigs’ skin to various temperatures of hot water for various periods of time and the severity of the burns were studied and recorded. These were the studies used to develop the time and temperature exposure charts.

There have been numerous white papers, seminars and reports since then discussing the fact that burns can occur quicker than those recorded in the Moritz and Henrique studies for adult males. The skin is thinner for children and the elderly and the amount of time to receive an irreversible second-degree burn injury is shorter because their skin is thinner. Many white papers reference the Moritz and Dr. Henrique burn studies, using a ratio of the skin thickness to come up with burn times for thinner skin of children and the elderly. Children, the elderly and handicapped are also slower to react because it takes them more time to realize what is happening and react to get out of harm’s way. Someone once told me an apartment complex was not intended for children or the elderly. I said everyone grows old and children often come visit so we need to consider prevention of scalding to children, the elderly and people with disabilities more so than burns to adults because burns can occur quicker for those groups.

The PIEV theory for reaction time

The PIEV theory is most commonly used to address braking distance in automobile accidents. It addresses the amount of time it takes a driver to sense a problem and decide to react. The reaction time is added to the braking time for the total distance that a car travels before stopping. The PIEV theory can also apply to reaction times for a bather with respect to hot water scalds.

PIEV relates to the amount of time it takes a person to react to a hazard. PIEV stands for Perception, Intellection, Emotion and Volition. Before we recognize and react to a hazard, four specific areas of activity need to be processed by the brain for the muscles to react. Those processes are:

1. Perception – We need to perceive or gain a perception of a hazard. There can be delays in the perception with limitation in sight, sound, feeling, or any other of our senses.

2. Intellection – We go through a period called intellection, or the act or process of using the intellect by thinking or reasoning. The bather must determine if the hazard is legitimate and deciding either move out of the way of the hazard or eliminate the hazard by adjusting the controls or in some cases where the bather may be sitting out of the reach of the controls the bather may choose to pull the shower curtain in front of them. If the adjustment of the controls is the choice one must decide which control to turn and try to remember which way to turn each control to adjust the temperature or turn the water off in order to eliminate the hazard. If a wrong choice is made during this process, it could compound the situation by making the water even hotter. I travel a lot and I often find that shower controls can be very confusing with respect to how to adjust the controls. I still find two handle shower controls that do not meet code requirements. This is critically important when there is no temperature limit on the shower controls. For example, if the shower has a two-handle shower valve and 160 ° F hot water is supplied to the system, then turning of the cold water first could lead to instant scalding injuries. Turning down the hot water to 120 ° F or below creates a system where it could incubate Legionella bacteria to very high levels.

3. Emotion – There is an emotion or evaluation factor that is defined as a conscious mental reaction subjectively experienced as a strong feeling usually directed toward a specific object and typically accompanied by physiological and behavioral changes in the body with respect to deciding or assessing how to react. A person with reduced mental capacity or elderly person will take longer to process this information and decide how to react.

4. Volition – There is the physical volition or deciding/choosing to act and acting. In the case of braking distance, it is when the choice is made to move the foot from the gas pedal to the brake pedal and pressing on the brake pedal. This can be related to the time the bather chooses to adjust the control, and they move their hand to the shower control valve, plus the time to rotate or re-adjust the shower valve plus the time from the adjustment until the water temperature changes coming out of the shower head. Often, it can take as much as 3-5 seconds to re-adjust the shower head and another few seconds until the water temperature changes. For ultra-low-flow (ULF) showers, the delay from the time of the adjustment of the shower valve until the water temperature changes coming out of the shower head can be even longer. So, burns can become more severe with ULF shower heads. This is one more area where water conservation measures can unintentionally make plumbing systems less safe.

As the temperature of the water increases this PIEV reaction time becomes more important. Using a bathtub/shower controller with a single handle would reduce the mental processing time and reduce the possibility of making an error when turning off the water. As Figure 1 shows, the higher the temperatures get, the quicker the burns can occur. Within seconds or less the degree and severity of the burn can be affected by this reaction time.

As you can see in the chart in Figure 1, if the water is at 140 ° F it will take about 0.8 seconds for a child to receive a second degree irreversible burn injury and it will take about 5.6 seconds for an adult male to receive an irreversible burn injury at 140 ° F. Everyone else will fall somewhere in between. An adult will often find it very difficult to react to a sudden change in temperature within 5 seconds. If the shower head is an Ultra-Low-Flow (ULF) shower head the delay can be several seconds longer before the water temperature is reduced because the mixed water temperature must evacuate or flush out the hot water in the pipe riser from the shower valve to the shower head.

There is basically very little to no time to react at higher temperatures. For a typical adult that is alert, the PIEV theory shows it can take well over 5 seconds to react to a sudden burst of hot water in a shower. For an elderly person or a small child that is confused, it could take several minutes or more before they are able to react.

Editor’s Note:?Ron George’s column, “A hot water system balancing act...Scald vs. legionella prevention,”?will continue next month in the April 2013 issue of Plumbing Engineer.

Ron George is president of Plumb-Tech Design and Consulting Services LLC. He has served as chairman of the International Residential Plumbing & Mechanical Code Committee. Visit www.Plumb-TechLLC.com, e-mail Ron@Plumb-TechLLC.com or phone 734/755-1908.