For those responsible for putting the system in place and maintaining it, the danger is in the potential lawsuits.

Ignoring the design, installation and operation of excessively hot water for bathing and showering can lead to severe scald burns that cause debilitating, permanent injuries or even death for the user. And for those responsible for installing and maintaining the hot-water system, the danger is in the potential for losing lawsuits-and rightfully so.

The problem with hot-water scald burns is that there's a drastic increase in injury risk with relatively small increases in water temperature. The data in Table 1 illustrate this burn-time-to-water-temperature relationship. The chance for survival and the mortality rate with thermal burns as a function of age and percent of second- and third-degree burns are shown in Figure 1. To really appreciate the drastic changes in risk, note, for example, that it takes about 60 minutes at 115¿F for an adult to get third-degree burns, compared to only five seconds at 140¿F. Even more startling: It takes about 11 minutes at 115¿F for a child to receive second-degree burns, compared to less than one second at 140¿F.

The extreme dangers of excessively hot water are not always obvious because you cannot see, hear or smell scalding water-but you feel it. The ordinary consumer and most ordinary, reasonable members of the community do not know about the exponential increase in risk with relatively small increases in hot-water temperature.

Causes and effects

The words hot, cold and scalding are difficult to describe. So a few definitions are in order: In simple terms, a first-degree burn is like a sunburn. A second-degree burn is burn injury through the epidermis (the outer skin layer) and partially into the dermis, causing blisters, which provide a haven for infection-the biggest problem with this type of burn. The thickness of the epidermis varies from about 0.05 mm, in areas such as the eyelids or other folds in the body, to about 1 mm on the soles of the foot. Third-degree burns mean that tissue decay has occurred essentially throughout the body skin, which will destroy all the nerves and will require skin grafts. The thickness of the dermis is about 10 times the thickness of the associated epidermis. Weight of the skin is about 15 percent of total body weight. A fourth-degree burn is injury completely through the skin and injury to the muscles and ligaments and tendons.

Average total skin depth is 1 to 2 mm, thicker for males than females and thinner for infants and the elderly. Total surface area varies from about 0.25 square meters for a newborn to about 1.75 square meters for an adult.

On a large number of manufactured water heater thermostats, the positions of WARM, NORMAL and HOT are marked for adjustment, but they are not clear descriptions for the average consumer. In fact, they are reminiscent of a statistician's conclusion: If you bathe the left foot in COLD water at 30¿F and the right foot in HOT water at 170¿F, then the average temperature of 100¿F will be just right.

Risky behavior

Children are especially at risk because of their lack of understanding of the grave dangers of excessive hot water. They could accidentally turn on the hot water in a bathtub for only a few seconds and incur severe or fatal injuries, even if the caretaker is in the bathroom or nearby.

Some of the statistics from the Consumer Product Safety Commission (CPSC) show that:

1. About 5,000 children per year are scalded by hot tap water, most often in the bathtub.

2. The hospital admission rate was 17 percent for tap-water scalds, compared with about 5.5 percent for kitchen scalds.

3. The average bathtub-scald-burn covers about 12 percent of a baby's skin surface area, with full-thickness, third-degree burns.

4. Scald burns kill 1,300 and disable about 4,000 children per year.

The elderly are also at high risk-mostly in the shower. If the tub floor is slippery, as is often the case in motels and hotels, they can slip and fall (with resulting severe injuries), catch the shower handle on the way down in an attempt to stabilize themselves and accidentally turn the mixture valve to scalding water.

But there are no exemptions on age, and there have been cases where a middle-aged man or woman has had a petite or grand mal seizure and accidentally turned on the hot water and injured themselves.

Your responsibility

Plumbing engineers and plumbers should be acutely aware of the inherent inaccuracies of water heater thermostats in general, especially with electric residential water heaters. The dangers are amplified when coupled with the layering or stacking scenario, where the hottest water is at the top of the heater, as in electric or gas-fired heaters. In one series of tests, the thermostat was set at 140¿F. With a controlled variable draw to induce maximum stacking, hot-water temperatures reached 195¿F.

In some of our test work, the temperature variation with time for draw-down conditions (continuous, maximum hot-water flow) is given in Figure 2 and shows a decreasing temperature in time. But trickle flow or step-draw gives totally different results, as shown in Figure 3. The results for three different sets of tests with the same draw condition, but different thermostat set points, are given in Figure 4. The differential increase in the delivered hot water temperature varied between 17¿F and 27¿F. The hot water can be controlled with tempering valves at the point of source (on the heater or boiler), but manufacturers rarely furnish this device. The alternative is to use point-of-use tempering, i.e., thermostatic valves on the shower head or bathtub spigot. One of the two types of valves should always be used, especially in motels and hotels. The plumbing engineer should always include in the specifications a provision for testing the system after installation. The test should include a trickle-flow test to induce maximum stacking.

To further insulate plumbing engineers from costly lawsuits, the engineer should furnish the installer and the owner with general safety data regarding excessive hot water. The recommended safety data sheets should be clear and concise and able to be passed on to maintenance personnel by the owner.

Recommended maximum

Several types of problems ensue in general maintenance of the hot-water system. Sometimes the point-of-use tempering valves get maladjusted during renovations or other changes. Sometimes the tempering valves at point-of-source deteriorate and can be overlooked by general maintenance. The valve may not fail safe, and severely hot water can develop, with a high probability of scald injury. In fact, it appears inevitable that scald burn injuries will occur with delivered hot water at any temperature above 140¿F, and sometimes lower.

The question then is what should be the maximum temperature of delivered hot water for showers and baths. We recommend a maximum of 115¿F. From Table 1, 130¿F is unquestionably too hot for showers and baths when escape time is considered. The recommended maximum temperature varies among statutes, codes, standards and guidelines range from 105¿F to 130¿F, with the lower values for good reasons. For example, California mandates that all of its jails and prisons have a delivered hot-water temperature not exceeding 105¿F. This prevents prisoners from holding another prisoner under hot water to torture or kill him.

Keep in mind that the body core temperature is normally 98.6¿F, with the skin temperature normally 1 or 2 degrees cooler. A comfortable shower-water temperature is maximum 102¿F, with 104¿F extremely high, and 100¿F average, but all depending upon ventilation air flow. The pain threshold is shower-water at about 106¿F, so a continuous supply of hot water above 105¿F is not necessary for bathing.

A consideration in delivered hot-water temperature is Legionnaire's disease. The pneumophilia is an airborne virus found most often in stagnated areas of water in office-building rooftop condensing units, and sometimes in dead-end pipes in building plumbing systems. There have been no reported incidences of Legionnaire's in the residential sector. But any large office building complex should routinely treat the produced hot water by ultraviolet or other means.

Washing bodies, clothes and dishes

Energy costs should also be a consideration-the lower the set point for the hot-water supply, the lower the energy costs will be. But some will argue that the hot-water supply should be at a high temperature to allow for lengthier showers. Percentages of hot water in the shower or bath as a function of hot and cold water temperatures are given in Figure 5. It is true that the shower time in a residential setting will be extended with high-temperature hot water, but nobody really needs to take a half-hour shower.

Furthermore, the longer the shower time at the normal, comfortable 100¿F, then the longer the heater recovery time. So the time required for consecutive showers by two or more people, when the recovery cycle is included, is also increased. This results in an increase of the total time for two or more showers when the waiting time for heater recovery is considered, as shown in Figure 6.

The argument for extremely long showering times is very much like the argument for high-temperature coffee served at drive-in windows of fast food restaurants. Does the motorist really need coffee served so hot, with possible risks of burn, that the coffee will stay warm for an hour or 60 miles down the highway?

The common expectation is that very hot water (130¿F to 140¿F) is needed in the residential sector for washing clothes or dishes. Actually, there are commercial soaps available for washing clothes in cold water. And for washing dishes, an excess of 115¿F is not necessary either. With new residential dishwashers, a water temperature booster is built into the washer. With old dishwashers, adequate results can be obtained with the proper choice of soap, namely those soaps containing a surfactant. Dish cleaning is, of course, affected by the water temperature, but more importantly, it's affected by the severity of agitation, length of the soak cycle, and the amount and type of soap.

In general, plumbing engineers and contractors can avoid the burden of unnecessary costs, time and business disruption for inevitable litigation over scald burn injuries if they adequately address the safety of the delivered hot-water temperature. This can be handled at relatively minimal costs by a judicious method of controlling the delivered hot-water temperature, guarding the system against tampering whenever possible and providing appropriate instructions with warnings that are readily understandable by the owner, maintenance personnel and the owner management.