Domestic hot water scald prevention
By Ron George, CIPE, CPD
President, Ron George Design & Consulting Services
There are two types of hazards from domestic hot water systems: scalding and thermal shock. In the last few decades, the plumbing industry has come a long way in protecting people from these hazards.
Doing a dance in the shower is a common occurrence when someone else in a building is using cold water during a shower, causing a pressure drop in the cold water system. The pressure imbalance leads to a sudden increase in the hot water temperature for the second bather. The reverse happens when there is increased usage of hot water - hot water pressure drops and there is a burst of cold water. This sudden change in temperature at the showerhead is referred to as thermal shock. One danger of thermal shock is that it may cause the affected bather to react suddenly, resulting in a slip-and-fall injury.
In the early 20th century, two-handled bathtub and shower valves were common and contributed to thermal shock and scalding incidents on a regular basis. To address the problem, some manufacturers developed single-handled shower valves. Early designs of these valves did not have maximum temperature limit stops, so they provided thermal shock protection but not anti-scald protection. In later years, set screws, limit stops and cams were added, allowing the valves to be set to limit the amount of rotation toward the hot water, thereby providing a safe temperature for showering or bathing.
The severity and risk associated with scalding increases drastically with relatively small increases in water temperature. An anti-scald shower valve will only protect a bather if the maximum temperature limit stop or set screw is adjusted when the valve is installed. Most valves need to be adjusted seasonally to compensate for seasonal differences in the incoming cold water temperature, which can affect the mixed water temperature.
History of hot water system controls
The federal plumbing specification, WW P 541, was the minimum standard for plumbing products used by the federal government. The federal specifications were the first to mandate single-handled shower mixing valves for federal government projects. Early valves came in three categories: type P, pressure balancing, type T, thermostatic and type M, mechanical mixers. The mechanical mixer was simply a single-handed valve that opened with cold water and added hot water as the valve rotated open. Although some manufacturers had the required devices available, they were not commonly used, because the new technology cost more.
In 1973,The American Society of Sanitary Engineering (ASSE) developed the 1016 standard to address requirements for showers to protect against thermal shock and scalding. The standard called for a maximum temperature limit stop of 120 F and has evolved to include type P, type T and type TP (combination thermostatic/pressure balancing) valves. Type P devices have a pressure balancing piston or diaphragm that equalizes the pressure between the two inlet ports and maintains the same outlet temperature, as long as the incoming temperatures remain the same. Type T devices have a temperature-sensing element that adjusts the inlet ports to maintain a relatively constant outlet temperature. Type TP devices have pressure-sensing and temperature-sensing elements and can compensate for both temperature and pressure disturbances.
The ASSE 1016 standard does not address mechanical mixers or two-handled shower or tub/shower controls. In-line devices such as ASSE 1070 thermostatic mixing valves limit the water temperature of the hot water supply, but they do not provide thermal shock protection, since additional unprotected cold water is mixed in downstream of the in-line device.
The scald burn studies done by doctors Moritz and Henriques showed that it took approximately eight minutes of exposure to temperatures in the range of 120 F for adults to develop a serious scald burn: Someone exposed to water at 120 F would have up to eight minutes to get out of harm's way before an injury started to develop. (It should be noted that children and infants skin is thinner and could develop burns sooner than eight minutes.) The 120-degree temperature has become an industry standard temperature limit for scald prevention in showers and combination bathtub showers. The American Society of Plumbing Engineers (ASPE) is nearing completion of a standard on temperature limits for domestic hot water systems that will cover a wide variety of plumbing fixtures, with a few fixtures having temperature limits below 120 F.
In the mid-1970s, the U.S. Consumer Product Safety Commission (CPSC) published a report titled, "A Systematic Program to Reduce Incidence and Severity of Bathtub and Shower Area Injuries." The CPSC report prompted the American Society of Testing Materials (ASTM) to develop ASTM F444 and F445. ASTM F444 included performance requirements related to protection of showers from scalding, while ASTM F445 addressed "thermal shock preventing mixing valves" or pressure balancing valves. Since then, the scald provisions in the ASTM standards have been incorporated in the ASSE 1016 standard for shower or tub/shower valves.
By 1987, most of the model codes covering the United States included protection against thermal shock and scalding with references to ASSE 1016. Today almost all codes in the United States and Canada have thermal shock and scalding protection.
By the mid-1990s, barrier-free proponents added requirements in the Americans with Disabilities Act Accessibility Guidelines (ADAAG) that were adopted in the ANSI standard A117.1. The water delivered to "shower and bathtub/shower facilities" had to be "thermal shock protected to 120 F maximum." Some local health departments have set minimum and maximum temperatures that are slightly different, with tighter tolerances, for hospitals, nursing homes and other facilities for persons with limited mobility or senses.
ASSE hot water system control standards
ASSE 1016 - Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations
Because showering occurs in a confined space while standing on a wet floor surface with a large portion of the body in contact with flowing water discharged from a fixed showerhead, a significant hazard is present. In addition to the risk of scalding, reacting abruptly by moving away from the flowing water can cause a serious injury from a slip or fall.
ASSE 1062 - Temperature Actuated Flow Reduction Devices
Older showers with two-handled valves or single-handled mechanical mixers that do not meet the requirements of ASSE 1016 do not provide scalding or thermal shock protection. These two-handled installations can be provided with a temperature actuated flow reduction (TAFR) valve, a device that complies with ASSE 1062 and reduces the scalding risk but does not provide thermal shock protection. The TAFR valve is installed between the showerhead and the shower arm. In combination tub/shower installations, a TAFR can be screwed into the tub spout. The TAFR senses the water flowing through the device. When the water temperature exceeds the setpoint (about 115 - 117 F), the valve closes and only allows a trickle of flow. The trickle of flow is designed to allow the bather to readjust the controls to a cooler setting and to let the cooler water reset the TAFR valve. The resetting process takes about 15 - 20 seconds, but that is a small inconvenience, considering that the device can protect the bather from severe scald burns.
ASSE 1066 - Individual Pressure Balancing Valves for Individual Fixture Fittings.
To reduce the risk of thermal shock in older two-handled shower valve installations, an ASSE 1066 in-line pressure-balancing valve can be installed in the hot and cold water supply lines to the faucet. This device controls pressure imbalances that lead to thermal shock; it does not provide scald protection. If there is a circulating pump in the system, care should be taken to make sure a mixing valve is piped properly.
ASSE 1017 - Temperature Actuated Mixing Valves for Hot Water Distribution Systems.
Thermostatic mixing valves complying with ASSE 1017 are designed to control temperature from +/- 3 - 7 F, depending on the size, when flowing at the required flow rate. It should be noted that ASSE 1017 has no test for compensation during pressure fluctuation, so in order to minimize pressure fluctuations between the hot and cold water lines, the mixing valve needs to be located at the hot water source.
ASSE 1069 - Automatic Temperature Control Mixing Valves.
ASSE 1069 mixing valves are designed for gang shower applications such as those in a school, prison or health club with On-Off controls at each shower station. A single thermostatic mixing valve is exposed to a wide range of flow rates. This valve standard was developed to deal with a wide range of flows with tight temperature tolerances at low flows. If only one shower is flowing, the flow rate could be as low as two gallons per minute. In larger installations, it may be necessary to install a hi-low system that incorporates two or more thermostatic mixing valves. A smaller valve is used to compensate and maintain temperature during low flow conditions, and multiple valves work together as the flow rate increases.
ASSE 1070 - Water Temperature Limiting Devices.
In-line devices such as the ASSE 1070 thermostatic mixing valve limit the water temperature of the hot water supply to a shower valve, but they do not provide thermal shock protection, since additional unprotected cold water is mixed in downstream of the device. This device limits the water temperature to fixtures such as sinks, lavatories or bathtubs and reduces the risk of scalding. The device shall be either the final temperature regulation or have water further tempered downstream of the device with the addition of cold water.
Water heater thermostats
I cringe every time I hear a radio talk show host tell listeners to turn their water heater thermostats down. I see the same misguided advice dispensed in newspaper handyman columns that talk about water heaters or home safety. Water heater thermostats cannot be relied upon to control the hot water temperature to a hot water system. Although water heater manufacturers recommend that installers set thermostats at 120 - 125 F, and although most of them ship the water heaters at lower temperature settings, the plumbing engineering community continues to recommend that, in order to reduce the threat of Legionella bacteria growth in the tank, and in order to reduce the size of the hot water tank, hot water systems be designed with higher temperatures.
Hot water systems are designed and water heaters are generally sized to store hot water at 140 F. When the thermostat is set at a lower temperature, the water heater has a reduced capacity to deliver hot water. As a result, when users run out of hot water, and the water heater thermostat is re-adjusted to a temperature above 120 F, the heater will continue to store and deliver water at scalding hot temperatures.
Water heater thermostats were never intended to provide precise temperature controls for hot water systems. For example: the thermostat dial calibration test of ANSI Z21.10.1-1998, which is the applicable standard for gas-fired water heaters, allows the temperature to vary 10 degrees above or below the thermostat setting. I have talked to water heater manufacturers that have indicated that the controls can vary as much as 15 - 18 F above or below the setpoint. The thermostat is inserted into the lower portion of a water heater tank and turns the fuel supply to the heater on and off. With most water heater thermostat dials, there is no way to know what the temperature in the tank is.
Theoretically, if the water heater thermostatic element is set at 120 F, the burner would come on when the temperature at the thermostat reaches about 105 F. The burner stays on until the water around the thermostat element near the bottom of the heater reaches about 135 F (15 degrees F above the setpoint of the thermostat).
Most people don't realize that the maximum temperature limit test of the ANSI Z21.10.1 Gas Water Heater Standard allows the outlet water temperature of the water heater to rise 30 F above the thermostat setting. This provision accounts for the phenomenon known as "stacking" or "layering" of hot water in the top of a water heater. Stacking or layering occurs when hot water rises to the top of the heater due to recurring short duration heating cycles caused by a frequent number of small quantity hot water uses. This phenomenon can occur in any type of storage water heater.
So, at the top of a water heater that is theoretically set for 120 F, the temperatures can easily reach 165 F. This is why I highly recommend installing an ASSE 1017 thermostatic mixing valve on the outlet piping of a water heater to limit the hot water distribution temperatures to a maximum safe delivery temperature of 120 F. If high temperature hot water uses are required in a building, I recommend installing an ASSE 1070 thermostatic mixing valve on the local branch piping serving a fixture or group of fixtures.
I hope that these recommendations will keep you out of hot water.
Ron George specializes in plumbing, piping, fire protection and hvac design. He also provides plumbing/mechanical code and product standard consulting services and forensic investigations of mechanical system failures.