Warning! Anti-scald shower valves can scald you!

Every year, thousands of people suffer serious thermal shock and/or scalding injuries in showers or combination tub/shower fixtures. Tens of millions of currently installed code compliant shower valves, referred to as anti-scald or safety shower valves, can potentially scald people.

The terms anti-scald and safety are misleading, because, if the valves are not adjusted properly, they are neither “anti-scald” nor “safe.” The correct terminology is compensating type shower valve, so named because the valves compensate for changes in the incoming pressure or temperature. Changes in the supply system pressure or temperature can affect the mixed water temperature. These valves must meet the industry standards required by the codes.

A lot of scald injuries occur in both non-code compliant and code compliant fixtures. An incident can happen when the water heater temperature is adjusted or if the temperature limit stop on a shower valve is not readjusted when a water heater is replaced. Many people in the plumbing industry do not realize that they need to check the limit stops when they make these system alterations.

The thermostat dial on the water heater cannot be relied on to control the hot water system temperature. The thermostat is located at the bottom of the heater to sense the incoming cold water. It controls the “on-off” function of the heating element or burner in the water heater; it does not sense the hot water temperature at the top of the water heater.

In many scald cases, a water heater had been replaced but the shower valve was not code compliant. Replacing a water heater changes the temperature in the hot water system. The limit stop should be readjusted on a compensating type shower valve and a non- compensating valve should be replaced with an ASSE 1016 code compliant shower valve. If the non-compensating valve is not replaced, some other form of temperature controls should be added to the system. Temperature control and/or pressure-balancing devices or a combination of devices conforming to ASSE 1017, ASSE 1070, ASSE 1066, or ASSE 1062 can be used.

The Model codes have the following language addressing safety and hazards related to existing installations, maintenance and alterations:

The 2009 International Plumbing Code.

IPC 102.2 Existing installations. Plumbing systems lawfully in existence at the time of the adoption of this code shall be permitted to have their use and maintenance continued if the use, maintenance or repair is in accordance with the original design and no hazard to life, health or property is created by such plumbing system.

IPC 102.3 Maintenance. All plumbing systems, materials and appurtenances, both existing and new, and all parts thereof, shall be maintained in proper operating condition in accordance with the original design in a safe and sanitary condition. All devices or safeguards required by this code shall be maintained in compliance with the code edition under which they were installed. The owner or the owner's designated agent shall be responsible for maintenance of plumbing systems.

IPC 102.4 Additions, alterations or repairs. Additions, alterations, renovations or repairs to any plumbing system shall conform to that required for a new plumbing system without requiring the existing plumbing system to comply with all the requirements of this code. Additions, alterations or repairs shall not cause an existing system to become unsafe, insanitary or overloaded.

Minor additions, alterations, renovations and repairs to existing plumbing systems shall meet the provisions for new construction, unless such work is done in the same manner and arrangement as was in the existing system, is not hazardous and is approved.

Hot water in excess of 120 F is a scald hazard and a thermal shock hazard. Altering a plumbing system by replacing components such as a faucet, mixing valve, water heater or piping changes the hydraulic and temperature performance of the system. Pressure imbalances from altered flow rates can cause thermal shock and scald hazards.

The Uniform Plumbing code has similar language:

The 2009 Uniform Plumbing code

UPC Repairs and Alterations.

UPC Maintenance. The plumbing and drainage system of any premises under the Authority Having Jurisdiction shall be maintained in a sanitary and safe operating condition by the owner or the owner’s agent.

UPC Existing Construction. No provision of this code shall be deemed to require a change in any portion of a plumbing or drainage system or any other work regulated by this code in or on an existing building or lot when such work was installed and is maintained in accordance with law in effect prior to the effective date of this code, except when any such plumbing or drainage system or other work regulated by this code is determined by the Authority having jurisdiction to be in fact dangerous, unsafe, insanitary, or a nuisance and a menace to life, health, or property.

During an addition or remodel, a code compliant valve should be installed or some other approved temperature and pressure control device should be utilized to minimize the hazard. A lot of scalds occur when unlicensed and untrained sewer and drain cleaning companies expand into the water heater replacement business. Most of the time they do not know what a temperature limit stop is, and they are not aware of any kind of available temperature control. They say, “We were just hired to replace the water heater.”

Often, either the installation does not meet the current code or the manufacturer’s installation instructions were not followed. A significant number of scald incidents involve two-handle, non-compensating type shower valves. It has been estimated that well in excess of 50 percent of all tub/shower valves in existence today were installed prior to the early 80s and are this type.

The Plumbing Codes are quite clear on the maximum temperature limits for showers and combination bathtub/showers; that does not mean that the water heater must be set to low storage temperatures.

Thermal shock

Pressure-balancing, thermostatic or combination pressure-balancing/thermostatic valves conforming to ASSE 1016 or CSA B125.1 are designed for controlling thermal shock and scalding in showers and tub/shower combination units. ASSE, ASME and CSA recently published a harmonized standard for shower valves, which should be adopted in the model codes within the next few years.

Thermal shock is a bather’s reaction to a sudden change in temperature. Thermal shock injuries are often caused by slips and falls in reaction to a sudden increase or decrease in water temperature. Many injuries occur when a bather steps into a tub of very hot water, slips and falls. Pressure-balancing valves address thermal shock by sensing the supply pressure of the cold or hot water; if either one drops, the water flow to the opposite temperature is reduced proportionally to maintain a constant outlet temperature. The balancing of pressure between the hot and cold water is accomplished with either a piston or diaphragm that reacts to changes in water pressure.


Scalding is a burn injury caused by exposure to hot water and can occur anywhere that a hot water system is not designed, installed or maintained in a safe condition. The degree of the scald burn injury is dependent on the temperature of the water, the amount of time the skin is exposed to the water and the thickness of the skin. Scald burn studies were conducted at Harvard Medical College in the 1940s by doctors Moritz & Henriquez.

Because the Moritz & Henriquez studies showed that it would take approximately five to eight minutes to get a serious second or third degree burn resulting in irreversible skin damage, the model codes and industry standards have standardized 120 F as the maximum safe temperature for showering and bathing activities. Thermostatic temperature limiting valves conforming to ASSE 1070 are designed for reducing the hot water supply temperature delivered to bathtubs or whirlpool tubs to a maximum of 120 F. There is an effort currently underway to harmonize the ASSE 1070 standard with a Canadian Standard Association (CSA) standard.

Domestic hot water (DHW) system design

There is a delicate balance between providing water that is hot enough to prevent the growth of Legionella bacteria and other organic pathogens, yet is below 120 F at showers and bathtubs and below 110 F at bidets. This can be accomplished by storing hot water at temperatures above 135 F, although I prefer to store hot water between 140 and 160 F (a thermostat setting of 150 F), using a master mixing valve to distribute the water at about 125 F and using compensating type shower valves with the maximum temperature limit stop adjusted to reduce the maximum temperature to about 110 to 115 F or the maximum temperature required by the local code

The Legionella bacteria issue

Legionella bacteria grow and multiply between 68 and 122 F; they reproduce rapidly and thrive between 95 and 115 F. Below 68 degrees the bacteria survives, but it does not multiply. From 115 degrees to about 122 degrees, the bacteria grow slowly. From 123 to 131 F, the bacteria survive but do not multiply. At 131 F, it takes about five to six hours for the bacteria to die. At 140 F, the bacteria dies in 32 minutes. At 151 F, it dies in two minutes. At 158 F and higher, the bacteria dies instantly. One of the most widely accepted and preferred methods of controlling Legionella bacteria is to maintain the hot water system storage tank temperature at or above 135 – 140F. Unfortunately, the elevated temperature necessary to minimize the growth of and kill Legionella bacteria has the potential to cause serious scalding injuries. (For more information on Legionella growth temperatures go to www.legionellaprevention.org.)

CSPC warning

The Consumer Product Safety Commission (CSPC) and manufacturers of shower valves joined together several years ago to warn the public of the potential scalding hazard associated with code compliant shower valves that were not properly installed and adjusted. Many manufacturers decided to ship the products without the limit stop adjusted to fully limit the hot water. It is possible to buy a code compliant temperature and/or pressure compensating type shower valve and still be in danger of scalding if it is installed without adjusting the temperature limit stop.

Pressure-balancing or thermostatic shower valves or a combination of the two are code compliant. Compensating type shower valves are used in all building types. These types of valves have been on the market since before the late 1940s, but they were not mandated in the codes until after the ASSE 1016 standard for shower valves was developed in 1976. Soon after that, the model codes adopted the language mandating that valves meet that standard.

Many manufacturers welcomed this as a great safety improvement toward scald prevention. Several manufacturers, wanting to be proactive, started shipping their shower valves with the limit stop adjusted to restrict or limit the valve rotation toward the hot water outlet of the shower valve. The problem was that the plumbing industry and the public were not educated on the existence of limit stops and on how the devices were to be adjusted. Manufacturers’ installation instructions included directions on how to set the limit stop, but guess what? Many installers and homeowners didn’t read the instructions. Manufacturers’ technical support phone lines were jammed with people calling and complaining about these “valves that don’t work.”

Shipping the valves with the limit stop adjusted so that they did not allow hot water created a lot of headaches. Water flowed when the valves were briefly turned on, but they were not properly commissioned or adjusted to set the maximum temperature limit stop to a safe temperature, so no hot water came out. Often, the installing plumber would not flow water long enough to check the temperature. If he did take the time to test the valve and no hot water came from the shower valve, he would just turn up the water heater temperature and leave. This increased the opportunity for scalds at other fixtures and did not solve the problem at the shower valve. Many plumbers felt that they did not have time or the money in their bids to check every shower to make sure the limit stops were adjusted properly.

Manufacturers’ tech support lines were inundated with calls. The solution was to ship the valves with the limit stop fully open and to include instructions and illustrations on how to properly adjust the limit stop. The limit stop is slightly different on each manufacturer’s shower valve. Some have a set screw that limits the distance the valve can open, other models have a rotational limit stop with a set screw and a key and still others have a tooth and spline gear arrangement with a limit stop protruding.

Setting the limit stop should be part of the commissioning of a building. To set the limit stop on a shower or tub/shower valve, follow these steps:

1. Let the water run long enough until both hot and cold water temperatures stabilize.
2. Rotate the handle fully to the hottest position.
3. Place a thermometer in a plastic container and hold the container in the water stream.

If the water temperature is above the maximum temperature allowed by the local plumbing code, remove the handle and adjust the limit stop to reduce the maximum water temperature to a safe temperature.

Plumbing inspector’s responsibility

I recently spoke to a group of about 80 at a code inspectors’ conference. I asked by a show of hands how many inspectors carried a temperature gauge or thermometer with them on a final inspection to check for the maximum temperature at a shower or tub/shower. Only a couple raised their hands. When I asked why, one person said that his municipality would not pay for expensive testing equipment. I explained that you can a food thermometer (available at the grocery store for about ten bucks) can be used. Another said he never thought to use a thermometer and that he tested the water temperature by feeling it; as long as it did not get too hot, he considered it all right. Most attendees said they had not given the problem much thought. Several said they were going to purchase thermometers.

Part 2 of this code column can be found in the April 2012 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, email Ron@Plumb-TechLLC.com or phone 734/755-1908.

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