I am often asked, “What is a safe hot water temperature for domestic hot water?” If you read the model codes, it states the maximum hot water temperature for a shower or bathtub is 120 degrees Fahrenheit. If you read the warning labels on the side of most water heaters the maximum hot water temperature is 120 degrees Fahrenheit on some labels and 125 degrees Fahrenheit on other labels. The 125 degree limit probably allows for some temperature loss before the hot water gets to the fixtures. Most water heater literature and warning labels mention the availability of thermostatic mixing valves or automatic temperature compensating valves and they recommend their use. If you look at many of the industry standards for shower mixing valves, they state the valves must have limit stops that are adjustable to limit the maximum hot water temperature to 120 degrees Fahrenheit. The testing in the standards gives test criteria for testing the shower valves to these limits.

I have served on the working groups for several plumbing industry standards committees for temperature actuated mixing valves and shower valves and it is generally agreed that 120 degrees is the maximum, safe hot water temperature. I also have served on hot water system design standards committees where the participants had agreed that maximum domestic hot water temperature from plumbing fixtures used for bathing and washing purposes should be 120 degrees Fahrenheit. There were a few exceptions for bidets, sitz baths and whirlpool tubs that had temperatures lower than 120 degrees Fahrenheit for the recommended maximum temperatures to prevent scalding. It also should be noted that some other uses like commercial dishwashers and laundries may need temperatures higher than 120 degrees Fahrenheit. There were two temperatures discussed for each fixture during the design standard meetings. One was the “use temperature” and the other was “the maximum temperature” to prevent scalding.

It’s generally agreed that 120 degrees Fahrenheit is the maximum safe hot water temperature that should be delivered from a fixture. Therefore hot water above 120 degrees Fahrenheit can be considered hazardous. Model codes address this in various code sections.

The 2009 edition of the International Plumbing Code has the following language:

2009 IPC Section 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.

2009 IPC Section 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.

2009 IPC Section 424.3 — Individual shower valves. Individual shower and tub-shower combination valves shall be balanced-pressure, thermostatic or combination balanced-pressure/thermostatic valves that conform to the requirements of ASSE 1016 or ASME A112.18.1/CSA B125.1 and shall be installed at the point of use. Shower and tub-shower combination valves required by this section shall be equipped with a means to limit the maximum setting of the valve to 120°F (49°C), which shall be field adjusted in accordance with the manufacturer’s instructions. In-line thermostatic valves shall not be utilized for compliance with this section.

The last sentence that states “in-line devices shall not be used for compliance with this section” does not mean in-line devices should not be used, it means the in-line devices will not protect against thermal shock and an automatic temperature or pressure compensating type shower valve conforming to ASSE 1016 must still be used to protect against pressure imbalances between the hot and cold water system which can lead to thermal shock incidents. A properly designed system would have a water heater set at 140 degrees Fahrenheit followed by a master thermostatic mixing valve set at a maximum of 120 degrees Fahrenheit and compensating type shower valves conforming to ASSE 1016 or CSA B125.1 located at each shower. Each shower valve should have the maximum temperature limit stop set to a safe temperature below 120 degrees Fahrenheit.

2009 IPC Section 424.5 — Bathtub and whirlpool bathtub valves. The hot water supplied to bathtubs and whirlpool bathtubs shall be limited to a maximum temperature of 120°F (49°C) by a water-temperature limiting device that conforms to ASSE 1070 or CSA B125.3, except where such protection is otherwise provided by a combination tub/shower valve in accordance with Section 424.3.

2009 IPC Section 424.7 — Temperature-actuated, flow reduction valves for individual fixture fittings. Temperature-actuated, flow reduction devices, where installed for individual fixture fittings, shall conform to ASSE 1062. Such valves shall not be used alone as a substitute for the balanced pressure, thermostatic or combination shower valves required in Section 424.3.

The 2009 edition of the Uniform Plumbing Code Has the following Language:

2009 UPC Section 101.4.1.2 — 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.

2009 UPC Section 101.4.1.3 — 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.

414.5 Limitation of Hot Water in Bathtubs and Whirlpool Bathtubs. The maximum hot water temperature discharging from the bathtub and whirlpool bathtub filler shall be limited to 120ºF (49ºC) by a device that conforms to ASSE 1070 or CSA B125.3. The water heater thermostat shall not be considered a control for meeting this provision.

416.3 Limitation of Water Temperature in Bidets.

The maximum hot water temperature discharging from a bidet shall be limited to 110ºF (43ºC) by a device that conforms to ASSE 1070 or CSA B125.3. The water heater thermostat shall not be considered a control for meeting this provision.

418.0 Shower and Tub-Shower Combination Control Valves.

Showers and tub-shower combinations in buildings shall be provided with individual control valves of the pressure balance, thermostatic, or combination pressure balance/thermostatic mixing valve type that provide scald and thermal shock protection. These valves shall conform to ASSE 1016 or ASME A112.18.1/CSA B125.1. Gang showers, when supplied with a single temperature-controlled water supply pipe, shall be controlled by a mixing valve that conforms to ASSE 1069. Handle position stops shall be provided on such valves and shall be adjusted per the manufacturer’s instructions to deliver a maximum mixed water setting of 120°F (49°C). The water heater thermostat shall not be considered a suitable control for meeting this provision.

The codes generally agree if there is a hazardous condition or a condition that is unsafe or a nuisance to life, health and property it should be corrected. It is also generally agreed that water above 120 degrees Fahrenheit at fixtures for bathing and washing with a few exceptions for lower temperatures can be considered dangerous and proper precautions should be taken to prevent the hot water from being a scalding hazard by using the proper safety devices.

When I hear about people setting their water heater to 120 degrees Fahrenheit to prevent scalding, I know they have good intentions, but most people do not know you cannot accurately control the hot water temperature leaving a water heater with the thermostat dial.

Water Heater Thermostats Do Not Control the Water Heater Outlet Temperatures!

If you adjust the water heater thermostat for the burner or heating element on a water heater down to 120 degrees, it will not prevent scalding. Water heater thermostats cannot be relied upon to control the hot water temperature leaving a water heater. Water heater manufacturers recommend that installers set thermostats at 120 - 125 F, and most of them ship the water heaters at an even lower temperature setting. It is not possible to set a water heater thermostat at a given temperature and get a relatively constant temperature of hot water from a water heater. The thermostat can not accurately control the water heater outlet temperature with a water heater thermostat.

My experience has been that not many people know that water heater thermostats cannot control the outlet temperature of a water heater. This warrants an explanation of how a water heater thermostat works so everyone understands the dial on the water heater does not have the accuracy to control the outlet temperature of storage type heater.

Water heater thermostats do not 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 to 18 degrees Fahrenheit above or below the set point of the thermostat. From my experience, I have recorded the temperature leaving the top portion of a water heater over a long period of time during intermittent uses and saw temperature swings over 40 degrees Fahrenheit leaving the water heater. The shower valve standards do not have this kind of temperature fluctuation included their testing for all types of shower valves. The significant temperature swings are because the thermostat is inserted into the lower portion of a water heater tank and turns the fuel supply to the heater on and off. Most new water heater thermostat dials have no way to know what the temperature in the tank is. There is rarely a fixed temperature indicated on the dial, however some manufacturers publish temperatures associated with various marks on the thermostat dial or in their literature even though the dial cannot not control the outlet temperature of the water heater, it only controls when the energy to the heater is turned “on” and “off” by sensing the cold water coming into the bottom of the heater.

Generally, if the water heater thermostat dial is set at 120 degrees Fahrenheit, the burner would come on when the temperature at the thermostat reaches about 105 degrees Fahrenheit. The burner stays on until the water around the thermostat which is near the bottom of the heater reaches about 135 degrees Fahrenheit. (The “burner off” temperature is about 30 degrees higher than when the burner came “on” and generally about 15 degrees above the theoretical set point 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 significantly above the thermostat setting. This provision in the standard accounts for the phenomenon known as "stacking" or "thermal layering". The hot water is less dense and rises to the top of the hot water tank. Just like hot air rises and lifts a hot air balloon, hot water rises to the top of the tank and the cooler water drops to the bottom of the tank. Stacking or thermal layering occurs when the 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. Frequent short draws cause cold water to enter the bottom of the water heater where the thermostatic element senses the cold water from the turbulent flow stirring in the bottom of the heater. The cold water causes the water heater to cycle on. This phenomenon can occur in any type of storage water heater and generally is more significant in vertical heaters. 

I have recorded temperatures as high as 150 to 166 degrees Fahrenheit at the top of water heaters that had the thermostats set between 120 to 125 degrees Fahrenheit. Temperatures over 151 degrees Fahrenheit are extremely high temperatures and can cause serious scald burns in only a two seconds of contact with the skin. (See Table 1 - Water Temperature Effects on Adult Skin) It should be noted that the time temperature relationships in Table 1 are based upon the thickness of the skin for adult males. Children and the elderly typically have a thinner layer of the skin or epidermis and the exposure times can be shorter or the same burns can occurs in a given time at slightly lower temperatures.

Time/Temperature Relationships for Water Temperature Effects on Adult Skin

The time/temperature relationship for scald burns comes from the burn studies done by Dr. Moritz and Dr. Henrique in the 1940s. Their studies were conducted on military soldiers for up to first degree burns and they used pigs for conducting time/temperature exposure tests for second degree burns. From their research the following chart was published

Kitchen sinks and Lavatories

Kitchen sinks and lavatories have been the source of many scalding injuries because the codes do not have specific temperature limits listed for those fixtures. However it has been recognized by the industry that 120 degrees Fahrenheit is the maximum safe hot water temperature for showers and bathtub showers.

This is why I always recommend installing the proper type of 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 is required for a specific use in a building, I recommend utilizing a piping branch with the 140 degree or higher temperature hot water for the specific uses and I use a master mixing valve conforming to ASSE 1017 or ASSE 1070 in-line which lowers the temperature for all of the hand washing and other sinks to a safe temperature and I use an ASSE 1016 or CSA B125.1 device for the individual showers or tub/shower combination fixtures in the building. You could use local mixing valve conforming to ASSE 1070 on the local branch piping serving a fixture or group of fixtures utilized for bathing or washing. Mixing valves can then reduce the hot water temperature to a safe temperature.

I always recommend setting the thermostat dial on the water heater at 140 degrees Fahrenheit and mixing the hot water down to a safe temperature with a master temperature actuated thermostatic mixing valve located near the water heater. The master mixing valve should conform to ASSE 1017 or CSA B-125.3. The mixing valve blends the hot water from the water heater storage tank with cold water and delivers a relatively constant temperature of tempered water to the plumbing system. The mixing valve can be set to deliver hot water to the system at or below 120 degrees Fahrenheit. This design reduces the threat of Legionella bacteria or any other organic pathogen growth in the water heater storage tank, and also storing hot water at 140 degrees Fahrenheit allows a smaller water heater than one with a lower stored temperature.

Hot water systems are generally 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 and therefore should be larger.

Often when the water heater thermostat is adjusted down users run out of hot water, and the water heater thermostat is generally re-adjusted to a temperature above 120 F to adjust for the shortage of hot water during peak periods. When adjustments to the thermostat setting are made, most maintenance personnel and most users do not know they must also readjust the maximum temperature limit stop on all anti-scald type shower valves. Adjusting the thermostat increases the temperature to a scalding temperature at the shower or at other fixtures.  In some cases, the thermostat is adjusted even higher than the original setting which creates an ever greater scalding risk.  Even with the thermostat turned lower, the water heater will continue to be capable of storing and delivering hot water at temperatures well above 120 degrees Fahrenheit.

In my opinion, the answer to the equestion “What are safe hot water temperatures?” is always: “The maximum safe hot water temperature to prevent scalding is 120 degrees Fahrenheit.” The solution is using a master thermostatic mixing valve to control the hot water temperature to the domestic hot water system and you should use anti-scald shower valves at showers and tub/showers. You should never use the water heater burner control thermostat to try and prevent scalding injuries.

Ron George is President of Ron George Design & Consulting Services. He has served as Chairman of the International Residential Plumbing & Mechanical Code Committee. He is active in plumbing code and plumbing product standard development committees with ICC, IAPMO, ASSE, ASME, ISEA and ASTM. His company specializes in plumbing, piping, fire protection and HVAC system design and consulting services. He also provides plumbing and mechanical code consulting services and he provides investigations of mechanical system failures and litigation support. His company also provides 3D cad services and Building Information Modeling (BIM) services. E-mail: rgdc@rongeorgedesign.com Web site: www.rongeorgedesign.com