Hot water system temperatures and the code

I have noticed a lot of misquoting of the plumbing codes when it comes to hot water system temperatures and hot water system controls. There are a lot of people that think the model plumbing codes address domestic hot water system maximum storage temperatures. In fact, I am not aware of any model plumbing code that addresses minimum or maximum storage temperatures.

Misinterpreted codes

There have been people claiming the maximum allowable storage temperature or thermostat setting on a water heater is 120°F). There is no such requirement in any model plumbing code. There have been claims the maximum temperature is 125°F based on the scalding temperature labels on the outside of most water heaters. Again, there is no such requirement in any model plumbing code addressing the hot water storage or distribution temperature from a water heater.

The warning labels simply notify the owner or tenant that water temperatures in excess of 125°F can cause serious injuries and death and that children, the elderly and disabled people are at most risk. The label also informs you that there are temperature limiting devices available and to refer to the installation and operation manual for more information.

Domestic hot water storage and distribution temperatures are often one of the most misunderstood areas of the plumbing codes. The model codes list maximum hot water temperatures that can be delivered from various fixtures, but the storage and distribution temperatures have historically been left unaddressed in the plumbing codes. From my attendance at the code hearings over the years, mandating temperatures can increase costs and has been seen as limiting design options. So, it is important for a domestic hot water system to be designed for the intended application, installed in accordance with the design and maintained in accordance with the design. If any one of the links in the chain fails there could be problems.

The code gives us temperature limitations for various plumbing fixtures like showers and bathtubs (a maximum of 120°F), and this is often misinterpreted as a maximum temperature setting for the water heater thermostat or the storage temperature. This is one of the most common misinterpretations.

The 2012 IPC has the following shower valve temperature requirement:

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.

In the language in section 424.3 of the 2012 IPC, it is clear that the shower valve has an adjustment on it called a “maximum temperature limit-stop.” It must be adjusted after installation and before occupancy of the building where the contractor or owner must set and maintain the limit-stops seasonally for scald protection. The reason they must be adjusted seasonally is because the incoming cold water temperature changes seasonally, which can affect the outlet or mixed water temperature setting.

In discussions with many people on this issue, I have discovered that many people do not have a working knowledge of how shower valves are manufactured, designed, installed and maintained. It is for this reason I have suggested ASSE work on a white paper to educate the public and the industry on how to set the maximum temperature limit-stops on shower valves and on other temperature limiting devices. There are a few people that erroneously believe that the language in the code that states, “means to limit the maximum setting of the valve to 120°F can be accomplished with an adjustment of the thermostat dial on the water heater.”

There are also people that believe that hot water system temperature control scald prevention can be accomplished by using a master thermostatic mixing valve at the water heater without setting the limit stops at the shower valves or without using point of use temperature limiting devices. Master mixing valves are not mandated in the model plumbing codes, but it is good design practice for a constant hot water supply temperature.

Domestic hot water systems are unique in that a change of equipment in one part of the system can and most likely will affect the performance of another part of the system. Simply replacing a leaky water heater, circulating pump, balancing valve or mixing valve can significantly alter the system’s performance. I have investigated many scald incidents where the water heater was replaced and the new temperature setting was much higher which caused scald incidents. There have also been incidents where the circulating pump stopped working, causing calls for “no hot water” the maintenance staff went to the mechanical room and cranked up the water heater thermostat. Then, when unsuspecting individuals finally got hot water, they were blasted with scalding hot water.

There are several design issues that a domestic hot water system must address. Some of these are covered in the codes and some are not covered in the codes at this time. They are as follows:

1. Maximum Temperature Flowing from Fixture Outlet - The codes address maximum temperatures from Showers, maximum temperature flowing from a fixture to prevent scalding. As stated in the plumbing codes, the maximum temperature flowing from the fixture is not the maximum storage or distribution temperature.

2. The minimum temperature of hot water to prevent Legionella bacteria growth - The minimum temperature to prevent Legionella Bacteria growth is 122°F. Above 122°F and up to 131°F, Legionella bacteria survives, but will not multiply. At 131°F, it takes about 5 to 6 hours for the bacteria to die. At 140°F, the bacteria dies in about 32 minutes. At 151°F, the bacteria dies instantly. The recommended minimum disinfection temperature is a few degrees above 151°F, which is 158°F for about 5 minutes. To prevent bacteria growth, a safety factor of a couple of degrees would require a minimum of 124°F at the coolest spot in the hot water distribution system. The lowest temperature of hot water in a circulated hot water distribution system is always in the hot water return piping right before it connects back to the cold water inlet to the water heater. A temperature gauge should be located just before the hot water return connection to be able to monitor the lowest hot water temperature in the system and allow maintenance staff to adjust the system temperatures at the source to maintain a temperature above the Legionella growth temperature. It is always a good idea to locate a temperature and pressure gauge at the top of each water riser and at the end of remote branches to record the temperatures and pressures in those locations for diagnostic purposes in larger buildings. (Generally, I recommend this where the end of the domestic hot water supply pipe branch is more than 100 feet from the source.)

Is there a minimum hot water storage temperature?

No there is no code language addressing a minimum domestic hot water storage temperature. The plumbing codes define hot water as follows:

Hot water - Water at a temperature greater than or equal to 110°F (43°C).

This does not mean it is good design to store the hot water at 110°F. Storing at 110°F would require a massive water heater tank, larger hot water pipes because there would be a greater percentage of hot water flowing through the hot water pipe vs the cold water pipe for the mixed temperature and it is in the ideal growth temperature range for Legionella bacteria growth. Legionella bacteria grow and multiply between 68°F and 122°F; they reproduce rapidly and thrive between 95°F and 115°F. Below 68°F, the bacteria survives, but it does not multiply. From 115°F to about 122°F, the bacteria grow slowly. From 123°F 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°F to 140°F. 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

There have been many green code and energy code proposals by well-intentioned individuals trying to limit hot water temperatures for energy conservation purposes. There are Legionella outbreaks that have been attributed to energy conservation efforts by maintenance staff and energy conservation programs that mandated turning the temperature down on water heaters to try and accomplish energy savings to reduce stand-by energy losses. In other cases, the temperatures were turned down to try to prevent scalding. I have investigated many Legionella outbreaks associated with low hot water storage temperatures.

There is no language in the model codes that addresses storage or distribution temperatures in domestic hot water systems. Unfortunately, there are some local codes that have allowed the water heater thermostat to be reduced to 120°F to provide scald protection in showers and bathtubs in lieu of requiring shower valve products that meet the industry standards for temperature and/or pressure compensating shower valves and have adjustable temperature limit-stops to prevent scalding in showers or requiring temperature limiting valves that conform to the industry standards listed in the codes.

There is not a storage type water heater that I am aware of that has a thermostat that is capable of maintaining a constant and safe hot water outlet temperature. Water heaters should have their temperatures turned up and they a temperature actuated mixing valve should be provided on the outlet piping of the water heater. This is because most of the thermostats on storage type water heaters are located near the bottom of the tank to sense the incoming cold water.

The cold water connection is connected to the bottom of the tank through a dip tube or through a tank connection near the bottom of the water heater tank. Most water heater thermostats are not designed to accurately control the outlet temperature of a water heater. They are designed to turn “on” and “off” the burner or energy input. There can be significant fluctuations of close to about 15 degrees above and below the set point, which can allow over 30 degrees of temperature fluctuation in the hot water tank from the “energy on” temperature to the “energy-off” temperature.

What causes changes in hot water system temperatures?

There are several factors that can cause changes in hot water system temperatures. These factors include: usage or draw-down, heater design, thermostat design, system balancing issues, circulating pump issues, mixing valve set points, improper mixing valve piping, flow velocity in instantaneous water heaters, Insulation thickness and type, and many more issues that can affect the hot water system temperature.

The inherent design of many water heater thermostats has a built-in delay in responding to the water temperature in addition, intermittent short draws of a gallons or two of hot water by building occupants can cause cold water to be drawn into the bottom of a water heater causing the thermostat to cycle “on” and heat up the few gallons of cold water at the bottom of the tank. The water heater is also overheating the less dense hot water that rises to the top of the tank. This phenomenon is known as thermal layering or stacking in a water heater. If the multiple short draws continue for several cycles, it can significantly overheat the water at the top of the tank. It is for this reason model plumbing codes do not allow the thermostat on the water heater to be the final temperature control for scald prevention purposes.

It is not unusual to see temperature swings in excess of 30°F at the top of a water heater. This is why it is a good idea to have a master mixing valve, also known as a temperature actuated mixing valve, which conforms to ASSE 1017 or CSA B125 on the water heater outlet pipe.

Is there a maximum temperature limit for domestic water heaters?

The model plumbing codes do not address a maximum storage temperature. For combined heating hot water and domestic hot water systems, when the heating hot water system exceeds 140°F the IPC code requires a temperature actuated mixing valve conforming to ASSE 1017 to limit the domestic hot water temperature to a maximum of 140°F. That still does not address the maximum storage temperature in the hot water tank.

Are there maximum temperature limits at various plumbing fixtures?

Yes, because of scalding concerns various fixtures have maximum temperature limits for the water leaving the fixture outlet. Over the years, I have served on many code ad hoc committees and standards committees for temperature control valve products and it has generally been agreed that the maximum safe hot water temperature for many fixtures was 120F°. This was because at 120°F it gives the bather about five minutes to get out of harm’s way before an irreversible burn injury would occur. The time varies based on the thickness of the skin. Children, the elderly and handicapped persons are generally more at risk because they generally have reduced ability to react.

Chapter 4 of the model plumbing codes cover specific requirements for plumbing fixtures. They are:

A. Bidets

Bidet’s are limited to 110°F in the model codes with the use of an ASSE 1070 device. This standard has recently been harmonized as ASSE 1070-2014/ASME A112.1070-2014/CSA B125.70-14 Performance Requirements for Water Temperature Limiting Devices. The discharge water temperature from a bidet fitting should be limited to a maximum temperature of 110°F (43°C) by a water temperature limiting device conforming to the temperature limiting valves listed in the codes. The ASSE 1070 standard was recently updated and Harmonized with CSA and ASME and will take several years to be listed in the model codes. Engineers can be proactive by specifying the new harmonized standard now instead of waiting for it to be required in the 2018 code which will be adopted in many jurisdictions in 2019.

B. Tempered water for public hand-washing facilities

Specifications for public hand washing facilities should require tempered water to public hand washing facilities should be limited to a maximum of 120°F and delivered through an approved water-temperature limiting device that conforms to ASSE 1070-2014/ASME A112.1070-2014/CSA B125.70-14.

C. Shower temperature limits

Chapter 4 of the model plumbing codes covers individual shower valves and tub-shower combination valves and requires them to 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 This standard was also recently harmonized and the new title is, “ASSE 1016-2011/ASME A112.1016-2011/CSA B125.16-2011. (ASSE 1016) Performance.” Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations 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 at the valve to 120°F (49°C) or lower, which shall be field adjusted in accordance with the manufacturer’s instructions. In-line thermostatic valves including ASSE 1070 point-of-use mixing valves and ASSE 1017 master mixing valves should not be utilized for compliance with this section because they cannot address thermal shock associated with pressure imbalances in the system associated with fixture use. This is especially important for this application because in a shower, the entire body is immersed in the stream of water and any sudden change in temperature can cause a slip and fall injury or a scald injury. Mixing valves can reduce the hazard for scalds, but not thermal shock hazards.

D. Gang showers

Temperature limits and allowable devices for gang showers are covered in section 424.4 of the IPC. For single temperature installations the valve is typically set to a comfortable bathing temperature by the facility personnel at about 100°F to 105°F. The bather typically does not have individual temperature controls when an ASSE 1069 device is installed. The bather would simply have an on/off valve or a metering button. Gang showers are common in schools, prisons, health clubs and other institutions applications. The temperature limit for this type of fixture is 120°Ffor gang showers, but that would not be a practical temperature setting. The typical temperature setting is at a comfortable bathing temperature between about 100°F and 105°F. If individual shower controls are used and the showers are piped with both hot and cold water, the shower must have both hot and cold water supplied to it and then an ASSE 1016-2011/ASME A112.1016-2011/CSA B125.16-2011. Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations (ASSE 1016) device would be appropriate to control the water temperature. An ASSE 1016 device has a maximum temperature limit stop that should be adjusted by the installer and seasonally readjusted by the maintenance staff to limit the maximum hot water temperature to 120°F it or less.

E. Bathtubs and whirlpool bathtubs

Bathtub and whirlpool bathtub valves should have the hot water supply 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.

Is there a minimum hot water storage temperature for water heaters?
The plumbing codes do not specifically address a minimum hot water storage temperature, but they define hot water as “water at a temperature greater than or equal to 110°F (43°C). Unfortunately, 110°F is an ideal temperature for Legionella bacteria and other pathogens to grow.

Also, if a water heater is set to 110°F, and designed for a 20 degree temperature difference across the system the fixtures at the end of the system would get water that is about 100°F and return water temperatures of about 90°F.

Is there a code requirement to readjust the temperature limit stops if the hot water system temperatures are changed?

That could be argued, but there is not any language that addresses it specifically. However, there is code language that requires the components to be maintained in accordance with the manufacturer’s installation instructions. So, it depends on if the manufacturer covers this language in their installation and maintenance literature. Some do and some don’t.

Regardless of what the manufacturer’s literature says, if there is something that causes the hot water system temperatures to change, every shower valve and temperature limiting valve in the system should be readjusted to limit the hot water system temperatures to a safe temperature. So, if a water heater is replaced, the installing contractor should wait until the hot water temperature is heated up to full hot water temperature and then go flow hot water from every shower valve in the facility and verify if the hot water limit stops are set properly. If not, he can offer to readjust them for the homeowner for a price, or provide written instruction to the owner on how this procedure should be performed. This will then remove the liability form the contractor if there is a scald incident associated with a limit stop after he has performed work on the hot water system.

Good contractors will have this paperwork already printed up with the instructions on how this should be done. If the owner does not want to take on this task, a fee can be charged for each valve that is readjusted to prevent scald injuries. The same procedure should be followed if someone simply readjusts the thermostat setting on the water heater.

Hot water system commissioning checklist:

1. Verify the circulating pump is operating if one is installed.

2. Verify that the circulating pump flow direction is correct.

3. Document head pressure, flow and horsepower, voltage, phase, manufacturer and model number of the circulating pump.

4. Verify and document the set point of the thermostatic mixing valve (if a mixing valve is installed). Document the incoming cold water temperature, the incoming hot water temperature and the outlet temperature with a single fixture flowing downstream. Verify that the temperature difference between the hot water and the tempered water meets the manufacturers required temperature differential requirements (5°F - 20°F). Bi-metal coil type mixing valves generally require a higher minimum flow rate or circulated flow and a temperature difference of at least 20°F.

5. Document manufacturer, model number and flow and temperature range of the mixing valve (if one is installed).

6. Verify if a thermal expansion tank or means to relieve thermal expansion is installed and properly sized and located in the cold water line to the water heater.

7. If both a circulating pump and thermostatic mixing valve are installed, verify that the return piping is properly installed in accordance with the mixing valve manufacturer’s installation instructions.

8. Run hot water from the farthest fixture from the hot water source until the hot water temperature stabilizes. Document and record the maximum hot water system temperatures using a calibrated thermometer in a cup within 6 inches of the fixture outlet at the farthest fixture every 15 seconds until the temperature stabilizes for five consecutive readings.

9. If there is a temperature gauge near the end of the hot water return system, verify that the hot water return temperature is above 124°F to prevent Legionella bacteria from growing.

10. If the hot water return temperature is too low, adjust the thermostat on the water heater or the thermostatic mixing valve as required to get the desired hot water return temperature to avoid hot water system temperatures that are conducive to Legionella and other organic pathogen growth.

11. Record the maximum hot water system temperatures using a thermometer in a cup within 6 inches of the fixture outlet at all fixtures utilizing hot water. Record the temperatures every 15 seconds for five minutes or until the temperature has stabilized for five consecutive readings.

12. If any showers, tub/showers or other fixtures with code required temperature limits have a maximum hot water temperature that exceeds the code limit, readjust the limit stops or limiting device and re-perform the maximum temperature limit test.

13. If the owner does not want to perform the maximum temperature limit-stop tests to verify the temperatures are safe, provide documentation to the owner on how it should be done. Also, provide the manufacturer’s installation and maintenance literature and have the owner sign off on a release advising them that you have made them aware of this requirement and that it is their responsibility to perform the limit-stop adjustments before occupancy and seasonally in accordance with the manufacturers installation instructions.

Good system design would follow the temperature setting listed. Hot water should be stored at or above 140°F to minimize Legionella bacteria growth. The system should have a mixing valve to establish a constant hot water delivery temperature with a circulating pump sized for a 10 degree temperature difference throughout the system. If the system utilizes a bi-metal coil technology in the master mixing valve, a 20 degree temperature differential may be required for the valve to operate properly. The temperatures throughout a system with a 10 degree system temperature differential design should be as follows:

1. Hot water storage temperature in water heater (WH) = 150°F.

2. Hot water distribution temperature at master mixing valve (MV) outlet = 134°F.

3. Minimum Hot water return temp. at HWR conn. to CW inlet of WH/MV = 124°F.

4. Showers - Each shower will have a slightly different hot water supply temperature between 134°F and 129°F depending on where it is in the distribution system, the maximum temperature limit stop should be adjusted on all showers and tub/showers to a maximum of 110°F-115°F or as required (120°F max.)

5. All other fixtures requiring temperature limit protection - Each fixture will have a slightly different hot water supply temperature between 134°F and 129°F, depending on where it is in the distribution system, if a temperature at or below 120°F is required for scald protection a point-of-use mixing valve conforming to ASSE 1070 should be used. The maximum temperature limit adjustment should be adjusted to limit the hot water to the required temperature at or below 120°F to prevent scalding (110°F at bidets. See code for various other temperature limits at fixtures).

Ron George, CPD, is president of Plumb-Tech Design & Consulting Services LLC. Visit

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