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Thermostatic Mixing Valves

The Dangers of Hot Water

Every year, thousands of people suffer serious thermal shock or scalding injuries in their bathtubs, sinks and showers. Additional untold numbers become ill due to water-borne bacteria, such as Legionella.

Problem: Thermal Shock

Often, injuries in the shower are caused by slips and falls in reaction to a sudden increase or decrease in water temperature. Ideally, if the supply of cold or hot water is lost, the water flow should be shut down immediately. Termination of flow would eliminate the instinctive but dangerous reaction to move quickly away from a too hot or too cold water stream.


Watts Thermostatic Mixing Valves maintain and limit mixed hot water to a desired, selectable temperature, helping to minimize thermal shock.

Problem: Scalding

More than 90% of scalding incidents occur in the home. The tender skin of very young children and the slow reaction time of the elderly and the handicapped make them most vulnerable to serious hot water burns.Scalding injuries are tremendously painful, and the effects can last for years. Scalding occurs for a variety of reasons. In some cases, water heater thermostats are faulty, or set too high. In others, temperature regulating valves at the domestic hot water source are either malfunctioning or missing altogether. Water heaters are normally set to temperatures above 55C (131F) to prevent development of harmful bacteria, such as Legionella, in the water supply. Water at temperatures above 41C (106F) are painful. At a temperature of 55C (131F), a child can be scalded in less than 4 seconds.


Watts Thermostatic Mixing Valves maintain and limit the mixed hot water to a desirable selectable temperature, helping to prevent scalding injury.

Problem: Legionella Bacteria

Legionella is the bacteria responsible for Legionnaires Disease, an acute bacterial infection of the lower respiratory tract. This bacterium was first identified in 1977 by the Centers for Disease Control as the cause of an outbreak of pneumonia that caused 34 deaths at a 1976 American Legion Convention in Philadelphia. Pontiac Fever is a less severe, non-pneumonia, flu-like disease that is associated with and likely caused by Legionella bacteria. Legionella is a fairly common water bacteria and has been found to exist widely in many surface water sources such as; lakes, rivers, streams and ponds. It can also be found in ground water sources and some soils. At the levels found in these naturally occurring sources it typically does not pose a threat to public health. When the bacterium enters a domestic water system

it can find an ideal host environment of warm water temperatures (40-46C), stagnant water areas (storage tanks and dead-end piping legs) and ample food sources (sediment, scale, deposits and biofilm). Under these conditions Legionella can rapidly colonize, forming higher concentrations that can pose the public health threat of Legionnaires Disease.

There are many methods of controlling colonization of Legionella bacteria. However, a widely accepted and preferred method is to maintain the hot water system storage temperature continually at or above 60C. Unfortunately, the elevated temperature necessary to minimize the growth of and kill Legionella bacteria has the potential to cause serious thermal shock and scalding injuries.


Watts Thermostatic Mixing Valves maintain and limit the mixed hot water to a desirable, selectable temperature allowing for the storage of domestic hot water at the higher temperatures necessary to reduce the risk of Legionella growth in the system.

The Limitations of Pressure Balanced Mixing Valves

Many shower and combination tub/shower mixing valves in todays homes use a pressure balanced design. Unfortunately, pressure balanced type valves do not automatically control outlet temperatures, nor do they automatically limit outlet temperatures should the supply temperatures dramatically increase or decrease. Such limitations are potentially dangerous. Further, to prevent users from adjusting the valve beyond a safe operating temperature, pressure balanced valves require setting of the limit stop by the installer. In addition, to accommodate future changes in the hot or cold water temperature, the limit stop would need to be re-set. By contrast, Watts Thermostatic Mixing Valves offer precise control of the outlet temperatures, no matter how the supply temperatures or pressures vary over time.

Watts Makes It Safe

Since 1874 Watts has set the standard for the latest valve technology. Whether it is our pressure reducing valves to regulate steam or water, relief valves to ensure safe operation of water heaters or boilers or backflow prevention devices to help prevent cross-contamination of water supplies, Watts has earned a reputation for safety, reliability and innovation.

With widespread industry attention over the safe storage and distribution of domestic hot water due to concerns of tap water scalding, thermal shock and Legionella, Watts has again taken a leadership role by providing a complete line of thermostatic mixing valves, which provide safe, accurate control of hot water from its source through to its final use.

If you have any questions or would like additional information on how to protect against the dangers of scalding, thermal shock and Legionella growth, please visit our Danger Scalding Lurks page, or contact your local authorized Watts Representative.

Governing Standards

The Canadian national code stipulates that hot water installations in new residential construction requires water delivered to fixtures to have a controlled maximum temperature of 49C (120F).

CSA B125 / ASSE 1016 covers the delivery of water at the individual fixtures that are adjusted and controlled by the user. Therefore, it addresses very precise and immediate temperature regulation requirements. This standard covers three types of valves: Pressure Balancing, Thermostatic and Combined Pressure Balancing/Thermostatic. Watts ASSE 1016 listed valves meet the thermostatic requirements of this standard.

CSA B125 / ASSE 1070 is a recently approved standard and the newest of the three standards. It covers control of maximum temperature to a fixture or group of fixtures; such as sinks, lavatories or bathtubs. The device may either be the final temperature regulation or have water further tempered downstream with the addition of cold water.

CSA B125 / ASSE 1017, on the other hand, does not address final temperature control at fixtures and appliances. It is concerned with valves used at the source of hot water for distribution to the supply system, and therefore allows wider variation of the outlet temperature at higher flow rates.

ASSE 1017 listed devices should never be used to deliver water directly to the user. These valves should be used in combination with an ASSE 1016 and/or ASSE 1070 listed devices.

For more information, please check the CSA web site at:

and the ASSE website at:

How They Work

Upon use of tempered water, a thermostat in the mixing chamber of the valve senses the outlet temperature. The thermostat automatically positions a seat assembly which controls the flow of hot and cold water supplied to the mixing chamber. If the mixed outlet temperature increases, the thermostat will expand moving the seat assembly to allow the cold water inlet port to open more fully and at the same time restricting the hot water inlet port. Conversely, if the mixed outlet temperature decreases, the thermostat will contract moving the seat assembly to allow the hot water inlet port to open more fully and at the same time restricting the cold water inlet port. In both cases the mixed outlet water temperature is automatically and continually maintained at the preset temperature within the tolerances of the valve. In the event of a cold or hot water supply failure, the seat assembly moves to an extreme position shutting off the hot or cold inlet water port. A mechanical adjustment permits selection of the desired outlet water temperature within range of the valve.

Please download Flash 8

Why They Are Used

Supply Systems

Mixing valves provide the following benefits:

  1. The water temperature that is discharged directly from a water heater can vary 10C or more. This can be due to tolerance of control devices, inlet water temperature changes and stacking (higher water temperatures at the top of the water heater). Use of a thermostatic mixing valve assures constant outlet water temperature even with variations in hot or cold water supply temperature.**
  2. If a water heater is operated at lower temperatures, high flow demand situations can result in a reduction of hot water supply temperatures if the recovery time of the water heater is not sufficient for the flow demand. Using a hot water extender or mixing valve allows the water heater to be operated at higher temperatures, extending the effective system flow rate and preventing the growth of Legionella.
  3. To be able to supply peak demands for domestic hot water it is often necessary to provide high capacity water heaters with high recovery rates. Use of a hot water extender can reduce the size/BTU requirements needed to provide capacity for peak system demands.

At the Fixture

Heat loss in the system piping can result in varying temperatures of the hot and cold water supplied to fixtures. The use of thermostatic mixing valves assures constant safe hot water temperature at the point of delivery to the fixture.

** Please refer to specification for the selected model to determine the allowable inlet temperature range to maintain a desired outlet temperature.

Design Considerations

Heat Traps

  • All mixing valves used at the source of supply should be trapped. Trapping the valve helps reduce mineral deposits and premature component wear.
  • Heat traps help reduce convection loss also know as thermosiphoning, increasing system efficiency.

Stop Valves

  • All mixing valves should be installed with a means to isolate the valve for service and repair.

Check Valves

  • Check valves assure flow of water in one direction. They prevent thermal siphoning of hot water to the cold water supply.
  • The addition of check valves should be considered whenever the potential exists for extreme differential pressures between the hot and cold water supply.
  • Check valves are a requirement for compliance with ASSE Standard 1017 applications. Watts Series USG, MMV and 1170 come standard with integral check valves.


  • Water supply systems should be thoroughly flushed prior to placing mixing valves in service.
  • Installation of additional strainers in the supply piping is recommended in systems where water can contain particles that could impede the proper operation of the mixing valve. Watts Series MMV and 1170 include inlet filters to provide protection against supply line debris.

Maximum Pressure

This is the maximum pressure that the valve could be subjected to during normal operation. Normal residential water system pressures should be limited to 80psi (552 kPa) as required by national codes.

Maximum Temperature

This is the maximum temperature that the valve could be subjected to during normal operation. Exceeding the maximum temperature could cause failure of internal components.

Minimum Flow

A minimum flow of water through the valve is required to provide accurate temperature control. Water flow below the listed minimum could result in increased outlet deviations.

Minimum Inlet Differential

This is the minimum difference between the hot and cold temperature that is required to produce the desired outlet temperature.


At the Fixture

Delivery of water to fixtures intended for use in bathing or washing should always be controlled by valves listed to ASSE Standard 1016 Type T or ASSE Standard 1070 such as Watts Series MMV, L111 and USG mixing valves. ASSE 1016 listed valves provide the user with both scald protection and thermal shock protection. ASSE 1070 listed valves provide the user with scald protection only. These valves should never be set to exceed a maximum temperature of 49C (120F). (Watts recommends the maximum temperature of 43C (110F) for shower and bathing fixtures.)

Supply Systems

The control of water temperature for the source of supply should be controlled by mixing valves such as ASSE 1017 listed Watts Series 1170, L1170 and MMV. These valves can also be used for re-circulation systems to maintain domestic hot water supply temperatures at levels (temperatures above 55C (131F) that prevent the growth of harmful bacteria such as Legionella.

Radiant Heat Systems

Either a Watts 1170 or an L1170 mixing valve can be used for radiant heat systems. However, it is important that a boilers high temperature limits do not exceed the maximum temperature rating of the system components. This will protect against component temperature failure should dirt, sediment or other mechanical failure cause a mixing valve to become inoperable.

Periodic Inspection

Regular inspection of mixing valves is recommended to assure maximum valve life and a properly functioning system. Corrosive water conditions, unauthorized adjustments or repairs can be detected during inspection and service of the valve. The frequency of cleaning depends upon local water conditions.

More Information

Danger Scalding Lurks!

Every year, thousands of people in the United States suffer serious thermal shock or scalding injuries in their bathtubs, sinks and showers.

Click here for informative resources to help you protect against these dangers.

Additional Information

Danger – Scalding Lurks!

Every year, thousands of people suffer serious thermal shock or scalding injuries in their bathtubs, sinks and showers.

Click here for informative resources to help you protect against these dangers.

Learn About

Thermostatic Mixing Valves

Every year thousands suffer injuries as a result of hot water scalding and thermal shock injuries. Thermostatic mixing valves protect against these dangers.

To learn more about thermostatic mixing valves and how they work, click here.