Thermostatic Mixing Valves

How They Work


Safe storage and distribution of domestic hot water is imperative in any application. We provide a complete line of thermostatic mixing valves to ensure safe, tempered water is delivered from source to use.

The Benefits

  • Assures constant outlet water temperature even with variations in hot or cold water supply temperature.**
  • Allows the water heater to be operated at higher temperatures, extending the effective system flow rate and preventing the growth of Legionella.
  • Can reduce the size/BTU requirements needed to provide capacity for peak system demands.
  • Assures constant safe hot water temperature at the point of delivery to the fixture.

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.


  • 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.

What You Need to Know


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. 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 ASSE 1017 listed mixing valves. 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

It is important that a boiler's 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.

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