How to Install a Thermostatic Mixing Valve

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Thermostatic mixing valves keep children safe from being scalded from the tap.
Thermostatic mixing valves keep children safe from being scalded from the tap. (Image: faucet image by Laura Dynan from Fotolia.com)

The water temperature in a hot water heater needs be kept at approximately 140 degrees Fahrenheit to limit the potential of the unit becoming a breeding ground for Legionella, the bacteria that cause Legionnaire’s disease. However, water can cause scalding at approximately 120 degrees Fahrenheit, creating a difficult dilemma for homeowners. To address this issue, many areas now require the installation of thermostatic mixing valves by code in residential applications, with exceptions such as a direct line to a dishwasher. Thermostatic valves attach to both the cold water line and the hot water line to alter the cold water to hot water ratio, which allows the temperature of water from the tap to be safe.

Things You'll Need

  • Wrench
  • Screwdriver
  • Solder
  • Torch
  • Thermometer

Installing a Thermostatic Mixing Valve

Check local building codes to ensure all laws are followed with regard to what qualifications are required by code when installing the mixing valve.

Turn off the main water supply and open all faucets to allow water in the lines to drain. Be patient as this may take some time.

Select a location that will be accessible for cleaning, adjustment and servicing. Also note that dishwashers may benefit from hotter water. Run a separate line directly from the water heater to the dishwasher or to other appliances that might require higher temperature water to avoid using the mixing valve in these cases.

Solder the connections to the thermostatic valve before attaching it to prevent damaging the gaskets. Ensure that the solder is safe for potable water to prevent harmful elements from having contact with water used for drinking.

Tighten the connections to the valve using a wrench.

Turn the water on and check the temperature of the water from a faucet using a thermometer. Allow some time for the hot water to reach the faucet before recording the temperature.

Turn the hand wheel to adjust the dial to the desired temperature. The instructions that come with the thermostatic valve should indicate what the current assumed water temperature is without the mixing valve attached and what difference a one-degree turn will equal in percent.

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