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What is a Mixing Valve?

By Paul Scott
Updated May 17, 2024
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A mixing valve is a fairly loose term used to describe many devices that combine hot and cold water to produce a comfortable and safe temperature. It is most commonly used to describe thermostatic mixing valves included as anti-scald safety devices in hot water systems. These safety valves automatically control the hot water temperature and may be centrally located to control flow to all outlets or installed at individual points of use. The term mixing valve may also apply to single spout, manual mixer faucets in sinks, baths, and showers. These valves have a larger range of adjustment but on their own offer no scalding and thermal shock protection.

It is becoming standard practice that water stored for domestic applications is kept at high temperatures to prevent the development of dangerous bacterial strains. Typically these temperatures exceed 140° Fahrenheit (60° Celsius) which is hot enough to cause severe scalding and thermal shock. It is generally accepted that the safe maximum temperature for water used in domestic systems should not exceed 122° Fahrenheit (48 ° Celsius). It is mandatory in many areas that hot water system outlets be regulated at no more than 122 degrees. This is why central thermostatic mixing valves may be required by law for new hot water installations.

Thermostatic mixing valves feature a central thermostat unit preset to the desired outlet temperature. The valve housing is fitted with hot and cold inlets and a mixed water outlet. The thermostat mechanism is mechanically connected to a moving plunger which regulates the amount of hot water it allows into the valve according to the outlet temperature. Should the cold water pressure drop significantly or be turned off, the valve will instantly shut off the hot water supply completely. This prevents any possibility of end users being scalded in the case of cold water supply failures.

A single central thermostatic mixing valve is the most common installation arrangement although there are several point-of-use faucets available which feature thermostatic control. Typically though, a single unit is installed on the main outlet from the water heater which then feeds water at below 122° to manual mixer valves on baths and showers. This allows the user to preset the flow temperature at will without any fear of a sudden surge of dangerously hot water.

Manual mixing valve faucets are still, however, very popular in kitchens and bathrooms. These devices contain the same internal layout and mechanisms as a thermostat mixer with the exception of manual adjustment. Manual mixers may consist of dual faucet controls or single, unidirectional controls with a single spout or outlet for the mixed water. Although generally a very attractive decor addition, a manual mixing valve on its own offers no protection against sudden drops or cessation of the cold water supply. To avoid the possibility of scalding injuries, at least one thermostatic mixer valve should be installed on all hot water systems.

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