What is a mixing valve?

A mixing valve combines hot and cold water before it comes out of the faucet.

A mixing valve is a rather vague 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-scalding safety devices in hot water systems. These safety valves automatically control hot water temperature and can be centrally located to control flow to all outlets or installed at individual points of use. The term mixing valve can also be applied to single spout, hand mixer taps in sinks, bathtubs and showers. These valves have a wider range of adjustment, but alone do not provide protection against scalding and thermal shock.

The mixing valve can be applied to single-spout hand mixer taps in showers.

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 scalding and thermal shock. It is generally accepted that the maximum safe temperature for water used in domestic systems should not exceed 122° Fahrenheit (48° Celsius). It is mandatory in many areas that the hot water system outlets be regulated to no more than 122 degrees. This is why central thermostatic mixing valves may be required by law for new hot water installations.

Without a mixing valve, if a person is using the shower and the flush is turned on or another faucet is turned on, the water temperature can change dramatically.

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Thermostatic mixing valves feature a central thermostat unit preset to the desired outlet temperature. The valve body is equipped with hot and cold inlets and a mixed water outlet. The thermostat mechanism is mechanically connected to a movable plunger that regulates the amount of hot water it lets into the valve according to the outlet temperature. If the cold water pressure drops significantly or is turned off, the valve will instantly shut off the hot water supply altogether. This avoids any possibility of end users being scalded in the event of a cold water supply failure.

Installing a thermostatic mixing valve in a domestic hot water system can help prevent scalding injuries.

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

Faucets with a manual mixing valve are, however, still 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. Hand mixers can consist of dual faucet controls or single one-way controls with a single spout or outlet for mixed water. While usually a very attractive decor addition, a manual mixing valve alone does not provide protection from sudden drops or interruption of cold water supply. To avoid the possibility of burns, at least one thermostatic mixing valve must be installed on all hot water systems.

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