Thermostatic taps allow you to obtain a constant temperature of the water they supply, automatically regulating the flow rates of the hot and cold water mixture, independent of the temperature and to a large extent also of the pressure of the supply lines.[1]​ They have two controls with clearly differentiated functions: one for the flow rate and the other for the temperature.

Although there are thermostatic taps for all applications, their use has basically become widespread in the shower and in the bath-shower, due to the obvious convenience of always having water at the same temperature, even if the tap is opened and closed or the flow rate is varied.

The key part of a thermostatic faucet is the regulator around which the mixed water flows. It consists of a bimetallic wire wound in the shape of a helical spring, or a capsule filled with paraffin, called wax expansion element. Both types of regulators change length proportionally to the temperature changes they experience. Versions that use a bimetallic cable can usually be designed with larger length changes, resulting in greater precision in water temperature control.[2]​.

If the inlet water conditions change (temperature or flow), the regulator contracts or lengthens, gradually opening or closing the holes that give way to the flow of the cold and hot water inlet pipes.

The hot water temperature can be kept constant, with a margin of error of the control system of approximately one degree Celsius. As with domestic hot water taps, thermostats are usually equipped with a locking mechanism on the operating lever, which must be unlocked to select temperatures above 38°C. Some thermostats have a water save button, which must be unlocked to obtain the maximum possible flow.

As a general rule, non-return valves are provided in both the cold water supply and the hot water supply, in order to avoid a hydraulic short circuit between the cold and hot water networks, which would otherwise occur when connecting the two networks at the thermostatic tap.[3][4]​.

There are special designs in which the mixing chamber is surrounded by cold water, so that the faucet always remains cold from the outside, avoiding possible burns when touching the faucet when hot water at very high temperatures is used in the supply network.