Thermostatic mixing valve

A thermostatic mixing valve (TMV) is a valve that blends hot and cold water to ensure a nearly constant output temperature, such as for safe shower and bath temperatures to prevent scalding. The storage of water at high temperature removes one possible breeding ground for Legionella; the use of a thermostat, rather than a static mixing valve, provides increased safety against scalding, and increased user comfort, because the hot-water temperature remains constant.

Source: Wikipedia — Thermostatic mixing valve (CC BY-SA 4.0)