A thermostat is designed to regulate the entire cooling system of an engine and, in particular, after a cold start, a reduced circuit between the engine and the unit heater is made possible by the thermostat.

This guarantees an even rise in temperature of the entire engine, including the necessary heating for inside the car itself.

However, when the engine has reached its optimum running temperature, the thermostat directs all or part of the coolant towards the radiator to facilitate the release of the surplus heat produced by the engine.