Unlike valves two-way, these mechanisms do not regulate one flow, but mix two or separate one into two. Hence the three pipes. In ventilation systems, three-way mixing valves are mainly used - to obtain the desired temperature when mixing hot and cold water in front of the heat exchanger (fan coil unit). Almost all models of these devices provide an inversely proportional change in incoming flows - so that the total output flow remains almost unchanged. This important property contributes to the stability of the pressure in the heat exchanger and eliminates water hammer at full overlap. Like all control valves, three-way valves can have mechanical (manual), electrical, pneumatic or hydraulic control of two types - continuous (analog, smooth) and discrete (several fixed positions). The second option does not always provide sufficient accuracy, but has a higher stability of characteristics compared to the continuous one, when the drift of parameters is possible due to temperature, service life and other factors.