Rehau Actuators are fitted to the manifold. When the room thermostats in each zone adjusted in temperature, the actuator on the manifold will adjust the amount of water flowing through each zone. If you turn the thermostat down, the actuator will automatically adjust the flow of hot water to the circuits of heating pipe in that zone. The opposite is true as well, when you turn the thermostat up the actuator will adjust the flow of hot water to that zone accordingly.