Many modern homes have mechanical ventilation, with a 3 position switch to control the speed. However, common ventilators allow stepless speed control. They set the speed corresponding the voltage of an input connector. If 0V is provided, the fan goes off, if it is +10V, the fan goes to maximum speed. If only there was a way to set a voltage between 0V and +10V wirelessly, by a popular home automation protocol such as Z-Wave.

Actually, there is. It is a brilliant device by Qubino, called the Flush Dimmer 0-10V. I installed it into our ventilation unit (a Stork CMFe), and connected it to my Z-Wave network and Domoticz.