If a cooker hood has been connected to the ventilation unit, extract air will increase during cooker hood operation in order to draw air through the cooker hood. At the same time, extract air from the bathroom and the lavatory will also increase, which is entirely unnecessary. 

However, by installing a EM-box, you can regulate air extraction, so it is not increased in other rooms but only in the kitchen via the cooker hood where it is necessary. 

The EM-box is fitted with a steel filter that serves as extra protection and helps prevent grease from entering the unit. 

The EM-box should be mounted in the extract air duct immediately before the ventilation unit so you can regulate the extract air for the rest of the extract air valves.