Technically a CEA is neither "open or closed loop", although it is more akin to a closed loop because it does not allow any outside air to be exhausted or taken in. CEA is a term to describe a highly engineered 3 room environment where each room can be individually controlled. CEA requires dedicated power circuits. It's the most expensive multi room system to set up because it also requires a dedicated heavy duty air conditioner and dedicated controllers for CO2, RH, and controllers that allow you to tweak every single aspect of every room to perfection. A proper CEA system must contain a dedicated third room that acts as the lungs of the entire system. Oversimplifying it but hopefully you get the idea!