I have heard that the airflow to the rear seats from the side channels was minimal, at best. The air goes through a mixing chamber on each side mixing outside air and air from the heater core. It then goes into a side channel, making a 90 degree turn. Halfway down the channel there is a tab protrusion that blocks a portion of the channel. Also, there have been reports the the supplied vents did not sufficiently close off the cold outside air in flight.
I had thought about adding something like CPU fans at the vents to draw air in but thought about the added weight, power problem, and complexity. While installing the avionics tray it became obvious that the air system was making things difficult.
I decided to simplify the system by removing the mixing chambers and side channel transitions. I purchased AveoMax vents that are now directly duct by CEET to the vents. I believe the airflow will be increased out of the vents. Heat is solely provided to the floor and defroster. I may run a smaller heat duct down the center channel to the rear floor.