Rotary extraction system
Most mechanical roof fans have a rotary exhaust system. This means that the rotating hood is driven by the driving wind, pulling air out of the cargo space. But actually, you don't achieve much with just a roof fan. No refreshment of air takes place.
Roof fan with floor outlet
If you want to have a constant air flow in your cargo area, you can combine a rotary roof fan with a floor exhaust. This creates an underpressure. The underpressure draws fresh air (but also dirt and exhaust gases) through the floor, after which the roof fan extracts gases. As a result, you create a constant airflow from the floor vent at the bottom of the cargo area to the roof fan at the top. A constant airflow does not yet mean that air exchange is taking place.
Roof fan with push and pull operation
If you want to refresh the air in the cargo area of your van, it is best to install a mechanical roof fan with push and pull operation, such as the Easy Aero. A roof fan with two separate holes provides powerful air circulation. This arguably refreshes the entire cargo area of your van. A big plus is that you no longer need a bottom drain, meaning you don't have to drill a hole in the floor of your van.