Roof fan CFM consideration

[Flyview]

I'm considering adding a roof fan to my pop-top. I'm looking at the MaxxAir fans. They have the regular big square fan (900CFM) and a smaller "dome" fan at 200CFM. This got me thinking about the CFM actually required in a Ford van. According to a quick search the interior space in a Ford van is about 250CF. Say minus all the cabinetry and plus the pop top that's 300CF of space.

Does that really mean that the large fan can move 3 times the air inside, in one minute!? That seems insane.

Can someone explain this please? haha

[BrianW]

Following, as I'm currently thinking along the same lines.

[Steve Hunt]

I'm sure its like most things, the rating is under perfect test cell conditions. Generally speaking the intake area would need to be 1.5 times the discharge area to have a chance of pulling the 900 cfm rating into the space and then push it out the roof opening. If you kept all the windows closed for example then the fan would be working really hard to draw air into the van from any little crack or crevice not sealed, would put a vacuum on the space and derate severely down from the 900cfm. I'm sure it has multiple speeds, right? As you play with it you may use low at night but even though your in So Cal i think San Diego tied the 1877 record at 93 degrees the other day so it might be nice to use it on high to evacuate the oven on occasion. Of course remember everything inside will be fully heat soaked so it's not like it will magically be cool once you exchange all the air a few times. I'd go with the 900cfm unit.

[BrianW]

Or maybe the big one in the front and the small one in the back to allow for airflow?