Where to place the vent-fan on roof of van
 

Can anyone provide guidance as to where I should install a vent fan on the roof of my van?

I have a GM full-size cargo van (Chevrolet Express/GMC Savana), standard wheelbase. The length of the roof is approximately 137 inches.

I will be installing a MaxxAir RV vent fan:
Attachment 30442

From a ventilation standpoint, I would like the vent fan to be as far back as possible because while the doors are closed the only inlet of fresh air are the two front windows. I don't think my intuition is worth much when it comes to aerodynamics, but I suspect the drag penalty will increase significantly the closer the vent is towards the very rear of the roof.

That being said, this model is designed in such a way that it can be left in the open/raised position while driving. I have no idea how I should factor that in! I'm inclined to think it will be closed the majority of times I'm driving at highway speeds.

Attachment 30444

Here's a profile of the van. If there's any templates I could superimpose that would be helpful, let me know. This van should not have anything else mounted on the roof for the majority of the time.

Back in the '70s Dodge Ford and Chevy stuck a square pattern in the back of their van roofs exactly for this purpose. Open up the max air a bit and it would double the airflow from the A/C. The front scoop/flap did nothing except seal off the front when it was open. Don't know what it did for roof airflow as I was driving an aerodynamic ugly box that at best got 18 mpg downhill with tailwind.

I would place it at the rear for best ventalation. With it closed, it's not going to make a big difference on van.

vent
 

perhaps air conditioning and no vent would provide less of an energy liability?

Sometimes you surprise me, aerohead.

Active vs passive cooling? Alternator load instead of drag?

I'd point to your data on blisters and canopies: ecomodder.com: Canopy drag

AC load
 

Some messengers, decades ago, reported that the energy balance, concerning the operation of an air conditioner, versus opening the cabin for ventilation, could favor air conditioning.
Booming, buffeting, resonant effects, road noise in general, were also considerations with respect to driver fatigue in addition to climate control. And modern heat pumps appear to have a thermal efficiency advantage compared to a shaft-powered compressor system.
The NACA submerged inlet would be the way to go if the internal space requirements didn't forbid it's use. That's what all NASCAR racers use.
Drive naked!:D

Sheepskin seat covers!

I should have made it clear, the ventilation is primarily going to be used when the vehicle is not moving/running. The 12v fan will be hooked up to a 'house battery' (not the car's battery; likely AGM).

In retrospect, perhaps I should have hunted around for several lower-profile vents, rather that this single large one - when considering aero. I know Sortimo and other van outfitters make them.

Chevy Express ceiling
 

Does the van have a headliner, or is it just a stripped cargo van?
The reason I'm asking is that, with a radiant barrier and some insulation, you could really knock down the radiant, infrared load during full sun.
A white exterior roof ( if it didn't already have it ) would reflect much energy away before it had any chance to convert to long-wave radiation.