With Bernoulli’s principle in mind (I think that’s the right one), is it more efficient to vent a room by putting a fan back from a window a bit, or right up next to it, and if so, is there a general sweet-spot as to how far back it should be?

Question

For context, this would be to vent a basement bathroom with a a window in an old house, and I can’t install a bathroom vent.

In: 857

Answer 1

Firefighter here.

We commonly blow air into a building, after we extinguish the fire, to vent the smoke and other products of combustion.

The fans we use are bigger and more powerful than household fans, but placement would be the same:

The cone of air, blown from the fan, should hit all four corners of the opening, while extending as little outside the corners as possible.

This is hard with most openings, because they are rectangular and there may not be a good position for the fan. So you have to compromise, but it is best to have no area, around the entire perimeter of the opening, which isn’t being hit by the fan.

The reason for not having any area which isn’t hit by the cone of air, is that air will swirl back into the cone, which reduces the amount of air being moved through the opening.

Anticipating that people will claim it is better to have the cone completely inside the opening, because of the venturi effect: the air you feel, around the outside of the cone is the air which has been caught in the venturi. Outside of what you feel moving, is air which isn’t being moved by the fan, either directly or via the venturi.

A simple way to think of it is, you want the highest percentage of the cone going through the window, with no area of the opening not getting hit by the cone.