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
This is called positive or negative pressure ventilation, and firefighters use this to vent smoke from buildings.
My fire training said 3 to 5 feet in or out of the opening (the cone of air pushed should overlap the entirety of the hole it is being pushed out), but that is dependent on the volume of air being pushed/pulled by the fan and the amount of air needing to be moved.
Make sure you open a window on the general opposite side of the building for more efficient venting.
How does a screen affect this?
The real trick is air flow. Blowing a fan into a closed room is pointless.
Open a second window, and better yet is yhave a dedicated fan blowing OUT from that second window.
To make a jet pump, which is the intended scope of using the fan blast to push additional air out through the window, I would recommend:
1 fan diameter 1/3 to 1/2 of the window opening.
2 Fan distance 3 to 6 times the fan diameter.
3 Fan speed max.
Note 1: For lower speeds, reduce fan distance.
Super important note 2: you can push air out only as much as fresh air can replace it. You need a big air entrance somewhere else, roughly 2 times bigger than the fan window.
If the window is the only opening, use the fan placed at the window, zero to two diameter distance. The fan must be placed on a side, pointing out at an angle so the flow of ejected air doesn’t impede or mix with inward flow.
Source: I have a shit fan and I make best use of it. Force hot air out in summer and shoot my wood burner heat around the house in winter. And yea there’s better ways but I work in aviation so this game is fun to me. I don’t want a better fan, I’m having fun.
>is it more efficient to vent a room by putting a fan back from a window a bit
Yes
>is there a general sweet-spot
The fast moving jet of air will expand into a cone, picking up more air and slowing down. The sweetspot is where that cone roughly fits the size of the window, but the actual distance depends on size of the fan, speed of the fan, window size, and window shape.
Smoking the reefer in the parents basement eh?
why cant you install a vent? without having another open window of comparable size in the same room to allow air in it likeley doesnt make that much of a difference having it right against or not.
I’m a firefighter and ventilation is part of our training.
The best way to vent a room is to place a fan back from the window, so that you can feel the fan moving air in an area just *slightly* larger than the opening of the window. This’ll be most effective.
[Here’s a neat video showing the effect.](https://www.youtube.com/watch?v=XP6oqIic4lo)
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.
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