See stack effect.
I would open windows on the top and bottom. If using mechanical ventilation, position a fan blowing inside at the bottom of the stairs, and one blowing out at the top.
this post was submitted on 08 Apr 2025
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It also helps to have the fan blowing out be a few feet from the window. The Bernoulli Effect will cause more air to be blown out the window than having the fan right in the window. This isn't always convenient, though.
My personal favorite fan for this is a Vortex DC fan. It comes in a few sizes, and I have a couple of the 533DC models. They move a ton of air for less power than an AC fan. Plus it has seemingly infinite speed settings. They're great on a desk on a low setting blowing right in your face when it's hot. When you aim one out a window a few feet away, you can stand in front of an open window on the opposite side of the room and feel a breeze of fresh air flowing in!
In my experience it's better than one of those twin-blade window fans by a mile, and better than a big box fan, too. But in a stairwell, it might not work because where are you going to put it? It'll be in the way.
Of course nothing beats taking an old HVAC centrifugal fan, covering the sides with a grill, and sticking it in the window, blowing out. That'll make a breeze in your whole apartment! I did that once when I lived on the third floor of a house. In my bedroom on the opposite side of the apt, I had my bed right next to a window. That breeze that blew in on warm summer night was wonderful!
I went to an HVAC shop and asked them take a fan and motor combo out of any old furnace they had replaced. I only paid $35 for it about a week later. Then I covered the sides with hardware cloth, using sheet metal screws, wired it to always be on high, and threw a switch on it. I used metal angle brackets to allow the window to shut down onto it. Best thing ever! I greatly prefer fresh air over AC unless it's peak summer.
Cold air will enter the stairwell at the bottom. If it is cold outside, it will probably be at least a bit warmer in the stairwell, so the air will warm and rise to the top of the well. It's probably still cooler than in anyone's room, though so that will be what the 4th floor residents are noticing.
To air the well most effectively, I'd suggest opening a window at the bottom and at the top and maybe putting a heater at the bottom as well. That would cause the cold air entering at the bottom to rise more rapidly and then exit through the top window. This may not be particularly efficient from an energy use POV though.
With the top window open and the warmer air in the well the 4th floor may even get less of a draft doing this, as the air would be getting out that way instead - unless there was a wind blowing in through that top window.
Okay but let's say I have to choose one window, which should it be?
The issue is that the cleaning people don't fucking open windows when they clean and that shit enters my apartment and makes me itch.
They refuse to change the garbage they use, refuse to fucking do anything.
Now I have to be the bad guy opening a window, so that everyone complains that they're fucking freezing and that I'm a menace.
If these were lab conditions with known airflow and volumes and window sizes and air temperatures and solar gain and so on and so on maybe someone could give you a useful answer, but without any of that, I've no idea and I doubt that anyone else will be able to tell you either.