Ask HN: Do I place the fan so that it blows air inwards or outwards?

When I had an apartment with several rooms and non-working air conditioning, what I did was to open a window near the front of the apartment, in the dining room, and set up a large fan to blow out the window. Then I would open a window in my bedroom, at the opposite end of the apartment, to provide inflow. This was very effective at reducing the summer inside temperature to that of the outside at night, where the purported "heat pump" built in did essentially nothing.

The only issue was that it also sucked rain and pollen in as well.

I agree on "maximally far apart", but more in the horizontal than the vertical direction.

Extract hot air in one location, preferably up high, and let cold air in with a window that is as far away as possible - it doesn't really matter high or low.

Fans achieve two effects: ventilation (air exchange between two voids) and circulation (air distribution within a void). The first will achieve the goal of replacing a fluid volume with another (hotter/colder), but may not evenly mix the fluid within the conditioned volume.

Overpressures tend to prduce airstreamss or turbulence downstream, but underpressures don't induce much flow beyond the immediate orifice. Unless convection, ducting and venting can reliably create flow patterns, blowing air in to a conditioned space is a better option thansucking ot out, though if you want to control for emissions (say, filtering exhaust), you might want to combile both.

There's a classic physics experiment showing what happens when you run a water sprinkler, the kind with a spinning arm and one or more jets, backwards (sucking rather than blowing), and hhow it doesn't spin. Feynman discusses this in a story.

The fan(s) themselves contribut some additional heat to the space, but this is fairly negligible, especially relative to circulation effects.

If the heat source is external (e.g., incident sunlight), then natural airflow patterns which might draw air through a hotter chamber (say, an attic or other roof void) could have the opposit of the intended effect. It's key to check overall circulatory flow.

And if internal systems, or people, have significant evaaporative or similar cooling mechanisms, increased internal circulation will greatly improve cooling capacity.

Forced inflow may increase dust or other factors, if not filtered.

TL;DR: If you've only one option, I'd blow in rather than suck out. You might want to consider overall airflow, circulation, and heat distribution or souces (and sinks) within the void. Blowing onto rather than away from the primary heat source (in a cooling scenario) is likely best. If heat sources are external, exhaust from the top of the void is optimal.

Intakes should generally be low, exhausts high (for coolng).

For a large enough and complex enough (multiple chambers, zone, multiple heat sources, significant convection effects, dead zones), you would likely want a combination of intake, circulation, and exhaust fans, though I'd prioritise them in that order.

Disclaimer: I'm a space alien cat, not a fluid thermodynamics engineer.

This questtion has also appeared elsewhere:

https://superuser.com/questions/192920/should-case-fans-blow...

https://physics.stackexchange.com/questions/71620/on-a-hot-d...

If this is because you want to feel cooler, place the fan so it blows cool air inwards onto you. You'll get far more benefit from having air blown on you than you will from just dropping the ambient temperature around you.

This assumes there are enough bugs to be a problem. Some places this isn't the case. Also more likely to have bugs at lower floors of buildings as they tend to hang around trees and plants.