Can anyone speak to the advice I received years ago (on move-in day at a college dorm with no AC) that two box fans--one pointed outward and one pointed inward--would be more effective than having them both aimed inward? Supposedly helps create circulation, but, I'm not sure I buy it.
If you think of your room like a rack mount server it totally makes sense. I've never seen a server with fans pulling air in from the front AND the back, only only ever in one side and out the other. You want to get air in, have it absorb heat, and then get rid of it. Having positive pressure in your room or server just results in high pressure warm air, not a cooling effect.
>If you think of your room like a rack mount server it totally makes sense.
I'm not sure how good that analogy is. Generally a box fan is loosely fit in the window and has an open interior with no shroud, as opposed to server fans which are tightly mounted to a fan-sized hole and have a close-fitting shroud around the blades. This has a big effect on the optimum configuration, because the box fan's suboptimal design happens to be more suboptimal at sucking than blowing (while a server fan is good at both, being closer to an ideal actuator disk)
I have observed that, in general, a box fans work better at blowing out a window than sucking in. At first this confused me. Why should this be the case, given time reversal symmetry?
Here's my hypothesis. The fan produces a high velocity jet of outlet air, but sucks in any air nearby (that's why you can blow paper across the room, but you can't suck paper across the room, another seeming violation of time symmetry). Since the fan is located nearer to the interior of the window, it can either A) blow an outward jet that maintains its momentum through the window, or B) attempt to suck an inward jet of air through the window. That doesn't work, so it mainly intakes warm interior air, resulting in mostly in-room recirculation with very little flow through the window and very little static pressure created to drive whole-house airflow.
Think I'm full of shit? Good, then your skepticism is working! Don't take my word for it. Anyone can test this using a phone with a barometer. Put the phone on a table (for constant altitude) in the room with the fan. Compare the pressure drop (exhaust fan on vs. off) with the pressure rise (intake fan on vs. off). If I'm right, the absolute magnitude of the pressure drop should be greater than the pressure rise, indicating greater airflow in an exhausting configuration.
If you only have one fan, definitely have it pointed out, not in. The moving column of cold air cools the entire place, not just a single room. Also this arrangement keeps the noisy fan in a separate room, while you enjoy the cool air quietly wafting through a nearby open window.
I've transformed many friend's apartments from sweltering to comfortable by implementing this tip.
Yes. One box fan pointed in is just pushing against a wall of air that has nowhere to go. It's completely possible for a fan to be spinning but move essentially no air. One pointed in and one pointed out gives that wall of air somewhere to go, which means the fan pointed in is able to push more air in.
Having grown up in a hot place (Australia) - we think about ventilation a lot.
You're right - cross ventilation is really important. Generally you want to pull air in as low as possible as it's cooler. And then push out as high as possible. As "cross ventilation" somewhat implies, usually this is across a room or building for the best airflow.
A fan in a regular-height window and an open vent (a higher window, door louver) generally works pretty well. Even if it's the same window, it'll work ok.
This measurably and repeatably works in my house. One side of my house gets cooler and one gets hotter depending on time of day. I setup one fan to blow in cooler air and turn the other one to blow out hot air, and I've seen the temperature drop by as much as 3c° over the course of an hour, when outside temperature did not change. Other configurations did not do this.
Anecdotally, it absolutely does - I had chalked it up to increased airflow, similar to optimizing airflow in a computer case - the exit fan position optimized to remove hotter air, the inlet pumping in cooler air wherever I'm sitting.
