Quick and simple fan direction test - to blow out or in?

[Wizardling]

So I recently bought this case https://www.pbtech.co.nz/product/SEVRBP0052/Raspberry-Pi-Transparent-Case-with-Brushless-DC-Co for my new Pi 3 model B.

FYI - not a very good case IMO, due to it requiring a very slight bend of the Pi's board (the physical stress surely can't be good for it) as well as setting it on a wee bit of a slant (making it aesthetically displeasing in a clear case), to make the ports line up. Plus the included fan is irritatingly loud in an otherwise very quiet home office. But if suffices for now. I doubt I'll stick with it past the end of the year, however.

Anyway, when I first assembled this case I followed the instructions to the letter, without thinking, and thus had the fan blowing out the top, and cooling by drawing air over the CPU. But instead, today I switched the fan to blow in, right onto the CPU.

Testing:

Under the exact same running conditions, using 'vcgencmd measure_temp', I get a mode value of 59.1ºc blowing air out, and only 52.6ºc blowing air in.

So I'm pretty happy with that difference of 6.5ºc. It's not huge, but it's slightly more than I've read (usually around 5ºc) most people get adding cheep regular Pi chip-sized heat sinks (which I've also installed on everything but the bottom chip, so far) - so that's nothing to scoff at, eh?

Potential downsides:

This fan (and I note the same for several others) only has protection from touching the blades when installed blowing out. But it's not like it could really hurt my adult calloused fingers. Still, I wouldn't let a small child near it while installed blowing inwards.

I anticipate greater dust accumulation over the top of the board with the fan drawing air in from the top, especially given there's something about my home that encourages dust to form sigh. Dust will be unsightly in a clear case - which is the type of case I prefer (even my original full size Game Boy as a kid was the transparent model). But is easily blown out with compressed air, or carefully sucked out with a vacuum cleaner. So no big deal.

Final thoughts:

I wonder how common my instinct to try the fan around the other way for greater cooling, is for others? And why is it nearly all fans I see installed in a variety of electronics cases, are set to blow outwards, even when the positioning and design would sometimes make the opposite direction a more effective solution?

[drake999]

@Wizardling I can't speak to the general design choice for an outward blowing fan in most consumer electronics blow outward. This won't prevent dust accumulation because what goes out must come in somewhere else. This is usually remedied with air filters of some kind.

I use an inward blower and it seems to be working great. My Pi3 generally runs about 20 degrees cooler with the fan. I also use a heatsink applied with arctic silver thermal adhesive so the cooling is quite efficient. I'm not concerned about dust buildup because I just use it for gaming sessions that last no more than a couple hours once or twice a week so I imagine to get enough dust buildup to cause a problem will take years.