One thing I wanted to add: I noticed that my mobo is constantly hotter than it's supposed to be! And I don't know why.

The appropriate snippet of my sensors output:

Remember to check all temps in the BIOS and make sure they correspond. I know for sure that on my system the temps don't correspond to the BIOS. They're about 15 degrees off.

If the output fans have a higher Cubic Feet per Minute (CFM) then the input fans this will produce a negative pressure and thus cool better. The negative is it will draw in more dust and dirt.

totally agreed on this front. water is not conducive to anything computer, ha ha. I have a hard enough time dealing with leaks with my toilet, and in that case, the water drips on tile, which does not instantly get destroyed when touched by water, ha ha. STAY AWAY MAN!!

Negative pressure does not cool better. Heat transfer from heat sinks to air is primarily conductive; the air contacts the heatsink and picks up energy. The heat is then transported away convectively; you move the heated air with a fan.

Lower pressure means less air. Means less mass to carry the heat away. Means reduced efficiency.

OK, well I changed the paste and now the temps are: 40, 38, 46, 40, the old ones were 46, 44, 51, 46. It's better, but it looks like it's not the paste. Oh well.

The original paste was not very good, it looks like it dried out and cracked and stuff.

It still made it a few degrees better.

Not really, IMHO it is debatable but do not claim to be an expert. With a standard case if you have a single fan sucking air out it will create a slight negative pressure due to the resistance of the air trying to get through the front cooling inlet. If the fan is turned around then the case will have a slightly higher pressure because the air can not exit as fast due to the resistance of the air inlet. With an inlet and outlet fan if the inlet fan has a higher CFM rating then the outlet fan the case will have a higher pressure and may not cool as well as a single fan.

OK. That's all fine. I have no problem with any of that.

And I don't see how this follows at all.

It isn't the pressure difference (plus or minus) in the case, it is the flow patterns inside the case. You want to move the air past the CPU heatsink, then remove the heated air from the case. Since the warm air will rise, putting an exhaust fan at the top of the case is usually the most efficient way to get the heated air out; it will come to the fan naturally and is then forcibly moved out. This enables you to take advantage of natural convection in addition to the forced convection of the fan, and the location of the fan preferentially removes the hottest air in the case.

Having just an inlet fan in the bottom front is less efficient because the positive pressure induced by that fan will cause air to leave the case everyplace it can, not merely after moving across the heat sink and then to the top of the case. The warm air at the top of the case is not forcibly removed, but only removed by natural convection slightly aided by the slight positive pressure in the case. So the top of the case gets warmer and the system doesn't cool as well.

If an exhaust fan was set at the bottom of the case it would be less efficient than an inlet fan on the bottom of the case and far less efficient than an exhaust fan at the top of the case. This is because it would preferentially remove the coolest air from the case, and cause cool air to enter the top vents, thus greatly slowing the convective removal of hot air at the top and introducing turbulence into that flow.

An inlet fan on top of the case would equally be inefficient because, again, it would be working against natural convection. Now, you can certainly do this; downflow furnaces are common, for instance, but you need big fans which make a lot of noise in order to accomplish it.

If the venting through the system is appropriate; a good thermal design and a nice clean flow path for air through the system, then a push-pull (inlet fan down low in front and exhaust fan high in back) will move a lot more air than an exhaust fan alone. Even if the flow path is not that good, the push-pull will move more air than the exhaust fan alone. Often enough, when the flow is restricted, you NEED the push-pull more because you need the increased pressure to move enough air past the restrictions.

It isn't the pressure in the case that dictates the cooling, it is the way convection is used, and the volume of air you can move past the hot parts of the system then move out of the system that dictates the cooling.

@jiml8

Nice explanation! But one more thing I am curious about:

I very often see cases with intake fans on the top (the top panel, not the front or back panel) blowing inward. Seems like it would just make things worse.

Without seeing the system, I can't say for sure. But usually such a system is internally ducted, having a duct running past the CPU with the heat sink in the duct. The fan on top blows the air down across the heat sink, then directly out in the back or the bottom or something. I know that Dell used to do this on some systems.

Like I said, you CAN make the air flow down if your fan is powerful enough, or if you duct it to make it go where you want. This has you working against natural convection but that usually isn't a big deal, if you design for it.

Most build-it-yourself boxes are NOT designed for it, though.

Also, of course, a desktop case (as opposed to a tower) is usually wider than tall so convection doesn't work so well. In those cases, you usually employ some form of ducting, and you force the air to go where you want it.

The case didn't have any ducts.

I made a little diagram using KolourPaint:

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Oh. All right.

Yeah, I would think it would be better to change the direction of that top fan.

Looks like a typical homebuilt; ad-hoc, shove fans in until it stays cool, "design". We've all done it.

Here's my layout, blue is anything that has a fan on it, arrows show air movement by fans, green are ports without fans = places where air can come in. X means air moving into the board (like in physics). All fans are output fans.

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Hi

All computer cooling fans should consider water cooling systems.... LMAO

I'm a computer cooling fan myself... who isn't...??!!