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How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 2

How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 2
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After discussing the theory of PC cooling, in Part One, we now move on to technology and implementation, discussing case fans, CPU coolers, and thermal grease. We even wrap up with a recipe you can use if you fail to cool your PC properly.

After explaining the theoretical background in Part One, we're ready to wrap things up in our quest to help you plan and pick the parts for your next air-cooled PC. Admittedly, this excursion also includes a bit of theory. But our main goal here really is to push forward with an actual build. We'll be talking about case fans, heat sinks, thermal grease, and graphics card cooling.

Why Do We Care About Case Fans?

In the last piece, we briefly discussed the chimney effect. However, convection on its own is not sufficient for cooling a desktop PC. The more heat your hardware dissipates, the more air is needed to exhaust it out of your enclosures. This is largely achieved using case fans, which come in many different sizes.

The right combination of case fans plays a huge role in determining the cooling performance of your PC, plus the noise it generates. There are a few rules to follow for maximizing air volume at modest noise levels, and we'll get into those as well.

Will A Small Fan Work, Or Is Bigger Better?

The size of your case fans is often dictated by the chassis you choose and the mounting holes it includes. Fans come in several standard sizes, but we're focusing on 60, 80, 92, 120, and 140 mm models. Larger fans do exist, but a majority of those come factory-installed.

Fans move air using an array of spinning blades, similar to an airplane propeller. When a fan has to spin faster to move more air, it makes more noise. Conversely, blades that turn slower are also more quiet. You can compensate for the loss in air volume from a slower-spinning fan by increasing its diameter. Here's the takeaway: whenever possible, favor a large, slow fan to a small, fast fan. Most case vendors follow this line of thinking and include 120 mm and larger coolers with their enclosures. In general, smaller 80 mm fans are falling out of vogue as a result of the noise they create.

Of course, you don't have to shun 80 mm fans entirely. High-quality coolers can easily run more quietly than less-precisely-manufactured fans, even when they're smaller. We include an affordable 80 mm model in our forthcoming recommendations, which could easily replace a noisy model, if that's all your chassis can accommodate.

Fan Connectors

Fans are either speed-controlled or they're not, and a fan's connector tells you the complete tale. We'll cover voltages, pin-out changes, and simple ways of controlling fan speed. But, in general, case fans run on 12 volts. This voltage is supplied either by the motherboard or directly from the power supply. In the latter case, big four-pin Molex connectors are used (though only two of the four pins, ground and 12 V, are actually needed). Smaller fan connectors are also standardized by Molex. They plug into outputs built onto motherboards or emanating from a dedicated fan controller.

The three-pin plug includes a tachometer feedback signal, which lets the motherboard read a fan's rotational speed. This can then be controlled by varying supply voltage. Fans with four-pin connectors are more common on CPU coolers, and their speed can be controlled with PWM (pulse-width modulation), typically temperature-dependent.

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    vx53c , 16 November 2011 16:03
    Totally rad! I especially enjoyed the ghetto solutions for card cooling.
    I felt that a mention to case modding should have been made, opening holes in the case, adding filters and cutting out grills. I always enjoyed turning an old but totally sturdy steel case into a proper ventilating case.
    I couldn't believe my eyes when i saw the thermal paste comparison.
    A minor thing too, excess negative pressure can create reverse in psu flow. I always found that slight negative pressure is good so that the case can breathe out of the gpu grill for the extra air it needs, if only to create a current between dissipated hot air from the gfx cards and the rest of the case air flow.
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    Anonymous , 25 November 2011 04:55
    Thanks for the article. About to build a computer. This is so informative. Helps a lot!