Building a modern desktop PC is surprisingly easy, thanks to modular parts and a lot of solid engineering. It’s often explained as “LEGO for adults.” But managing the air cooling system within a PC is considerably more complex.  We’re talking about physics, thermodynamics, all sorts of fun stuff. But there are a few basic principles you can apply to almost any build to get optimal airflow, and thus, optimal cooling.

Pick the Best Fans for Your PC

Any desktop PC with standard case fan mounts will work (80mm, 120mm, 140mm, 200mm—it doesn’t mater as long as they’re consistent). Deciding on a cooling approach that matches your case and your components before you go shopping for fans and coolers can be helpful.

That said, cooling fans come with a surprising amount of variation. You’ll need to make sure they’re sized right to fit the screw mounts on your case, obviously, but beyond that you’ll also want to consider:

Big or little: Generally bigger fans can move the same amount of air as smaller fans at lower revolutions per minute. Since the tiny electric motors in the fan mechanism doesn’t need to spin so fast, bigger case fans are quieter than smaller ones—and thus more desirable, if your case supports them. RELATED: How to Auto-Control Your PC’s Fans for Cool, Quiet Operation Fast or slow: Case fans are rated at a maximum revolutions per minute, or RPM. Faster fans move more air, but slower fans are much quieter. With a compatible motherboard or a fan controller, though, you should be able to adjust your fans’ speed for the perfect balance, so this won’t matter as much. Some fans and cases even come with manual switches for basic fan control. Airflow or static pressure: Case fans generally come with two types of fins: those designed for airflow, and those designed for static pressure.  Airflow-optimized fans are quieter and great for unrestricted areas, like the front of your case. Static pressure fans are designed to pull or push air with extra force, which is ideal for areas with more restricted airflow—like a water cooling radiator or a large CPU cooler with lots of fins. That said, some basic tests on these “high static pressure” models show that their benefit is questionable in standard air-cooled builds. RELATED: How to Pimp Your Gaming PC: A Guide to Lights, Colors, and Other Mods LEDs and other aesthetics: some case fans use the power supplied to the fan motor to also light up LEDs, either in a single color or in a multi-colored RGB array. These look cool—especially when coupled with a “tricked out” overall build—but don’t add or detract from performance in any meaningful way. Splurge on LED fans if you want, or save some money and keep your build low-key.

If you don’t want to do a ton of research, we highly recommend Noctua fans for great noise-to-performance ratio—though some of their models are on the pricier side (not to mention the standard line is butt-ugly). But there are a lot of great fans out there, so dig around sites like Newegg to see what you can find.

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The Basics: Cool Air Comes In, Hot Air Goes Out

The central concept of air cooling is very simple. As the components in your computer operate, they build up heat, which can lower performance and eventually damage the hardware if it goes unchecked. The fans on the front of your PC’s case are typically intake fans, drawing in the relatively cool air of the surrounding room to lower the temperature inside the case. Fans on the rear and of the case are usually exhaust fans, expelling the hot air warmed up by the components back into the room.

This might seem obvious, but an air cooling setup relies on cooler air outside the case than inside. Since the inside of the case is usually quite warm indeed, this isn’t really a problem, but if you use the PC in a particularly hot room (like an un-air-conditioned garage in the summer) you’ll see less effective cooling. If you can, move your desk and your PC to a cooler room.

Avoid placing your PC directly on a carpeted floor, as this will block any intake from fans placed on the bottom of the case (and often the power supply output, too). Put it on your desk or a small side table if you don’t have wood or tile floors. Some office desks include a large cubby designed to “hide” a PC—don’t use these. The enclosed nature of the cabinet will limit the available air to your case fans, making them less effective.

Got all those basics covered? Alright, let’s talk about how to place your fans for optimal airflow.

Plan Out Your Airflow

Before you begin, you’ll want to look at your available fan mounts and decide the best way to plan out your airflow. Here are some things to keep in mind.

Air Should Flow from Front-to-Back, and Bottom-to-Top

When mounting case fans, air flows across the open side towards the side with the protective grille, like so:

So the open side of the fan should face outside the case for intake fans on the front or the bottom, and it should face inside the case for fans on the rear or top.

Most cases are designed with a certain directional airflow in mind–usually front to back, and bottom to top. That means you should mount your intake fans on the front of the case, or occasionally (if you have a multi-fan setup or the front mounting brackets are blocked) on the bottom.

Exhaust fans go on the rear or the top. Do not mount exhaust fans on the bottom of the case; since hot air rises, a bottom-firing exhaust fan will be working against physics by expelling slightly cooler air instead of warmer air. The Intake-exhaust direction should go front-to-back and bottom-to-top. Side-mounted fans can be intake or exhaust, depending on the setup.

Manage Your Cables and Other Obstructions

Generally, it’s best to have as few obstacles as possible between the intake fans on the front of the case and the exhaust fans on the rear and top of the case. This creates faster and more efficient airflow, more effectively cooling your components. Try to mount all long, flat components like CD drives, hard drives, and GPUs horizontally—this is the default configuration on most PC cases.

Cables, especially the large bundled rails from a power supply, can be especially troublesome. Most large cases include a system of holes and guides that allow users to thread these cables away from the main open area of the case, often behind the motherboard tray. Get as many of these cables out of the way as you can. Here’s a really nice example of a case with good cable management creating open airflow.

…And a not-so-nice example. The stock case doesn’t offer many options for placing the unused power supply cables out of the way, but you should still try to tuck them away somewhere as best you can.

Most cases include multiple mount points for case fans—sometimes even more mount points than included fans. If vent blockers are included, use them: it might seem tempting to keep them open for more hot air to escape, but it’s much more efficient to direct air through the exhaust fans instead, and that’s just one more place where dust can get in. Similarly, be sure to make use of all of the spacers that came with your case for unused PCIe slots, 5.25″ drive bays, and so on.

Target the Hot Spots

Your CPU has its own heatsink and fan, even if you haven’t added one yourself—it’s the only fan directly mounted onto a motherboard component. This fan is expelling heat directly from the CPU into the case’s main airflow lane. Ideally, you want to place an exhaust fan as close to the CPU as possible to quickly expel this hot air. A side-mounted fan (expelling or drawing in air in a direction perpendicular to the motherboard) might be useful here, but not all cases support that.

If you have a large aftermarket CPU cooler, it probably has one or more fans of its own. Try to direct the output of these fans to align with one of the case’s exhaust fans, sending heat directly from the CPU to the outside of the case. Most CPU coolers can be mounted in any cardinal direction to help achieve this (and to make it easier to clear other internal components). Remember, case fans draw in air on the open side and expel air on the grille side.

Balance Your Air Pressure

Think of a PC case as an enclosed box, and the air going in or out of each fan as roughly equal. (It isn’t totally enclosed, and the airflow generally isn’t equal, but we’re talking in generalities here.) Assuming all the fans are the same size and speed, then you have one of three possible options for the air pressure inside the case:

Positive air pressure: More fans are drawing air into the case than blowing air out of the case. Negative air pressure: More fans are blowing air out of the case than drawing air in, causing a slight vacuum effect. Equal air pressure: The same amount of fans are blowing air in and out, creating approximately the same air pressure as the surrounding room.

Because of the way the internal components create blocks in airflow, it’s more or less impossible to achieve a truly equal air pressure inside a case. You want at least one intake and one exhaust fan at the very least, so assuming you have more, which is better, drawing in more air for positive pressure or blowing more out for negative pressure?

Both approaches have their advantages. Negative air pressure should create a slightly cooler environment (at least in theory), as the fans are working harder to expel hot air. But the drawback is that the slight vacuum it creates inside the case tends to draw in air from all the unsealed areas: the vents, unused PCIe slots on the rear panel, even the seams of metal in the case itself. Positive air pressure won’t cool quite as well, but—combine with dust filters (see below)—will take in less dust since those vents and seams will expel air rather than suck it in.

Opinions on positive versus negative pressure is mixed. Most people opt for a more balanced approach, leaning slightly toward negative air pressure (for theoretical cooling) or positive air pressure (for less dust buildup), and we’d probably recommend something in the middle there. In reality, PC cases are so far from being a sealed environment that the difference is probably negligible. If you’re seeing too much dust buildup, move one of your output fans to an input position. If you’re purely concerned with temperatures, check CPU and GPU temps with a software monitor and try some different configurations.

Dust: The Silent Killer

Even the most carefully-constructed build will accumulate dust from the surrounding room, and if you live in an especially dry, dusty environment, (or you smoke, or have pets, etc.) you’ll need to be extra vigilant. Check your PC for dust buildup regularly. More dust means less efficient cooling…not to mention looking totally gross.

Every six months or so, or more often if you live in a particularly dusty area, open up your computer and blow it out with some compressed air to get rid of any lingering dust. If it’s been a while, you you might need to remove the fans from their mounting screws and wipe the plastic blades as well.

To prevent dust, slap some dust filters on your intake fans. Clean them out with water and dry them completely every few months to keep dust from flowing into your case (again, slightly positive air pressure can help here too). Most cases sold for system builders come with some kind of dust filter, but if you need more, you can buy some nice magnetic ones in different sizes for your intake fans. If you’re desperate or frugal, you can even make them yourself with some panty hose.

What About Water Cooling?

If you’re looking into a water cooled setup that uses liquid convention to draw heat directly from your CPU or GPU to a radiator, odds are that you’re already working on a pretty advanced build. But for the sake of completeness: water cooled components have a minimal effect on the internal airflow of a case. The radiator and fan combo itself can be mounted to the front or bottom for intake or the rear or top for exhaust, but it will be less efficient than a fan alone.

If possible, mount your radiator and fans as exhaust fans. Putting them in an intake position will warm up the air via the radiator as it comes into your PC…which is basically defeating the purpose of water cooling your components in the first place.

Image Credits: Newegg, CyberPowerPC, Corsair, Cooler Master, Garry dr/Flickr, Vinni Malek/Flickr, Atredl/Imgur, lungstruck/Flickr