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PC Fans & Airflow: Everything You Need to Know

If you’re reading this article on a desktop computer, you can probably hear a fan spinning at this very moment. It’s probably the fan in the back of your computer case.

Do you know whether that fan is blowing air out of the case or taking it in?

What about the other fans in your computer? Your power supply has one, as well as your CPU.

If you’re curious about how the system of fans in a computer works to keep everything cool, read on, because in this article we’ll discuss everything you need to know about airflow in your PC.

How to Determine Fan Airflow Direction

How to Determine Fan Airflow Direction

A computer case fan can either be an intake, meaning it sucks air into the case, or an exhaust, meaning it blows air out of the case.

There are a few ways you can determine the airflow direction of any fan:

  1. Place your hand in front of the fan. If you feel air blowing on your hand, the fan is an exhaust. Otherwise it’s an intake.
  2. Look on the side of the fan case. There may be a small arrow. If present, this arrow indicates the direction of airflow through the fan.
  3. If the fan has a protective grill, it will be on the side of the fan where air exits.

Next, we’ll talk about general recommendations for fan location and airflow direction.

Guidelines for PC Case Airflow Design

Guidelines for PC Case Airflow Design

Just about every computer case has a fan in the back panel; this should be an exhaust fan.

Additionally, the fan mounted to your CPU should be configured as an exhaust fan so that it directs the hot air generated by your CPU away from it.

Take a look at this guide if you’re wondering how hot your CPU can safely run.

Many computer cases contain only these two fans and rely on cool air naturally entering through a side vent as the intake. You can improve airflow by adding an intake fan to either this side vent or to the front of the case, provided there is a fan mount point there.

Finally, if your case has a fan mount point on the top, this should be for exhaust, since warm air rises.

Now that we have some guidelines for the placement and direction of fans, let’s talk about some of the key parameters that define fan performance.

If you’re interested in other cooling systems and wonder if they’re worth it, also check out this guide.

What to Look for When Choosing PC Fans

What to Look for When Choosing PC Fans

RPM: This stands for revolutions per minute, and is a measure of how many times per minute the fan blade makes one complete rotation. Obviously, the higher the RPM spec a fan has, the faster it can spin.

CFM: This stands for cubic feet per minute, and indicates the volume of air a fan can move each minute. The higher the CFM, the more cooling power a fan has.

Fan Size: you may think that a larger fan is a more powerful fan, but this is not necessarily true. The important feature of larger fans is that they can move the same CFM of air as a smaller fan, but at less RPM than the smaller fan. The reason that this is desirable is that a fan operating at a lower RPM is generally quieter than a fan operating at a higher RPM. So think about larger fans as being quieter fans.

Fin Type: Fins are generally optimized for one of two things: airflow or static pressure. Fins optimized for airflow are typically quieter, whereas fins optimized for static pressure provide more force to move air. This is sometimes desirable for airflow-restricted areas such as large heat sinks with a lot of fins. One note about static pressure optimized fans is that their benefits are widely disputed for air-cooled systems. If your system features water cooling, you may want to look into them though.

Now that we know a bit more about fans, we can talk about how they are used to create air pressure in the case.

If you’re confused about different fan types and their abilities, we also have a guide to the pros, cons, and differences of CPU fans versus chassis fans.

PC Case Air Pressure

PC Case Air Pressure

The pressure inside your case can be in one of three states: positive, neutral, or negative.

Positive Pressure occurs when the air pressure inside the case is greater than the air pressure outside the case. This happens when there is more air coming into the case than going out.

Neutral Pressure occurs when the air pressure inside the case is equal to the air pressure outside the case, meaning that the airflow in equals the airflow out. In practice, neutral pressure is difficult to achieve.

Negative Pressure occurs when the air pressure inside the case is less than the air pressure outside the case. This happens when there is less air coming into the case than going out.

Here’s an easy way to determine the pressure of your case:

  • Add all of the CFM specs of your intake fans. Call this CFM_in.
  • Add all of the CFM specs of your exhaust fans. Call this CFM_out.
  • The air pressure in your case is calculated as CFM_in – CFM_out.

The value you calculated tells you about the kind of pressure you have.

If the value is positive, you have positive pressure.

If it’s negative, you have negative pressure.

If it’s zero, you have neutral pressure.

But which is best? What should you aim for?

In theory, neutral PC case pressure is viewed to be optimal, but as stated, it’s very hard to achieve. Even if you try to design a system with neutral pressure, you’ll very likely end up with a system that operates at slightly positive or slightly negative pressure.

That’s where the debate comes in. Opinions vary, because while negative pressure technically creates a cooler environment inside the case, it also leads to unfiltered air entering through nooks and crannies in the case.

Unfiltered air is dusty air, and dust is one of the biggest enemies of a cooling system. Positive pressure, on the other hand, is a great way to ensure that all air entering the case is filtered and therefore contains minimal dust.

For this reason, positive pressure is more typically recommended for the majority of cooling systems.

One final note in our air pressure discussion – you may have seen enthusiasts operating their computer with the case open in an effort to create the coolest environment possible; perhaps you’ve even done this yourself.

In reality, operating with an open case does more harm than good. It actually reduces the efficiency of airflow through the case in addition to creating a huge area for dust to enter.

For the best results, keep the case closed, and ensure that all intake fans and vents have dust filters installed.

To make filtering easy, consider purchasing magnetic filters. That way you don’t have to worry about different screw sizes and mounting patterns. In a pinch, even pantyhose will work!

If your case has more mount points than fans, install vent blockers in the unused locations. Also, ensure unused drive bays, PCI slots, and other openings are blocked.

No matter what case pressure you decide on, it’s important to make sure that all of the other components inside your case are arranged in a way that creates optimal airflow. That’s what we’ll talk about next.

Optimizing the Case Interior for Airflow

Optimizing the Case Interior for Airflow

The biggest impediment to airflow inside the case is a tangled mess of cables.

At the very least, you should get rid of any cables that you aren’t using and tie wrap the rest of them to organize them and make them as compact as possible.

You may also be able to purchase cables that are less bulky.

With most stock cases, this is probably about the limit of what you’ll be able to do. With many larger or premium cases, a system of holes and guides may be built into the case for the express purpose of managing cables.

If you’re serious about optimizing your cooling setup, consider upgrading your case to one of these models.

Another thing to keep in mind is the mounting orientation of drives and other long, flat components. They should be mounted horizontally (parallel to the case floor) to allow maximum airflow around them. This is the default configuration in most cases, but it’s something you should be aware of.

Once you’ve optimized the interior of your case for airflow, there’s one thing left to consider: location.

PC Location in a Room

You may be tempted to place your computer in a “cubby” or cabinet in order to hide it for aesthetic or noise reasons.

Avoid this temptation.

Closed locations like these restrict airflow and greatly decrease the effectiveness of your cooling system, because the supply of cool, outside air is limited and hot air from your system has nowhere to go.

Keep your computer in an open, well-ventilated area.

Don’t place your computer on carpet, especially if it has vents of fans on the bottom of the case. Carpet will impede the flow of air into these openings.

Finally, keep your computer in as cool a room as is practical. The cooler the air outside your computer is, the more effectively it will be able to cool the inside of your system.

Final Recommendations

You should now have a great idea of how fans work to cool a computer. With a little planning, you can ensure that your system is optimized for the appropriate airflow.

If you want a quieter cooling system, consider larger fans.

Keep air pressure in mind too, especially with regards to how it affects dust buildup. Make sure that unused openings are blocked and vents and intakes are filtered to keep dust to a minimum.

Finally, keep the inside of your case tidy and give it a home in a cool, well-ventilated location.

Keep these tips in mind to ensure a cool, dust-free system that’s ready for any computing task you can throw at it.

Also, we always highly recommend using a program to monitor your PC’s internal temperatures. For two lightweight, easy to use options, glance over our full software comparison of Core Temp and Real Temp.

There’s no doubt that choosing cooling components/layouts can be a pain point for anyone planning a DIY PC build (something we cover in our guide to How Hard/Easy It Is To Build Your Own PC). But don’t be overwhelmed!

If you use the information in this guide (and the associated guides linked throughout) as a starting point in your research, you’ll have no problem getting your cooling just right.

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