This article serves as an introduction to airflow in PC cases and as a guide to the first steps towards optimising your system. From defining airflow, to the choice of fans, and the consequences of various pressure scenarios, the main topics surrounding airflow are discussed here.

Airflow — the basics

When building a PC, the primary way to maintain low temperatures is to ensure good airflow through the entire case. This means that a continuous stream of cool air is always supplied to all components. Furthermore, warm air must be exhausted from the case at nearly the same rate. However, not only the performance, but also the noise level of cooling products plays a significant role in the choice of fans.

In general, a higher airflow (i.e. more air moved in a certain time period) will also lead to more noise. 

To find a good airflow-to-noise balance, please keep the following rules of thumb in mind, assuming all other variables remain constant:

• Rotations per minute (RPM) & noise level: The faster the fan spins, the louder it gets
• RPM & airflow: The higher the RPM, the more airflow the fan produces
• Size & noise level: The bigger the fan, the quieter it runs (producing the same airflow) 
• Size & airflow: The bigger the fan, the higher the airflow (at the same noise level)
• Number of fans & total noise level: The more fans running at the same RPM, the louder the system. However, if you have more fans, you can set them to a lower RPM, decreasing the noise level, but retaining the same airflow performance you would have with fewer fans rotating faster.
• Number of fans & total airflow: The more fans, the more airflow in the case

As you already might be able to tell, there is no one-size-fits-all setting for fan speeds. These should be determined by how much noise is acceptable to you and how much airflow performance you need from your fans to cool your hardware.

Achieving good airflow

First, it is necessary to make sure that the case you chose allows for a sufficient supply of air. This is achieved by a (relatively open) mesh on all sides that have fan mounts. Ideally, the sides where intake fans are used could be covered by a filter, as to prevent dust and debris from entering the case. This, however, reduces the airflow by a certain amount. So, it is important to consider environmental factors, such as dust (less dust means less need for filters, which in turn provides higher airflow), humidity (more humidity requires more regular cleaning), room temperature (with a lower room temperature, less cooling is required so fans can run at lower RPM), etc.

Two important factors come into play to ensure that enough air is supplied to all of your hardware, namely an adequate number of fans running at appropriate speeds.

When picking the number of fans needed or wanted in your chassis, multiple factors influence this decision that will be covered in part 2 of this guide. To keep it simple for now, we will assume a total number of six fan slots in an average chassis, three in front, two on top and one in the back. For this example, two fans in front and one in the back would be a good basis. Furthermore, as a general tip for beginners, we recommend using one face of the case for either intake or exhaust, and not mixing them for the time being.

Controlling the RPM of your fans is essential as well. This means that the exact rotational speed of the fans should be set depending on the temperatures of your key components, such as the CPU or GPU. You might have heard of “fan curves”, these are the profiles that determine the rotational speed of the fans in relation to temperature (either CPU, GPU or case). Setting up your fan curves is explained in our FAQ entry on this subject.

The importance of (case) pressure

Roughly depending on the number of fans that pull air into the case and those that push air out of the case, a certain amount of pressure is present inside the PC chassis.

Positive pressure is present when more air enters the case than exits it. This can be beneficial in dust management since the excess pressure inside the case forces air through any gap. Therefore, dust and debris can be hindered from entering the case through these openings. A potential downside to this approach might be the build-up of increasingly warm air inside the case, which is not exhausted immediately.

Negative pressure occurs when more air is forced out of the case than enters it. This configuration can be useful when using hardware that produces a lot of heat, since warm air is continuously pushed out of the chassis. However, this setup will also pull in air and small particles through small openings like gaps between the side panel and the case, or the expansion slot covers. Dust management can therefore be more challenging with negative pressure.

Neutral pressure occurs when the same amount of air enters as is exhausted. This can be considered a compromise, since fresh air continuously flows through the case without warm air building up, but dust and debris are not pulled into the case.

While all these configurations have their pros and cons, more consideration must be given to decide on one of them, since they depend a lot on the use case. 

For more information on all these subjects, including how to achieve positive, negative or neutral pressure, read the second part of this series.