For the uninitiated, the debate of whether positive or negative pressure is optimal for a PC has been ongoing for many years. For those that are unfamiliar with what I am talking about, let’s start with the basics.

Why Does a PC Get Hot?

Electronic equipment is comprised of circuits — a collection of components such as resistors, inductors, capacitors, etc. — which are all connected together with wiring that electricity flows through. All of these components provide some measure of resistance, which means that they resist the flow of electricity in some way. That seems simple enough, right? Well, next we need to consider the first law of thermodynamics, which is a version of the law of the conservation of energy that has been adapted for thermodynamic systems. Whilst there are many definitions or interpretations of this law, to put it simply:

Energy cannot be created nor destroyed, but it can change forms or move around.

This applies to everything within our universe, whether living (even us) or not (light bulbs, vehicles). So, within the context of a PC, when electricity flows through the circuit and encounters resistance, the energy is not lost; it simply changes to a different form of energy — heat.

As technology has advanced, modern computers have come to contain billions of transistors within their CPU (central processing unit) alone! Thankfully, most components these days are designed to dissipate the heat rather than store it, otherwise they would just melt. Heat dissipation is a type of heat transfer, in this case the heat of the hotter object (CPU, GPU, etc.) is transferred to a colder object (heatsink) and the surrounding environment (the inside of your PC case).

Heat transfer is reliant on two key factors:

A lower ambient temperature (how hot the air around the heatsink is)

Movement of air (hence why we tend to use fans)

In order to control the first factor (ambient temperature), the easiest solution is to directly manipulate the second factor by using fans. But herein lies the problem, by introducing air movement we can also introduce the silent killer of PCs all over the world:

DUST

PC Air Pressure: Positive vs Negative

Now that the impromptu physics class is over and done with, let us get back to the issue at hand. Cooling fans on a PC case can be installed either as an intake (bringing air into the case) or as an exhaust (moving air out of the case). In a perfect environment we would have an equal balance between the two, as this would mean plenty of fresh cool air is drawn in to aid with the heat transfer and then all that hot air is blown out to prevent the ambient temperature within the case from rising too much. This can somewhat be calculated by using some simple math. All fans have a CFM (Cubic Feet per Minute) rating which signifies the air volume that a fan delivers. If we add the CFM ratings from our intake fans together and likewise add up all the CFM ratings from exhaust fans, we have an idea of how much air is entering — as well as exiting — the case. If the volume of air coming into the case is equal to the volume of air exiting the case, then a perfect balance has been achieved, right?

Unfortunately it is more complicated than that. You are not wrong for thinking that should work, but that would only apply if the PC case is completely air-tight. Most cases have vent holes, mesh, or even gaps between the panels; so those factors would have to be incorporated into the air flow calculations. This is why the argument of positive vs negative pressure is important. If we attain positive pressure (which means there is more air entering the case than exiting) then air will escape the case via those gaps. Whereas if we achieve negative pressure (more air exiting the case than entering) then air would be drawn in through those gaps. Depending on which we want, we either ensure our intake CFM is higher (for positive pressure) or that our exhaust CFM is higher (for negative pressure).

If you are unsure of whether your PC currently has positive or negative pressure, I have a little tip that will help you find out relatively quickly:

By using an incense stick, you can visually detect where air is being either sucked in or blown out of your PC case. Obviously your fans will either suck or blow the smoke, but this tactic is great for testing all the other places around your case: mesh, vent holes, panel seams. If the smoke is being drawn into the case through these gaps, then you have negative pressure. If the smoke however is being blown away from the case then you have positive pressure.

In my old PC I used slight positive pressure for one key reason: dust prevention. (Whereas now, my current build requires a bit more maintenance. Update: my current build is fully water-cooled.)

Previously I had purposely created a negative pressure environment, as I believed this would give me the lowest temperatures possible. Sure enough the system ran nice and cool, but I found myself having to clean my PC quite often due to the accumulation of dust. After changing out a few components (replacing the huge fan on the top of the case with a window), installing filters on all of the intake fans, and configuring my fans so I had more air entering the case than exiting; my system still has not required any dusting since. The ambient temperature within the case has only risen by a degree or two, and the end result is much more optimal for my configuration.

But which should I use?

You will hate this part. It was not my intention to be ambiguous by any means, but it truly depends on a lot of factors. In my case (pardon the pun) positive pressure works best for me. With a full house, pets, and our climate, dust is a significant issue for me, so positive pressure does wonders for keeping my system clean and healthy. If dust is not an issue for you — whether due to climate or environment — or you have higher ambient temperatures, then slight negative pressure may work out better for you. The key word is slight, if you do not have any intake fans at all you will create a whole range of problems within your case: deadzones/hot spots, turbulence, etc.

There are other factors to consider as well when it comes to your fan choices — static pressure fans vs high air flow versions — but that will have to wait for another time!