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Antec PSU Myth 4: Larger Fans Are Better

Antec PSU Myth 4: Larger Fans Are Better
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Well, actually, it depends. As a blanket statement, this isn’t true at all. Whether a certain fan-size is good or bad depends on a number of factors, like the location of the power supply’s fan, the interior design of the power supply, the bearing type used for the fan and the power supply’s expected lifetime. Let’s start with the basics first.

Fan layout matters for PSU performance
First and foremost, it is wrong to say that larger fans are generally better. Of course it seems intuitive to people that a larger fan would transport more air with one rotation than a smaller one would. But there’s a big problem – larger fans in power supplies are almost always mounted on the top of the power supply, meaning that the air needs to be redirected by 90 degrees when travelling through the power supply. This top-fan layout is very common. Many companies are struggling with the design of these power supplies because the design doesn’t permit sufficient airflow. The 90-degree redirection of air creates a lot of turbulence, causing components to receive insufficient cooling and therefore heat up. There are nonetheless advantages to this layout – having an entire side of the PCB empty allowing the placement of detachable cables – but then again, there is also the chance of stagnant air being stuck in the back of the PSU if there aren’t any ventilation holes.

Interior layout produces big differences in PSU performance
The internal configuration of the power supply’s components has tremendous importance on its performance – more importance than small or large fans. Smaller fans, in contrast to larger fans, tend to be used in horizontal pull or push-pull configurations that mount fans at the back of the power supply – not the top.

This kind of configuration allows air to travel easily through the power supply housing without being diverted or disrupted. And the components and heatsinks can be built bigger, since the fan doesn’t take space on top of the power supply any more. These power supplies do tend to be a little longer since the fan needs to be attached to the front or the back of the power supply, but this is in fact the ideal cooling configuration for the power supply. The one disadvantage is that there is less space in the back of the unit for cable management.

As you can see now, it is factually incorrect to say one fan-size is better than the other without considering the details of the inner layout of the PSU. There are good PSUs with larger fans and good PSUs with smaller fans, and both layouts also have bad versions in which the airflow is being blocked. It’s also not correct to say that smaller fans are louder than big ones, because this depends on the type of bearing in use. Let’s take a closer look at those now.

The top-fan layout. Note the 90-degree redirection of air drawn in from the top of the power supply which must exit through the back. This produces counter-productive turbulence that reduces cooling.

Push-pull airflow through an Antec TPQ-1200 power supply with an exhaust fan. Air is pulled from the interior of the case and exhausted out the back. Note the smooth, undisrupted horizontal airflow – less turbulence means better cooling

The bearing is a very important factor in fans!
Fan bearings have everything to do with fan acoustics and reliability, and are therefore one of the most important factors that come with a fan – no matter which size!

Bearing type can be a bigger factor than fan size when it comes to determining noise levels and reliability. There are many different kinds of fan bearings. Many of them are virtually the same and just come with a different name. Let’s take a look at the two main bearing types used in power supplies today:

These two bearing types are the most common, and most others derive from them. Sleeve bearings are a type of plain bearings, which use two rubbing surfaces (usually with lubricant). These are considered technically inferior to ball bearings, in which spherical “ball” shapes roll in between the two surfaces. Sleeve bearings have much higher friction between the two surfaces, making them more inefficient and also shortening lifetime. Many companies constantly work on making sleeve bearing better – for example, by adding fluids between the shaft and the rotor to lower friction – but sleeve bearings’ lifetime is still not as good as ball-bearings.

An additional factor with fans in power supplies is the higher temperatures inside the case and ultimately inside the power supply. The average temperature that a fan works at has significant bearing (no pun intended) on overall lifetime. If the lifetime is specified at 25°C as above, then you can cut the lifetime in half with every additional 10°C. So if you have a sleeve bearing fan with only 30,000 hours lifetime at 25°C and you run this fan constantly at 35°C, you will only have 15,000 hours lifetime left – a reduction of 1.7 years in how long you can expect your power supply to run!

When you decide on a power supply, in addition to fan lifetime, you should also consider the overall lifetime of the components inside of the power supply. The slower the fan runs, the less heat it will exhaust from the power supply. This brings us to the last factor we’ll discuss: desired overall lifetime.

Lifetime is very important for fans being installed in a power supply because companies should build power supplies to last. This is why there should be no sleeve bearing fans installed in power supplies – nobody wants a fan that might just give up half way through the lifetime of the power supply and might take out the power supply itself while failing and ultimately shutting down the PC it is running in. But this doesn’t mean that ball bearings are the perfect answer - there is a downside to ball bearing fans, which is acoustic noise. Ball bearing fans produce more noise because two rows of balls inside of the fan literally roll on the shaft, and each point of contact produces noise. Sleeve bearing fans have no rolling elements and can work therefore much more quietly, but don’t have a lifetime even close to ball bearing fans.

In overview:

Bearing TypeProCon
BallLong lifetime
(~100,000hrs/25°C)
Higher acoustic noise
SleeveLower acoustic noiseShorter lifetime
(~30,000hrs/25°C)

The impact of fans on expected lifetime
When you decide on a power supply, in addition to fan lifetime, you should also consider the desired overall lifetime of the components inside of the power supply. The slower the fan runs, the less heat will it exhaust from the power supply. The less heat is exhausted, the warmer the components get. The warmer the components get, the more they decrease in lifetime. Even a single degree of increased temperature can have a tremendous impact on component lifetime. This means with higher load (and therefore rising temperatures) the fan needs to run faster to cool the unit and preserve the desired lifetime. Unfortunately, this also means that ultimately the power supply will be louder with increasing loads as well. And this means every power supply (except for fan-less power supplies of course) will get louder with increasing loads. From the perspective of maximal component lifetime, increasing acoustic noise is an unfortunate but necessary result from the important increase of fan speed that is necessary to cool your power supply to enable a long lifetime.

You may recall that the biggest argument against 80 mm fans is that they are supposedly noisier than other sizes. This is not entirely true - smaller fans can also run at lower RPM and are therefore producing similar noise levels at lower speeds. Pulse width modulation (PWM) fans, featured on select Antec power supplies, are a little more expensive but allow decreasing RPM to an even lower level that virtually eliminates noise. In fact, a PWM 80 mm fan at low CFM is as quiet as a 120 mm fan. For noise level, the most tangible aspect of power supply performance, the desired lifetime of your power supply and the PC that it powers is much more important – fan size just doesn’t matter!

Conclusions
Ultimately, whether a small or large fan is better depends upon a complex set of factors that include not only fan size but also fan placement, airflow design, bearing type, expected load, desired component lifetime, intended interior temperature and noise level. It’s not as simple as just saying one size is better than the other.

Myth Busted.