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FANLESS PSU IN THE ATX CASE
The defining quality of a fanless PSU is its fanlessness. Obvious, but thermally, what does this mean? A fanless PSU is a source of heat, though the three we consider best produce very little heat. They are also very well ventilated. Combine the existing PSU exhaust vent in the case with the openness of these fanless PSUs, and what you end up with is... a vent. Basically, from an airflow point of view, the fanless Seasonic or Kingwin PSU is a somewhat restricted vent in the case.
So what is the function of a vent in a PC case? It allows air to flow in or out. Which way the air flows depends on the location, direction and intensity of other fans in the case. Of course, if there are no other fans, then natural convection and rising heat have their way, but for the time being, we're only considering system with some forced air cooling.
Let's consider what happens in an old school ATX tower case like the Cooler Master Sileo 500 pictured on the previous page. When a fan is spinning, air is displaced from one side of the fan to the other. The displaced air is continuously replaced by air flowing from elsewhere, and this flow always follows the path of least resistance. When that back panel fan is spinning without a PSU mounted in place, air flows into the case (and to the intake side of the fan) via the back panel hole for the PSU this is the closest hole with the least amount of resistance. Mount a fanless Seasonic X there, and the outside air now flows through the PSU to the back panel exhaust fan.
If a fanless PSU like a Seasonic X400 or Kingwin STR-500 is used, then it becomes an intake vent for the exhaust fan on the back panel.
Got that? The PSU itself becomes an intake vent for the back panel exhaust fan!
This phenomenon is easily tested by holding a strip of tissue paper at the exhaust vent side of the PSU. If the back panel exhaust fan is blowing out, the air flowing in will force the tissue paper against the PSU vent. This is not to suggest that the Seasonic fanless PSU becomes the only intake vent but one of many, unless all other vents in the case are closed off. However, its close proximity and the openness of the back and side panels of the Seasonic certainly makes it a prime intake vent.
Having said that, does reversing the flow direction of the back panel fan force air out through the PSU? Here, the answer is generally not as clear, because the airflow pattern on the exhaust side of a fan is usually more directed and focused than on the intake side. If you develop a positive pressure flow by making all fans mounted on the case panels blow inwards, then for sure, the PSU will become one of the exhaust vents. This will have some effect on CPU and GPU cooling, which needs to be checked empirically.
Empirical testing, by the way, is fundamental for thorough PC cooling setup, especially if you have not ventured into the balancing game of silent cooling. It means experimenting systematically before finalizing the hardware settings, including fan positions, speeds and directions.
FANLESS PSU IN THE BOTTOM POSITION
What happens to airflow when one of these fanlesss PSUs is mounted at the bottom/back of the case? Well, unless there is a fan fairly nearby creating some kind of forced airflow, convection rules. This is something you need to keep in mind: A fan's cooling airflow "reach" is limited in distance, especially when run at the very slow speeds which make them silent (typically, under 800 RPM).
Anyone who takes delivery of a Seasonic X-series fanless PSU will see the big warning about mounting the PSU in the case with its vented top facing up.
Seasonic recommends mouting the PSU with its ventilated top facing up in order to allow convection to help with cooling.
If you have ever examined the interior of a standard ATX PSU, you know that the normal "up" position puts the PCB on the top, with all the components hanging upside down off it. A 120mm fan is mounted on the "underside" of the PSU, blowing into the PSU, and the air makes a 90 degree turn to exit out the back. So Seasonic's mounting recommendation for its X-400 and X-460 is the reverse of the norm, in order to use rising heat of convection to help cool the PSU.
But as we've just explained, a single back panel exhaust fan in the case reverses the airflow through the PSU. The rising heat effect of convection in a typical computer is easily dismissed by the forced airflow from any nearby case fan. [Note: We could get hung up in a major defense of this point, but for now, take our word for it. We've played with things like this for a decade, and we are completely convinced that the airflow from any 80mm or larger diameter fan at even minimal speed provides many, many times greater cooling effect than convection in any PC using typical heatsinks.]
Let's look again at the Coolermaster Silencio 550, which has a bottom PSU mounting position with an intake ven driectly below the PSU.
Coolermaster Silencio 550 more or less leaves the PSU to cool itself. For a fanless PSU, this could spell trouble under high load.
Neither the back panel exhaust fan nor the front panel fan are in close proximity to the PSU, so they will not impact the PSU's internal cooling much, unless the fans are set to high speed, which is anathema to a silent PC. If the front fan was positioned lower, and nothing put in front of it to impede airflow, its flow might help the PSU a little. But even so, with the X-400/460 mounted facing up as recommended by Seasonic, the most likely scenario is that the hot air from the PSU would rise up, with a bit of the heat going out the back vent of the PSU, and more of it going up towards the CPU/GPU area, and be evacuated by the top back panel case fan. This is not a bad scenario, but it does increase the thermal load on the main CPU/GPU cooling system. Our torture testing showed that the Seasonic X-400/460 can withstand very high loads and temperatures without skipping a beat, so perhaps that's the only real downside.
Flipping the Seasonic PSU would allow some outside air to get into the PSU via the bottom vent of the case, but this might trap some pockets of hot air in the PSU, which is possibly what Seasonic is trying to avoid with its strong recommendation about keeping the vented top up. In any case, you can see that the bottom PSU position does not actually offer any clear thermal advantages for a generously vented fanless PSU.
IN A 90 DEGREE ROTATED CASE?
The SilverStone cases with rotated motherboard trays, the Raven RV01/02 and Fortress FT02 seem suitable for use with a fanless PSU. As the GPU, CPU, and PSU are positioned horizontally to one another, the heat generated from each component radiates upward, having a minimal effect on adjacent parts. In addition, natural convection is helped along by three 18 cm intake fans on the case floor and a 12 cm exhaust on top. With the open vent top of the Seasonic X-400/460 facing the back panel vent, the air could flow through the PSU easily, cooling its internal components, and blow out both the top and the back. However, these large cases are tailored for high-end gaming systems which are inherently louder. It might be a bit wasteful to pair a passively cooled power supply with such a system, and the 400~500W rating of these fanless PSUs might not be adequate.
SilverStone Fortress FT02.
MODIFIED STANDARD ATX TOWER
The new Antec Solo II is a conventional ATX tower with one significant tweak: A large top panel vent directly over the PSU. As far as we know, it is the only ATX mid-tower case with a standard PSU position to offer this feature. The open face of the power supply is intended to face the top panel vent. If a fan cooled PSU is used, that vent is an intake from drawing in air from outside the case. If a fanless PSU is used, then top vent can be an exhaust or an intake, depending on how other fans in the case are deployed. Regardless, the extra vent is a benefit for cooling a fanless PSU, more obviously useful than the bottom panel vent in a bottom PSU case.
Any PSU in the Solo II is still subject to the hot air flowing upwards from the CPU and GPU, but the PSU panel facing the bottom is sealed; the heat doesn't have an easy access vent like a fan intake. The rear panel exhaust fan also helps keep this heat at bay, particularly if it is complemented by a front intake fan on the opposite side, or an in-line fan on a tower CPU heatsink that keeps all the air flowing in a line front-to-back. The Solo II also happens to have a number of quiet features heavy 1mm panels, very sturdy construction, polycarbonate sheets on panels for vibration damping, and a hard drive floating suspension system. A fanless power supply suggests a silent system, and silence is the Solo II's raison d'être.
Antec Solo II.
The best microATX candidate is the SilverStone Temjin TJ08-E. The layout is odd, similar to a modern tower only flipped upside-down. The end result is well suited for a fanless power supply as it sits at the top with a dedicated exhaust vent. It has a massive 18 cm intake fan at the front, and a 12 cm position at the rear next to the CPU, so not much heat will waft up from the graphics card and processor. The fan setup might be better than the Solo II, but the top vent is smaller and the entire case is more cramped (see how the hard drive cage overhangs the motherboard tray).
SilverStone Temjin TJ08-E.
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