All-in-one (AIO) liquid coolers are very popular amongst advanced PC users for a variety of reasons, with a current market potential that easily rivals that of top tier air coolers. As such, nearly all of the companies involved in the production of advanced cooling products for consumer PCs are currently marketing AIO coolers. However, AIO coolers are so successful that even companies who do not currently produce any air-based cooling solutions have decided to market their own liquid-based AIO coolers.

Fractal Design, the renowned Swedish designer and manufacturer of PC cases, is one such company who has decided to offer their own AIO coolers. A couple of years ago they released the Kelvin series, a simple-looking AIO liquid cooler design, yet it was one of the few expandable kits. It also was available with a 360 mm radiator, which is very rare even nowadays.

Fractal Design just released an upgrade of the Kelvin series, the Celsius. The Celsius is based on proven hardware (i.e. Asetek parts), comes with Fractal Design’s X2 PWM fans, features interesting fan speed controls and is expandable. The new AIO cooler is available in two versions - the S24 and the S36. The only difference between these two AIO coolers is the size of the radiator that, as their names suggest, can support up to two 120 mm fans or three 120 mm fans respectively. We will examine both coolers closely in this review.

Packaging & Bundle

We received the Fractal Design Celsius coolers in large, strong cardboard packaging. The artwork on the packaging is subtle and clean, focused on pointing out the main features of the coolers. Inside the box, the coolers are well protected into a custom cardboard insert.

Both coolers share an essentially identical bundle, with the only exception being the manual. The S36 also has a few more screws for the third cooling fan. Other than that, inside each box we found the necessary CPU socket mounting hardware, screws and washers for the fans and for mounting the radiator on the case, and two cable management clips.

The fans provided with the Celsius AIO liquid cooling kits – two with the S24 and three with the S36 – are Fractal Design’s Dynamic X2 GP-12 fans. These PWM models have a very broad speed range, allowing the fans to operate anywhere between 500 and 2000 RPM. Furthermore, the serrated blades of the fans are supposed to reduce aerodynamic noise. Their engines feature a “LLS” bearing that we weren't able to find any information on – not even what the acronym stands for. A (catastrophic) disassembly of one of the fans revealed that it has what it seems to be an almost typical rifle bearing but with a magnet attached near the top. It would seem that the designer’s simple, yet effective concept was to attach a magnet at the edge of the bearing, using the magnetic force to repel the frame, reducing the friction between the stationary and rotating parts, thus significantly improving the longevity of the design. As such, the “LLS” bearing would typically classify as a “maglev” bearing engine.

The Fractal Design Celsius S24 & S36 AIO Coolers

As we mentioned before, both the Celsius S24 and Celsius S36 are essentially identical, with the sole exception being the size of the radiator. The Celsius is a common configuration of a single radiator, two hoses and one block that combines the CPU contact plate, the pump, and the speed control electronics. Fractal Design went with a subtle, all-black design, so you will find no fancy colors or RGB lighting here. The coolers however are very attractive, with a symmetric, classy appearance. The use of braiding on the cooling tubes that perfectly matches that of the power wire (and cable sleeving in general) is a nice touch.

 
The Fractal Design Celsius S24 (left) and S36 (right)

The radiators of the S24 and S36 differ only in terms of length, with the former being 284 mm long and has space for two 120 mm fans, while the latter is 403 mm long and has space for three 120 mm fans. Both radiators are 30 mm thick and 123 mm wide. It is a common, proven core design, with wavy fins attached to thin oblong tubes. In theory, it is possible to install double the number of fans in a push-pull configuration, but that would not have a significant effect on radiators as narrow as these are, as the airflow impedance is not significant to begin with.

One of the simple features of the Celsius coolers that do stand out is the integration of a “power hub” on the radiators. This is meant only for the fans of the cooling kit, so the hub on the S24 has two headers, while that of the S36 has three. The hubs allow the users to connect the fans on the radiator and allow them to be controlled in parallel with the kit’s liquid pump from the motherboard’s header. With the hub’s cable routed through one of the hoses, this is exceptionally useful for users who are trying to build ultra-clean systems, without any visible or loose cables.

The hoses of both coolers are removable, allowing for the expansion of the kits if required. We found warranty seals on the hoses and Fractal Design states that removing them will limit the terms of the warranty, but not negate it completely. However, we would like to stress that there are two issues with these kits, should the user decides to go ahead with an upgrade. The first is that there are no fill ports and no reservoir, therefore proceeding with the upgrade without adding a reservoir would be a nearly impossible task for the average user. The second is the small ceramic pump that is integrated into the CPU block, which will be significantly overworked if it is called to drive the flow via several cooling blocks and a reservoir. It will undoubtedly work, but the overall performance will not be anywhere close to that of a stand-alone DIY liquid cooling system with a serious pump.

The circular CPU blocks betray Asetek’s involvement with these kits. The CPU block of the Celsius has a glossy top and matte black surround, without any lighting or complex geometric shapes. Only the company’s logo is partially etched at the top of the blocks. The matte black surround actually is a dial, allowing the user to switch between Auto and PWM speed control. It may feel like an overkill but it makes it exceptionally easy to reach inside the case, find the block and rotate the dial, even without any visual contact.

The copper contact plate is attached to the circular base of the block with eight screws. Thermal material is pre-applied to it and it comes with the Intel CPU mounting retention bracket installed from the factory. A retention bracket for AMD CPUs is included in the bundle. The Celsius is fully compatible with AMD’s latest Ryzen processors.

Testing Methodology

Although the testing of a cooler appears to be a simple task, that could not be much further from the truth. Proper thermal testing cannot be performed with a cooler mounted on a single chip, for multiple reasons. Some of these reasons include the instability of the thermal load and the inability to fully control and or monitor it, as well as the inaccuracy of the chip-integrated sensors. It is also impossible to compare results taken on different chips, let alone entirely different systems, which is a great problem when testing computer coolers, as the hardware changes every several months. Finally, testing a cooler on a typical system prevents the tester from assessing the most vital characteristic of a cooler, its absolute thermal resistance.

The absolute thermal resistance defines the absolute performance of a heatsink by indicating the temperature rise per unit of power, in our case in degrees Celsius per Watt (°C/W). In layman's terms, if the thermal resistance of a heatsink is known, the user can assess the highest possible temperature rise of a chip over ambient by simply multiplying the maximum thermal design power (TDP) rating of the chip with it. Extracting the absolute thermal resistance of a cooler however is no simple task, as the load has to be perfectly even, steady and variable, as the thermal resistance also varies depending on the magnitude of the thermal load. Therefore, even if it would be possible to assess the thermal resistance of a cooler while it is mounted on a working chip, it would not suffice, as a large change of the thermal load can yield much different results.

Appropriate thermal testing requires the creation of a proper testing station and the use of laboratory-grade equipment. Therefore, we created a thermal testing platform with a fully controllable thermal energy source that may be used to test any kind of cooler, regardless of its design and or compatibility. The thermal cartridge inside the core of our testing station can have its power adjusted between 60 W and 340 W, in 2 W increments (and it never throttles). Furthermore, monitoring and logging of the testing process via software minimizes the possibility of human errors during testing. A multifunction data acquisition module (DAQ) is responsible for the automatic or the manual control of the testing equipment, the acquisition of the ambient and the in-core temperatures via PT100 sensors, the logging of the test results and the mathematical extraction of performance figures.

Finally, as noise measurements are a bit tricky, their measurement is being performed only manually. Fans can have significant variations in speed from their rated values, thus their actual speed during the thermal testing is being acquired via a laser tachometer. The fans (and pumps, when applicable) are being powered via an adjustable, fanless desktop DC power supply and noise measurements are being taken 1 meter away from the cooler, in a straight line ahead from its fan engine. At this point we should also note that the Decibel scale is logarithmic, which means that roughly every 3 dB(A) the sound pressure doubles. Therefore, the difference of sound pressure between 30 dB(A) and 60 dB(A) is not "twice as much" but nearly a thousand times greater. The table below should help you cross-reference our test results with real-life situations.

The noise floor of our recording equipment is 30.2-30.4 dB(A), which represents a medium-sized room without any active noise sources. All of our acoustic testing takes place during night hours, minimizing the possibility of external disruptions.

*noise levels above this are not suggested for daily use

Testing Results, Maximum Fan Speed

Starting things off, let's take a look at cooler performance with the fans at their full, 12 V fan speed.

Our concurrent testing of the two Celsius variants gave us some very interesting results. With their fans running at their maximum rated speeds, the performance of the Celsius S24 is not record-breaking, yet it definitely is competitive, especially when taking the lower noise figures into account. The average thermal resistance of the Celsius S24 is 0.082 °C/W, which is slightly higher than that of most other dual 120 mm fan coolers, but our sound pressure meter was also recording only 38.7 dB(A) at the time of the test, which is the lowest figure that we have recorded up to this date from any comparative AIO cooler.

Comparing the performance of the Celsius S24 to its larger S36 is not as straightforward as most people would assume. Despite its significantly greater mass, the S36 delivers virtually the same performance at lower load levels. As the load increases, the performance gap between the two coolers widens slightly, but only at very high power loads does the S36 display a true performance advantage. Ultimately, the average thermal resistance of the Celsius S36 is slightly better, at 0.0797 °C/W, but only because of its ability to handle very high thermal loads more efficiently. Meanwhile, the noise level rises to 40.6 dB(A), diminishing the thermal performance gains even further.

Testing Results, Low Fan Speed

Before dissecting our low fan speed results, we should begin by noting that the design of the Celsius AIO coolers suggests that the fans will be powered by the cooler itself, from the hub on the radiator. With the whole assembly powered by a single cable, applying any fan speed control to the cooler will also affect the liquid pump as well. This particular pump does not react properly to a reduced voltage input, essentially requiring PWM control. Using a PWM voltage regulator, we reduced the speed of both kits down to their minimum operating point, which brought the speed of the fans down to 550-560 RPM.

The thermal performance of the Celsius kits with their fans and pumps driven down to minimum speed produced some very interesting results. Individually, the Celsius S24 displayed comparatively good thermal performance in conjunction with exceptionally low noise levels. The average thermal resistance of 0.1154 °C/W is mediocre when compared to that of other similarly sized coolers, but the sound pressure reading of 32.1 dB(A) is the lowest that we have recorded up to this date. Other recently released products could possibly rival the acoustics performance of the Celsius S24 if both the fans and the pump were driven via PWM controllers, but the Celsius series offers the convenience of concurrently driving everything via a single PWM signal/cable.

However, the reduction of the pump’s speed alongside with the fans does not work as well for the larger version of the Celsius. The Celsius S36 not only did not perform significantly better than the S24 during this test, but it actually performed worse, especially at low load levels. Our assumption is that the small ceramic pump is not powerful enough to have its speed reduced that much and still be able to cope with the resistance of such a large radiator, meaning that the liquid flow was lowered to the point that outweighed the benefit of the greater heat exchange area. Regardless of equipment, measuring the flow without interfering with the factory configuration was all but impossible, so we cannot currently put our theory to the test. Nonetheless, irrespectively of the correct answer, the fact is that the Celsius S24 performs better than the large S36 with the speed of the pump and fans down to their minimum setting. 

Conclusion

Fractal Design released the new Celsius coolers with the aim of bringing an all-around competitive product into the market while staying true to the company’s motto - “less is more”. The Celsius coolers are aesthetically simple, without RGB lighting and fancy colors; on the contrary, the designer of the Celsius has been trying to make the otherwise large devices as inconspicuous as possible and blending in with the environment inside a high-end PC. The all-black design and the minimal appearance and travel of cables will definitely help advanced users and modders assemble some of the cleanest-looking systems out there.

With that said, no cooler could succeed while focused on aesthetics alone and Fractal Design certainly did not forget about performance. Although the Celsius S24 or S36 will not be breaking any thermal performance records, the Scandinavian coolers seem to deliver very good thermal performance while maintaining exceptionally low noise levels. The ceramic pump is exceptionally balanced, very quiet, with no whining noises coming from it even when it is running at maximum speed. Fractal Design’s Dynamic X2 fans appear to have been an excellent choice for these coolers, providing good performance with relatively very low noise levels.

The discussion on thermal performance becomes complicated not when trying to compare the Celsius coolers to other products, but when trying to compare them between themselves. In our testing, the huge radiator of the Celsius S36 only has a performance advantage when the cooler's fans/pump are running at high speeds, and even then the advantage is apparent only when the thermal load is very high. This scenario hardly makes any sense with a single modern CPU – even one that is heavily overclocked – as even HEDT CPUs don't product the 300W+ of heat it takes to really dfiferentiate these coolers.

On the other hand, with the fan/pump speeds lowered, the Celsius S24 not only delivers just as good of thermal performance as its bigger S36 counterpart, but it actually manages to outperform the S36 during most of the tests, all while producing significantly lower noise levels. We believe that the only scenario that would give the S36 a sizable advantage is the expansion of the kit to cover more energy-hungry parts, but we also feel that the small ceramic pump will be outclassed if it finds itself having to deal with a 400 mm long radiator and multiple cooling blocks. Meanwhile the S36 has the inherent disadvantage of the extra size and higher noise levels that the third cooling fan unavoidably introduces, making it an even less appealing choice. Alone it might be a more interesting cooler, but the S24 performs so well that it puts the S36 in a bit of a tough spot.

Availability of the Fractal Design Celsius in North America is relatively limited, which is not unusual for a product from a European company that has just launched. The Celsius S24 and S36 are currently available in Amazon for $109 and $119 respectively, which are not very competitive prices considering the overwhelming competition that they have to overcome. They may be two of the least noisy AIO coolers that we have ever tested, but retailing that much higher than similar (and very popular) products is not going to help Fractal with their sales. The price of the kits will most definitely come down as more vendors bring in some stock, and the Fractal Design Celsius S24 will most likely become a favorite for users whom prioritize simplicity and quiet operation.

As for the Fractal Design Celsius S36, we will leave it up to the reader to decide if the extra size, noise and cost are worth the slightly better performance under very heavy thermal loads. But, according to our test results, the Celsius S24 definitely is a more sensible choice for most users.