For the average water cooled system, distilled or de-ionized water is the coolant of choice. Of course, if sub-zero cooling is a goal, then coolant with a lower freezing point than water is an obvious requirement. Distilled water is a good coolant because it has excellent heat transfer qualities, is inexpensive, and is a non-conductor. Unless you plan on flushing your system on a monthly or even weekly basis, some other additives will need to be added to the coolant mix.

Primarily, additives serve two main purposes. The can augment the performance of the coolant as a heat conduit, and they can add some aesthetic appeal to the case design. From a performance standpoint, there are several products on the market which can increase the thermal properties of the coolant. This may be by lowering its freezing point or by decreasing its surface tension, to name only two. What is surface tension and why is it beneficial to decrease it? Take a clear glass, pour water in it, and then look at it with the water level at your eye level. You’ll notice that the water is slightly higher at the sides of the cup than in the middle. It almost looks as if the water is "grabbing" the edges of the cup. That is a physical demonstration of surface tension. Now, why would a decrease in surface tension be a good thing in a water cooled system? Just like in the example of the cup, coolant will tend to "grab" the sides of the tubing walls, creating two layers of coolant; fast moving coolant running though the middle of the tubes/blocks/radiator and slower moving coolant clinging to the sides. In this scenario, heat transfer is inefficient, as a lot of the coolant moving through the system barely gets any heat transfer, and the coolant that does is not pulled away from the water block quickly enough to maximize cooling. By lowering the surface tension using an additive, this effect of surface tension can be reduced.

Other performance issues also include the fact that in just about every water cooled system out there, there are mixed metals, usually copper and aluminum. Since the water acts as a catalyst between the two metals, corrosion becomes an issue that needs to be addressed. Glycol (a.k.a common antifreeze) has excellent anti-corrosion qualities, but decreases some thermal properties of water and increases surface tension. As such, you’ll typically find a mix of distilled water, glycol and a performance additive in most water cooled rigs.

Also, since warm water is a natural breeding ground for microbes of all kinds, an anti-algae/anti-biological agent is also a good recommendation for users who might only flush their systems once or twice a year. There are a few different additives being employed in water cooled systems. Swiftech includes a product called HydrX coolant, while some users employ products from the automotive industry such as Redline Water Wetter or Royal Purple’s Purple Ice. Purple Ice claims that it provides corrosion resistance and decreases surface temperatures, as well as being a lubricant for seals in water pumps. HydrX is listed as an anti-corrosion, anti-algae agent that improves heat transfer. Having never worked with Water Wetter personally, I can only quote the manufacturer by saying that it is principally designed for increased thermal transfer and as an anti-corrosion agent. For the test setup, I decided to work with my de-ionized water and the recommended dosage of HydrX extreme duty coolant.