Air or Water?

A perspective on choosing between the two – Joe

SUMMARY: Air cooled heatsink are competitive with water cooling kits, with some caveats.

I routinely get emails remarking on the similarity of C/Ws between air cooled heatsinks and water cooled kits, so I thought a more detailed look was in order.

First, direct C/W comparisons between water cooling kits and P4 heatsinks are not valid – the air cooled heatsinks are tested using a P4 test bed and the water cooling kits use the die simulator. The die measures 10 x 13 mm and as such is not comparable to the P4’s IHS.

However, there have been times when I tested P4 heatsinks using both methods. I looked back to these tests and on average, the difference between the two is about 0.10 +/- 0.02, with the P4 results being lower than the die simulator. I want to caution that this is an average difference – please view what follows with that caveat.

The table below ranks P4 heatsinks and water cooling kits using the conversion factor above – if, for example, a water cooling kit C/W was 0.20, it adjusts to 0.10 to compare against P4 air cooled heatsinks. I have indicated the adjusted C/W in parenthesis in the water cooling kit results (water kits in BLUE); I have limited the selections to P4 heatsinks with C/Ws of 0.20 or better and water cooling kits with adjusted C/Ws of 0.20 or better.

While not a perfect comparison, results indicate that there are indeed air cooled heatsinks that are competitive with water cooling kits. However, note that for extreme performance, high-end water cooling kits dominate – not a surprise by any means. While some air cooled heatsinks attain top results, many times it’s with very aggressive (noisy) fans – not necessarily the case for water.

Air cooled results must be tempered by taking into account case temps – an excellent air cooled heatsink in a poorly ventilated case will NOT deliver the C/Ws shown in this table (for more on this, go HERE.) The higher the case temp, the larger the discrepancy between test results and performance on screen.

Water cooling kits, in comparison, tend to pull cooling air from outside the case, so that case temps have much less impact on performance.

One major difference between air and water is the amount of heat which can be handled; the “overhead” with water is much higher compared to air cooled solutions. It is not uncommon to see water cooling systems with TECs used to cool CPUs to below-ambient temps, a situation not feasible for air cooling.

All my desktops are water cooled, first and foremost for low noise. Newer heatpipes can effectively compete on this basis and deliver extremely good cooling performance, assuming excellent case airflow. Water will always trump air for the load it can handle.

Users pondering air vs water must first consider their objectives in CPU cooling – low noise with high heat loads tips the balance to water, while low noise and “normal” heat loads may tip the balance to air, although other factors, such as case airflow, must also be taken into account.

Finally, the cost difference between the two solutions is significant – expect water to cost about five times more than air for high-end kits.

Email Joe