My loop has been running 24/7 for the most part since 2003. Started with one DDC-1T and now have three in the loop.
The only times the computer and loop were down is due to power failure or making upgrades. My system runs like this for F@H for team 31630.
The pumps however are powered not from the PSU, but from a dedicated wall plug into a Tripp-Lite surge protector. So, if something flaky happens to the power of the computer, the pumps are well protected to keep running even if the computer turns off. Therefore, the hardware is being cooled as the WCing loop remains flowing. This is sort of like newer vehicles that have the engine cooling fans running still after the engine turns off.
My computer can stay off while my WCing still circulates. Thus, no worries about stagnant coolant to grow the nasties. DDC pumps work well beyond the 50,000+ hour run life if maintained really well. Using about 20% Sierra PG coolant + distilled water that lubricates the pumps really helps for run life along with 7.5VDC @ 950mA.
.Edited by bmaverick - 1/27/13 at 10:05am