Constant speed, YES! Do not try to have its speed vary according to CPU temperature. The reason has nothing to do with wear and lifetime. In a liquid-cooled system there are TWO things that impact the rate of heat removal from the CPU. PUMP speed determines how fast heat is moved from CPU to rad. FAN speed determines how fast heat is moved from rad into air and expelled. Each if these has a different response time or "delay time". IF you set your system to use the CPU temperature to alter the speed of BOTH of these items, a temp increase will tell BOTH to speed up. But one will act faster than the other, and the temp will start to drop before the second one has its full impact. Then that second impact hits and over-cools the CPU, so the system reacts by telling BOTH to slow down. Again, the pair of actions results in over-correction, and the system just keeps "chasing" itself up and down. The solution is to have ONLY ONE element's speed altered automatically in response to CPU temperature. Normally it is RAD FAN speed that is varied by the control system, and PUMP speed is kept constant.
What constant speed is needed? The simple answer is let it run full speed all the time. This guarantees that the rate of moving heat from CPU to rad will always be at max and will NOT limit the cooling capacity of the system. The SLIGHT downside is that this will have an impact on pump lifetime, but good design can ensure that pump lifetime under these conditions will not be so short (compared to the lifetimes of other system components) that it limits the overall system.
SOME people who worry about pump lifetime may opt for a slightly different strategy. They MAY arrange to fix the PUMP speed at some manually-set constant value slower than max to increase lifetime. They MAY even set up some "rules" for themselves to go in and alter the fixed speed setting from time to time depending on their own assessment of workload and heat removal load. How much this can extend pump lifetime cannot really be determined.
By the way, HOW to set a less-than-max speed for a PUMP motor is not obvious to many. Because most AIO systems are designed to be used with the pump running full speed all the time, the pump is designed for that. The most common way is that it is built exactly like an older 3-pin fan in which the speed can be controlled only by reducing the VOLTAGE supplied to the motor. So IF the pump is connected to a mobo header that is using the more modern PWM Mode of control suited to 4-pin fans, that pump will always run full speed. OR, if the pump is connected to a header set to supply a PUMP in particular, that setting means the header will always send out the max 12 VDC power supply needed from Pin #2. If you actually WANT to set you own PUMP speed, you must connect it to a header that is set to the older Voltage Control Mode (aka DC Mode) and then set the fixed speed that way.
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