What loop did you use for the testing? At least a decent length of tubing with a couple of blocks in I hope. It sounds like you ran a small loop to and from a reservoir then left it running for 3 hours. If so, consider
Pumps run hotter when not loaded. If it's pumping against considerable back pressure it runs more efficiently as the impeller speed remains within sensible limits. The majority of the electrical power drawn is used to pump through the resistance.
If pumping against minimal pressure, it'll spin faster but won't be able to do enough work on the water. Consequently of the power used (lets say 18W for the '18W ddc'), a smaller fraction goes into the water. The pump then runs hotter, and no doubt heats the water more.
We therefore need more information about your experimental set up before it can be considered conclusive. I applaud you actually testing, that's definitely the right way to go about things. An alternative explanation at the moment is that the CPX-Pro, being a weaker pump, was running closer to its expected load and therefore at a higher efficiency than the laing ddc.
Pumps run hotter when not loaded. If it's pumping against considerable back pressure it runs more efficiently as the impeller speed remains within sensible limits. The majority of the electrical power drawn is used to pump through the resistance.
If pumping against minimal pressure, it'll spin faster but won't be able to do enough work on the water. Consequently of the power used (lets say 18W for the '18W ddc'), a smaller fraction goes into the water. The pump then runs hotter, and no doubt heats the water more.
We therefore need more information about your experimental set up before it can be considered conclusive. I applaud you actually testing, that's definitely the right way to go about things. An alternative explanation at the moment is that the CPX-Pro, being a weaker pump, was running closer to its expected load and therefore at a higher efficiency than the laing ddc.
