[pHaestus]

Why would the flow resistance part of the pump's heat change from pump to pump Ben? I don't follow. Wouldn't that only be a function of flow rate and the resistances in the loop? Now there is a portion of the heat dumped into water directly by the pump. THAT would vary from pump to pump. I don't see how a pump's efficiency could affect an equation for secondary heat production due to friction though. I also don't think you can measure the deltaT across a restriction with any kind of accuracy at all: across a waterblock or a radiator you can do ok (and probably across a pump) but across something that isn't actively dumping or removing heat then I don't think you'll see a difference with any equipment we can afford. You'd need better than 0.01C res at any rate to pick up any change.