The pump isn't dumping ALL of the energy into the water as heat. Some is used as mech energy, some as heat. Some of the heat gets transferred to the water through the casing of the pump, and about as much gets transferred to the air/water around it. IF you are using a submerged unit, ALL heat generated by the pump is being transmitted into your water (you were warned
). By nature, centrifugal pumps are VERY inefficient, and quite a bit of the energy is converted to heat (which is why many require submersion, or else the engine would BURN UP).
Pumps add heat. It's a fact. If you are adding more than 3-4C to water temp, there is something drastically wrong. I'm using an extremely overrated pump, and it does raise the water temp by a couple degs C when in use with the rad fans on but the CPU off. Most rads can handle the excess heat easily, especially since they are more efficient with a greater temperature differential between the fluids (water/air).