Think of a heater core in this way... If you're hot in the summertime and don't have a pool or A/C, what do you do? You get a fan. Does the fan warm you or cool you? Of course, it actually makes you feel cooler. The air blowing over you blows away the heat surrounding your skin and allows it to put out more heat and you feel cooler. Now, how long does it take for you to feel cooler in this situation if you are outside in your swim shorts and the air temp. is 4 degrees. Your blood (the flowing heated liquid) will not be able to keep up with the cooling effects of the fan (blower). Thus your body (the core) will eventually grow cold. If you shut down the fan for a while, the heat will start to warm up the core once again until you turn the fan back on. There is an old law of thermodynamics (mine, I think) which points out that the output (of your core) will never exceed the input (from your engine).