Something is wrong with your heater or installation. I'm wondering if you have an air pocket in the top of the chamber. It is easy to purge the air by just loosening the watertight fitting a tad while the pump is on (assuming your heater is on the pressure side of the pump). You should hear the air being forced out and when a few drops of water leak out just screw the seal tight again.
I have two of these using 250W Ebo Jager heaters and both work as expected. I do have the advantage of being able to see the heater indicator light (earlier in this thread).
I agree the pump shouldnt impart much heat at all.