I have two identical, and burned, flo-thru heaters. One is a Vulcan, the other an Aetna. I believe the heater elements in them are replaceable, but I can't get the one I've tried to turn loose. There is no visible nut or screw, so perhaps the element is welded in place. I've rapped with a small hammer and used a pin punch beside the connector nubs. Nothing so far, and I'm afraid to hit it harder. Do those elements come out, or do I need to buy the whole manifold with element?

The element can't be replaced in it.

I always recommend against replacing those heaters due to the expense. The best thing for you to do is invest in a replacement controller that uses a less expensive element.