I don't know much about heat exchangers. However, my understanding is that if water is heated in a small diameter copper tube, you can get some combination of steam bubbles and liquid water that will block the tube, and can then build up enough pressure to burst the copper.
I'd be comfortable with a home made heat exchanger in either of these two circumstances:
1) Temperature in tubing is never high enough to turn water into steam.
2) Water is heated in an open container with no way for pressure to build up. Hot water may be circulated through tubing, but never steam.
It's possible that I'm being overly conservative since I don't have a good grasp of the thermodynamics involved. Just be careful if you do try this.