I've been thinking along those lines. I picked up a "small refrig compressor" a couple of weeks ago.
(It's actually an old dehumidifier).
I've tested it, and it still works okay. but it's very old looking..
I think that I can pump it down safely, for a '"refrigerant salvage pump" project.
I've got all kinds of interesting stuff going on, now that it's getting warmer.
Bike riding is high on my daily To-Do-List..
And now we are just starting the process of buying a new car (Prius C2) for the wife.
I'm totally distracted from the hot water problem. Might be another month (or 2)
before I get back on that project..