I built a glycol fermenter from the design given in BYO's "25 great homebrew projects" issue. The design is to put the cooling coil of a window A/C unit into a glycol bath (I used RV antifreeze), and fountain pumps to circulate the cold glycol through copper tubing surrounding two fermenters. Finally, Love controllers run the fountain pumps and A/C unit to ensure that the glycol bath and both fermenters stay at the correct temperatures.
It ran great for the first three batches. I then had some house guests, didn't brew for a month, and tonight powered it back up. It didn't cool the glycol bath -- after an hour of the A/C unit running it had cooled by two degrees. (Everything was on and running -- the fan was running, compressor got hot, etc. Just that the hot air that should have been blowing out the back wasn't, and the coil didn't get cold.)
I've got a couple guesses about the problem. Could be that during the process of moving the A/C coil from inside the unit to outside, I damaged the tubing in a way that is imperceptible to the eye but allowed the refrigerant to escape.
Or, I might have damaged the A/C unit in some other way.
Could be that RV antifreeze damaged the coils, though it's just propylene glycol, water, and some chemicals designed to make it less corrosive.
I'm curious if anyone else has had issues with this design, or might have some insight into this problem.