I came across this the other day on a jobsite and had to do a little research. A guy had a Solar heat exchanger installed by a professional company that specializes in this type of system. The problem was the single wall heat exchanger. The company used Propylene Glycol saying it was non-toxic but the msds gave it a "1" for health. The UPC prohibits toxic substances in heat exchangers unless it is double walled. So I had do some research and found out that the Propylene Glycol cannot be used in a single wall exchanger because it IS toxic. The company said that they've never been called on it before and the package says "non-toxic" so what's the big deal. The UPC references some old archaic toxicology book that we had to find to confirm the code call. What a pain in the arse! So the company is now going to use a Glycerin based antifreeze and since Glycerin is ok by code we have to allow it. I can't seem to find what the other ingredients are, probably proprietary information, but still, I don't know if I'd want something that's going to make me just a "little" sick even close to my potable water. Now I'm wondering how many of those systems have been installed with Propylene Glycol? Any thoughts on this one?