Originally Posted by glennjanie
Copper comes several different ways: soft (in a coil), hard copper which comes in straight lengths, classified as K, L, M and DWV. The Drain Waste and Vent copper is the thinnest grade, obviously it doesn't carry pressure. The other 3 types all have the same outside dimensions but the grades are thicker or thinner. L is the most used grade of copper for water lines and comes in both coil and straight lengths. L is also the type used for HVAC-R, the only difference is the tube has been evacuated and capped to prevent oxidation inside.
Kentucky's Plumbing Department is deadly serious about cross-connections, back flow and back pressure. It is so important to us that I used to give my students the day off if they could quote (word for word) the definition of cross-connection. One of them really worked at it; he even quoted it backward without missing a word.
In the case of the DWHR one of the lines is already violated (the drain) and the supply line may not always have pressure on it; thus the possibility of backflow.
Probably the best way to do this is with two separate coils, one for wastewater and one for potable, both of which are installed in a small tank filled with a transfer medium, such as RV antifreeze. In order for wastewater to contaminate the potable water, both coils would have to leak. Either one leaking would cause a level change in the transfer medium. As to cost effective??? Perhaps, perhaps not.