With all this response I decided last night to take a pair of huge pipe pliers and a hammer to a spare piece (pipe & ball valve) that I had cemented together with the ABS glue. It did not give way easily, the pipe was pretty much mangled before the glue let go. That being said though when it did fail the piece came out of the joint like the glue didn't exist. The inside of the ball value looks like new, it seems the only place the glue attached to was on the very end of the pipe. It kind of reminds me of the behaviour of some glues on metal or glass, once you unseat it a little it just completely releases.
In the end I could easily see the glue releasing if it was expanding and contracting a lot but as mentioned in the posts I'm not sure if that would even occur if you're running a constant temperature through the pipes. As suggested I'm thinking for the drains I’m just going to leave them the way it is. The return I may redo as it runs along the outside of the tank but I will put that off till I have to move the tank again once the floors have been replaced.
Maybe the Rona or Home Depot guys will give me deal, they pretty much know me by name now...
