Otto is correct (as far as what I was saying.) That puck that's attached to the tank itself is probably marginally attached to the tank sheet metal, and may be sealed with solder. If you get it too hot, you run the danger of letting the solder run out of the joint between it and the sheet metal. You'd have to make sure that there's a solid fillet of solder around both that piece and the one that's threaded for the outlet nipple.
