A thermal expansion tank is need only if there is a check valve in the supply line that would prevent the expansion from being absorbed in the water supply. This usually applies to city water supply when there is a pressure regulator valve or a check valve in the water meter. Neither of these apply to your situation. Frankly, I don't even know what you mean by "bonding" pipes together. I have a professionally installed gas heater and the lines are simply connected to the heater with flex copper. Perhaps some of the learned pros will know what you are referring to.