FWIW, we had a vacuum chamber built at work to perform electrical tests at a simulated altitude. The chamber was intende to be good to at least 70,000 ft altitude equivalent pressure. It's a cube about 40 inches on a side, the walls are 1 inch thick 6061-T651 aluminum plate. For electrical and fluid pass through connection areas we used 1.25 inch thick Lexan. I've seen this chamber go to ~1 psia, an equivalent pressure differential of ~13.5 psi inside to outside. Based on this construction, I think you will need thicker walls for your pressure tank. Even if it's made of steel, 3/16 starts looking pretty thin when you consider the forces.
As everyone else has said, aluminum and copper don't mix with a water heating system. I've been involved in designing aluminum heat exchangers to run with liquid cooling systems. Even with corrosion inhibitor, using aluminum and copper with any water based working fluid (usually propylene or ethylene glycol based) is a bad idea. This goes double at high temperatures, like where you would run a heating system. The last propylene glycol/water system we set up, we had to go through every piece of equipment that would ever come in contact with one of the heat exchangers and make sure there was no copper anywhere in the system. Even to the point of using nickel brazed flat plate heat exchangers in our test equipment instead of copper brazed flat plates. If there was any copper present, the filters would plug with blueish-grey paste.
