Which is it, water pressue or air pressue? I don’t use hot seltzer so I imagine it is water pressure. ROB, you seem to be missing something very essential here. Water pressure means that there is a load of cold water waiting to enter the tank, but can’t because it is full. When I open my hot water valve, hereby releasing some hot water from the tank, that allows in new cold water, which is what pushes out the hot water.
This is the same pressure that drives my cold water up the pipe. If I shut my cold water in the basement, the remaining water in the pipes will not go up because nothing is pushing it anymore. The same goes for hot water.