Time for a visual aid.
Here are some small cross sectional mockups I did.
Looks like 2 inch pipe on 4 inch pipe wont work. Its going to have to be 2.5 inch pipe.
I used actual 0.080 wall 4 inch pipe and standard 0.062 wall 2 and 2.5 inch pipe.
2.5 inch pipe on 4 inch pipe:
2 inch pipe on 4 inch pipe:
Some welds. Miller maxstar, 70 amps, 2% Thoriated tungsten, very pointy, 20SCFM.
To further improve the welds I am thinking about getting plasma cutter torch wheels and putting them on the tig torch that way I can roll the torch along the weld and get a super smooth evenly heated weld almost as if a laser wielding robot welded it.
Next step will be to build about a 1 foot long functional prototype, that wont be put in the ground, just torcher tested to failure.
Where I will do actual back purging the fresh water pipe and develop a good way to split 2.5 inch pipe down the middle and fit up the welding joint a little tighter.
Then I can measure the volume of the 1 foot section of fresh water pipe and see how much of a hot water reservoir I have.