Many years ago, i played with the idea of getting hot clean water from my car, i knew there were commercial water heaters for campervans etc, but i wanted to build this very cheap
I can't remember the exact fittings i used, but essetially i wanted to be able to connect it into the 1/2 inch heater hose circuit, a length of 15mm copper pipe inside a 22mm length of copper pipe was my first thought.
So you need a 22mm copper tee, at this stage i can't remember if i was able to purchase an unequal Tee with one side and one end with 15mm outlets, and one side 22mm (if you can't find this, then use a 22mm unequal with just the one 15mm outlet at the side, plus a 22 - 15 adaptor)
You need to get the 15mm pipe all the way through the pipe,so you'll need to file away the internal stop.
One other thing you will need is something that will fit in the space between the 2 pipes to make a spiral, i used some small bore copper pipe, but some thick copper wire should do the same job
When asssembled like the diagram below, insert into the heater line of your car, when the engine is up to temperature, you can feed cold water into one port, and get very hot water out, handy for washing up etc.
If you want to use this for hand washing, either mix cold into a bowl, or add a mixing circuit into the inlet side, it it important to leave the outlet open as a vent to prevent pressurisation.
Also note that the heating flow and cold water flow are in opposite directions, i think this gives better heat transfer.
It is also important to note that once you finish using it, the residual water left in the heat exchanger can turn to steam.