The "wetback" stove has a heat exchanger that's plumbed directly into the (otherwise electric) hot water heater in the cabinet just to the right. It's electric most of the time, and the stove just takes the water up to much higher temperatures than the thermostat would.

So our hot water is nearly boiling some of the time. And there's another widget downstairs called a "temperer" that automagically mixes it down to non-searing temperatures, more or less. It's mechanical, so it doesn't respond instantly.

Oh, and the whole hot water/temperer setup is run at low pressure (like 10psi), so there's a pressure reducer and a pressure relief valve.

This whole crazy thing is a particularly kiwi invention, not generally used in other parts of the world, and no longer installed in new houses.