Now that's like a small version of the water heater at my grand-dad's house in Broken Hill. It was called a chip heater, and was permanently plumbed in to the cold water, with a simple shower head on the end of a tube. My dad remembers as a boy dropping little bits of kindling, straw, twigs etc down it to get it lit, and then after a few minutes having plenty of hot water for a shower, the water pressure of the cold water coming in pushing the hot water out. Apparently you could have a couple of minutes shower before the water went too cold.