From top of my head: my grandma used to have one. There was always a little flame running for safety in case of leaks, but when she used hoy water, I think the geyser just heated it on the go, instead of preheating a reservoir.

Could be wrong though.

A device for heating water on-demand, usually a gas burner with a spiraling water pipe surrounding/above it. As opposed to a boiler, which pre-heats water and stores it for later use (and also needs to keep reheating the water as it cools if not used).

Spanish wikipedia is the only with a picture of the internals: https://es.wikipedia.org/wiki/Calentador_de_agua_circulante?...

In the US, this is called a tankless water heater.

I’m not sure where in the world calls them geysers, but I agree it’s not a term you’ll hear in the US!

Water heater for bath/showers