This is the American version of the Ewart (British) geyser. Operation was similar except that this one kept flue gases and water separate, so you could cook with or drink the hot water safely. It dates from 1902.

People figured out that you could, with enough valves, use heaters like this for more than one thing. This one came with sufficient plumbing to supply a bathtub and sink.

They could also be used to run the newfangled "rain bath" (shower). Originally rain baths were only proper for sick people. Bathing in general was frowned upon as being effeminate and unhealthy. It was only after 1863, when a bathtub was installed in the White House that bathing began to lose its bad image.