Calling it a device that heated water was a bit of an oversimplification, to be honest. Realistically, it was more like a giant kettle with a space to put an Energia Crystal in the bottom.
Think of a propane camping kettle on a larger scale, considering it was large enough to be called a boiler.
Add 10 gallons of water, slot in a Fire Elementalized Energia Crystal, and within 5 minutes, the water is piping hot.
Setting it up was easy because the shed he'd brought with him to Genovia, courtesy of his thoughtful Mom, was insulated and could keep all the warmth inside, turning the small 7x7 room into a practical sauna.
With that all set, Aoni moved to the other side of the shed and began setting up some basic PVC piping, connecting the tub to the heater. Although a bit crude and by no means aesthetically pleasing, it served the purpose of creating a usable bathing station.