Welcome to the Tankless water Heater group of SmartGuy.com. Feel free to interact with other professionals from across the world. You can find professionals, find deals on services, find professionals who might barter, or simple post questions, comments, or suggestions. Tankless water heaters, also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters, are water heaters that heat water instantly as it flows through the device. They do not retain any water internally except for what is in the heat exchanger coil.