A heat-sink type of water heater unit is disclosed wherein the unit is designed for external installation on a stove to transfer heat from the stove to water flowing through the unit, to pre-heat the water for subsequent use. The unit has a flat surface which can be bolted to the outside of the stove, an inlet which can be attached to a cold-water source, a U-shaped inner passage in which water can flow and be heated by the stove, and a discharge which can be hooked up to a hot-water tank. In addition, the unit has an adjustment feature which enables the unit to be moved laterally, closer to or farther away from the hot stove wall to which the unit is connected. By regulating the distance between the unit and the stove, the water temperature produced by the unit can be selectively controlled.