Abstract: A water distiller provides distilled water from an input water supply for domestic, clinic, studio, hospital or factory applications with greatly improved energy efficiency. Stages of the distiller may employ under pressure conditions and/or various vapor compression, heat pump or intermediate process arrangements to provide efficient heat transfer and heating, without requiring boiling of the full input water volume. Enhanced mixing and/or heat transfer may be effected by passing water vapor through the water heating pool and result in clean, efficient distillation in a unit designed for a defined rate or volume of distillate or small system. Embodiments of the invention require less energy than required for comparably-sized direct boiling systems, and may be configured to purge gases or impurities to attain, or maintain, clean heat transfer surfaces and a desired level of product purity.