Tankless water heaters—also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters—are gaining in popularity. These high-power water heaters instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication.
Tankless heaters may be installed throughout a household at more than one point-of-use (POU), far from a central water heater, or larger centralized models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are a plentiful continuous flow of hot water (as compared to a limited flow of continuously heated hot water from conventional tank water heaters), and potential energy savings under some conditions.
Stand-alone appliances for quickly heating water for DHW are known in North America as tankless or on demand water heaters. In some places, they’re called multipoint heaters, geysers or ascots. In Australia and New Zealand they are called instantaneous hot water units. In Argentina they are called “calefones”. In that country “calefones” use gas instead of electricity.