Water heaters and boilers are residential and commercial appliances that provide a continuous supply of hot water throughout a building for conditioning or washing. The variation in names and functions of hot water heaters is attributed to their capacity to heat drinking water, usage in the a range of purposes in the domestic or industrial sphere, and their respective energy sources. Fossil fuels are commonly used for heating water, but alternative energy, backed with fossil fuels or electricity, can also be implemented instead. Electric heat pump hot water heaters are another more efficient, but more expensive, option. Hot water for the purposes of space heating is heated in a boiler.

Water heaters and boilers have come a long way. Initially, people heated water over open flames, which was tedious and time-consuming. The 19th century saw the development of rudimentary boilers, gradually replaced by more efficient models. By the 20th century, electricity and gas-powered options became common, offering quicker and more convenient ways to get hot water. Tankless designs emerged, providing instant heat without the need for storage. These advancements greatly improved home comfort and convenience.

As we move forward, water heaters and boilers are becoming more eco-friendly and efficient. Solar-powered units and heat pump technologies are gaining traction, reducing carbon footprints. Smart tech integration allows for remote control and monitoring. The challenge lies in retrofitting old infrastructure with these new systems and ensuring they're accessible to all, despite the higher initial costs. The emphasis is on sustainability, energy-saving, and user convenience.