Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Domestically, water is customarily heated in vessels known as water heaters, kettles, cauldrons, pots, or coppers.

Appliances for providing a more-or-less stable supply of hot water are variously known as water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, calorifiers, or geysers depending on whether they are heating filtered or non-potable water, in domestic or industrial use, their energy source, and in which part of the world they are found.

These metal vessels heat a batch of water, but do not produce a continual supply of heated water at a preset temperature.