Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.

Appliances that provide a continual supply of hot water are called water heaters, hot water heaters, hot water tanks, boilers and tankless hot water heaters. In domestic installations, potable water heated for uses other than space heating is also called domestic hot water (DHW).

In household and commercial usage, water heaters are the tank type, also called storage water heaters. These consist of a cylindrical vessel or container that keep water continuously hot and ready to use.

Typical sizes for household use range from 75 to 400 liters (20 to 100 US gallons). These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular in the US and most European countries since the gas is often conveniently piped throughout cities and towns and is currently the cheapest to use.

In the United States, natural gas water heaters for households without unusual typically need are 40 or 50 US gallons with a burner rated at 34,000 to 40,000 BTU/hour. Some models offer “High Efficiency and Ultra Low NOx” emissions.