A hot water heater is an appliance for heating water above its ambient temperature. In industrial usage, large hot water heaters are called boilers.

In household and commercial usage, most hot water heaters are of the tank type. These consist of tanks in which a given amount of water is kept continuously hot and ready for use. Typical sizes for household use are 20, 30, or 40 gallons. These may run on electricity, natural gas, propane gas , fuel oil, or other energy sources. The most popular in the United States is the natural gas type.

Tank-type hot water heaters can be made more efficient by installation of additional insulation jackets around the tank, flow valve devices at their inlet and outlet, cycle timers, electronic ignition (in the case of fuel-using models), sealed air intake systems (in the case of fuel-using models), and pipe insulation. The sealed air-intake system types are sometimes called "band-joist" intake units. "High efficiency" units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cool and are mechanically ventillated without the need of a chimney.

Tankless instantaneous or "instant-on" hot water heaters are also available and gaining in popularity. These water heaters heat the water as the water flows through the device, and do not retain any water internally except for what is in the pipe. Installed at the point of use, these heaters can be far more efficient than the tank type. They are usually available to use electricity, natural gas or propane.

In British English, hot water heaters are called immersion tanks or geysers. Other common terms are hot water tank, water heater and hot water system.

External links

• Howstuffworks "How Water Heaters Work"