In the McGraw-Hill Dictionary of Architecture & Construction the definition of water heater is: a device for heating water for domestic use, usually supplied at a temperature in the range between 120°F and 140°F
Tank Water Heaters
Most water heaters are tank water heaters which keep a cylindrical tank full of hot water in your home at all times. When hot water is required the water is circulated to the open faucet. Conventional tanks utilize electric, natural gas, propane, oil, solar or geothermal power as an energy source to heat the water.
Tankless Water Heaters
Tankless water heaters are more expensive to install than other models, but remember, keeping up with your water heater repairs or replacing an old water heater with nearly any newer model will generally lead to a more efficient, less costly hot water system overall.
Electric Water Heaters
The typical electric water heater is wired to a 220-volt circuit. To heat the water, the current passes through electrical-resistance heating elements—usually two, one at the middle of the tank and one at the bottom. Power is delivered to each element through a thermostat—a switch that senses the water temperature. When the temperature drops, the switch closes to allow current flow, and it opens when the temperature reaches itspreset limit. Thermostats have a dial for setting the maximum water temperature--generally between 130 degrees and 140 degrees F, or as low as about 120 degrees F for increased energy savings and scald protection.
Gas-Fired Water Heaters
Gas-fired heaters have a burner that's fed gas through a control valve and a thermostat switch. The burner is usually situated to throw a flame under the tank. The exhaust gases are vented either through a hollow core at the center of the tank or around the tank sides. Because gas-fired heaters heat the tank, which inturn heats the water, there will be more wear and tear on the tank than with electric heat. A gas-fired heater, therefore, may have a shorter life expectancy than an electric heater.