Full size electric water heaters typically have two heating elements, the upper and the lower. Most models are designed for the elements to operate independently, but in some models they may only operate in tandem. When the thermostats are closed, current flows to the immersion heating elements inside the tank.
Details Unique to a Gas Water Heater
The thermostat on a gas heater is an integral part of the gas control valve and is immersed inside the tank. When the temperature drops, the thermostat signals the gas control valve to supply gas to the burner. However, before it allows gas to flow, the thermocouple must signal that the pilot light is lit, otherwise a dangerous gas leak would occur.
The thermocouple is situated in or near the pilot flame. The heat of the flame creates millivolt current which engages a magnet in the gas valve. The gas valve releases gas in a burner adjacent to the pilot flame and is ignited. If the pilot goes out, the current stops and the magnet disengages, preventing the gas valve from opening.
Gas water heaters must be vented to the outdoors. The combustion of natural gas results in toxic carbon monoxide. In the center of the tank is a flew to carry the gases away to the vent mounted on top of the tank. The vent is critical to your safety because it carries the carbon monoxide outside your home.