There are several parts to a gas fired water heater’s temperature control system. On some models they are all integrated into one unit called a combination gas valve, on others they are in separate units throughout the water heater. Gas water heaters use heat from a gas burner flame, transmitted by conduction through the bottom of the tank, to bring household water up to usable temperatures and maintain it there. Doing so in the safest and most energy efficient orderly manner is the work of the control system. The components of the system are listed and described briefly below.
Thermostatic Valve: These automatically adjust gas input to the burner(s) with respect to the temperature of the water in the tank. Working like a thermostat, they open a valve that supplies gas into the burner when the water temperature drops below a preset point, shutting it when the water temperature rises above a certain point, thus maintaining the water at a set temperature range. Thermostatic valves consist of a thin copper tube, protruding into the tank, which acts a sensor, attached mechanically to a bimetallic snap clicker mechanism that opens and closes the gas valve. The copper tube reacts to a drop in temperature by contracting inward toward the snap mechanism to close the valve. The opposite action happens when the water temperature rises.
Automatic Pilot Valve: This is a spring-loaded valve which automatically shuts off gas supply to the pilot valve and main gas burner in case of improper lighting or pilot flame outage. It is powered by a thermocouple linked to an electromagnet attached to the pilot valve. The thermocouple produces electrical current when temperature differential between a junction at the point of pilot flame and a junction at a non-heated point is sufficient. Lack of the correct temperature differential will cut off current to the electromagnet, in turn causing the spring loaded valve to close. In order to relight the pilot flame, a reset button must be pressed.
Manual Gas Valve: Also called a gas cock or cock valve, this acts as a manual safety backup for the automatic pilot valve, as well as provides the means of shutting off gas to the main burner during pilot lighting.
Pressure Regulator: Gas pressure regulators are mandatory on residential water heaters, and regulate pressure of gas supplied to both the main burner and pilot burner. They consist of enclosures containing pairs of diaphragms which balance out differences in pressure from upstream and downstream, using the balanced pressure principle.
Automatic Gas Shutoff Valve: The function of this safety device is to shut off all gas supply to the water heater if temperature in the tank exceeds certain temperatures, usually above 210 F. Automatic gas shutoff valves typically work in combination with the automatic pilot valve, being connected in series with them in the pilot safety shutoff circuit. The actual component is not typically a valve, but rather a closed electrical switch mounted on the water tank’s side towards it’s top. The switch is set to open when the tank reaches a given temperature; when it does, the automatic pilot valve shuts, turning off gas to pilot and main burners.
Temperature Relief Valve: Designed to relieve water pressure in the tank when it builds up due to excessive temperature, this safety device releases water and introduces cold water supply into the tank to keep the temperature below 210 degrees F.
Pressure Relief Valve: Provides a release of water from the tank when pressure builds up beyond acceptable levels. These valves are required in residential water heaters. More on pressure relief valves here.
Combination Gas Valve: A combination gas valve contains all the manual and automatic gas and temperature controls in one unit. Typical combination valves include a thermostatic valve, automatic gas shutoff, automatic pilot valve, gas pressure regulator, and manual valve.