How Gas Water Heaters Work

Standard Gas Water Heater Operation

This section provides an overview of gas water heaters. The first part of this section has illustrations, photographs, and a brief description of each part. The second part of this section describes the operation of FLAME LOCK® gas water heaters. When this section is completed, the reader should have a basic understanding of how a gas water heater transfers heat into water.

Burner - The burner is centered under the bottom head and flue. Gas is ignited at the burner by the pilot light. The resulting combustion transfers heat to the water through the bottom head and the flue. Gas water heaters burn either natural gas or propane (also known as L.P., for Liquid Propane) fuel.

Standard Gas Water Heater Burner

Venting - The combustion process creates by-product gasses such as carbon dioxide, carbon monoxide, and nitrogen oxide. These gasses are harmful to breathe and need to be vented outside of the home. On standard gas models a draft hood is positioned at the top of the flue. The draft hood prevents the backflow of air, called a back draft, into the flue. A back draft could interfere with proper venting of harmful gasses, and could blow out the pilot light or burner. Some models use direct, power or power direct venting to exhaust flue gasses. Standard atmospheric water heaters can be vented in combination with gas furnaces, heaters and boilers, but not with gas cooking appliances, clothes dryers, or incinerators. Direct vent, Power Vent, and Power Direct Vent models may not be vented in combination with any other gas appliance.

Standard Gas Water Heater Venting

Gas water heaters transfer heat to the water by means of combustion of either natural or L.P. (liquid propane) gas. Typically, a burner in a combustion chamber ignites the gas through the use of a pilot light, with combustion air supplied through a safety feature in the bottom of the combustion chamber called the flame trap. The resulting heat is transferred to the water through direct contact with the bottom of the tank, as well as through the flue, which leads hot exhaust gasses out of the water heater and to the exterior of the home. Specially designed baffles may be included in the flue, which are designed to direct the flow of the exhaust gas so as to insure maximum exposure to the flue walls and therefore transfer the maximum amount of heat into the water.

Just like with electric water heaters, many options exist for gas water heaters, including the type of gas used, the amount of heat generated (referred to as Btu output), the altitude at which the unit may be operated, the type and sizing of the venting, and whether the unit has power venting or not (determines where in the residence the water heater may be located).

In order for a standard gas water heater to operate, the pilot must be lit.

The heat from the pilot flame creates an electrical current in the thermopile circuit that operates a small spring-loaded electromagnet in the gas valve. The electromagnet holds the main gas supply line interrupter open, allowing gas to flow to the pilot and main burner. If the pilot goes out, the electromagnet will no longer receive a current and the spring-loaded interrupter will shut off the gas supply.

If the pilot is lit, the gas valve's thermostat controls gas flow to the main burner. The thermostat's sensor is a small probe. It is mounted to the back-side of the gas valve and is inserted inside the water heater when the gas valve is installed. A knob on the front of the gas valve allows the user to adjust the temperature.

An energy cut off (ECO) switch is mounted inside the probe attached to the back of the gas valve and extends inside the water heater. The ECO functions as a high limit switch. If water temperature inside the tank reaches about 190°F, the ECO disables the water heater. The entire gas valve must be replaced if this happens.


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Spills of gasoline, or the presence of other flammable vapors, near a standard gas water heater can be very dangerous.

The American National Standards Institute (ANSI) now requires all residential water heaters to be flammable vapor ignition resistant (FVIR). All FLAME LOCK® water heaters are fully compliant with ANSI FVIR standards.

FLAME LOCK® water heaters have a sealed combustion chamber. Combustion air and flammable vapors can only enter through a specially designed flame trap. The flame trap is located in the bottom of the combustion chamber, under the main burner and pilot flame. If flammable vapors are present, the pilot or main burner immediately ignites them inside the combustion chamber.

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The vapors burn on top of the flame trap and are prevented from escaping back into the room. When the vapors become too rich or too lean to burn, they snuff themselves out.

If flammable vapors are ignited, they will trigger the thermal switch, cutting off the gas supply to the pilot and burner. This prevents the vapors from being reignited after they have self-extinguished. A water heater that has experienced a flammable vapor event, in which the thermal switch has cut off the gas supply, the water heater should be replaced. In such situations consumers should call the support number provided on their water heater jacket, or in their warranty documentation, for details.

Insufficient combustion air supply or blocked ventilation can also activate the thermal cutoff switch. When these conditions have been corrected, the thermal switch can be reset, and the heater returned to normal operation.