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.
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 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
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