Water Heater Components
Storage water
heaters operate on the principle of heating water via either
electric elements, or through combustion of natural or liquid
propane (L.P.) gas, and the resulting transfer of generated heat
into the water. The storage tank is designed with special features
to ensure reliable, safe operation and long tank life, as well as
to efficiently collect and retain the heat transferred to the
water.
Primary features of the tank are the anode rod and the ceramic
lining, designed to resist the corrosive effects of water
(especially heated water) upon the steel tank. All tanks include
safety measures to avoid pressure buildup beyond the tank
limitations, in the form of a Temperature & Pressure Relief
Valve, often referred to as the T&P Valve. The storage tank is
well insulated to minimize heat loss while the water waits to be
used by consumers. And finally, every water heater includes
features for bringing cold water into the unit, sending hot water
out of the unit, and draining the unit in case of service
procedures. The features common to both electric and gas water
heaters are explained in the following section.
Outer
Jacket - The outer jacket of a water heater consists
of three pieces: jacket top, jacket skirt, and jacket bottom. The
jacket provides a space for insulation around the tank, preventing
the consumer from coming into contact with hot surfaces and to
reduce energy loss. Connections extend from the tank through the
outer jacket to provide plumbing connections for the cold-water
inlet and the hot water outlet. The jacket base assembly is where
combustion takes place in a gas water heater, and it is designed to
shield the floor from excessive temperatures.
Ceramic Lined Tank - A water heater tank
is constructed of three pieces of steel: a top head, a bottom head,
and a shell. These three pieces are welded together to form a tank
that will withstand a working pressure of 150 pounds per square
inch. The interior of the tank is coated with a ceramic material,
baked on at about 1600°F, which will not break or crack, unless it
receives a blow severe enough to dent the steel tank. This coating
helps protect the tank from the corrosive effects of hot water. In
addition to the top head, bottom head, and shell, the gas water
heater also has a flue running through the center. As combustion
occurs below the bottom head, heat rises into the flue. The flue
contains a baffle, which is designed to increase the transfer of
heat along the flue pipe and decrease the temperature of gasses as
they exit. Both the tank and the flue are ceramic coated to prevent
corrosion. On FLAME LOCK® water heaters the flue is ceramic coated
on both the inside and outside. 
Insulation - Water heaters have a layer of
insulation between the tank and the outer jacket. Three materials
are used as insulation in water heaters: non-CFC polyurethane foam,
fiberglass, and polyester. Non-CFC polyurethane foam is the more
efficient of these materials and is injected through a port in the
outer jacket in liquid form. This liquid expands quickly, filling
in the space between the tank and jacket, and then dries. Polyester
or fiberglass is used around all openings and controls to ensure
that the non-CFC polyurethane foam does not cover these devices. A
fiberglass blanket is used around the combustion chamber of gas
models due to the high temperatures radiated through the combustion
chamber walls during burner operation. 
Dip Tube - The dip tube moves incoming
cold water to the bottom of the tank, preventing it from mixing
with the stored hot water at the top of the tank. Dip tubes are
designed to withstand high water temperatures and are made of
materials such as polypropylene, polysulfone, and cross-linked
polyethylene. Some dip tubes include integral heat traps that
prevent heat from transferring back into the plumbing lines, while
others are designed to reduce sediment buildup in the tank. In gas
water heaters, incoming water sometimes causes condensation of flue
gasses to occur, which can cause a sizzling sound when water
droplets fall onto the burner below. This occurrence is not
detrimental to the operation of the water heater.
Heat Traps - In some models, heat traps
are installed at the hot water outlet and cold-water inlet to
reduce standby heat loss. The heat trap saves energy by preventing
hot water in the tank from rising into the plumbing lines. Heat
traps can be installed internally or externally. 
Anode Rod - The anode rod is a sacrificial
part, factory installed in the tank to reduce tank corrosion. A
tank full of hot water acts very much like a battery, with small
electrical currents flowing between the different types of metals.
The anode rod is made of aluminum, magnesium or other alloys, which
is more conductive than the steel of the tank. The electrical
currents corrode the anode rod rather than the tank. 
T&P Valve - The T&P Valve, or
Temperature and Pressure Relief Valve, is a safety device designed
to relieve excessive pressure on the tank, preventing it from
bursting. This valve activates by discharging water if the
temperature inside the tank reaches 210°F. The T&P valve must
never be altered, restricted, or blocked, as this creates a
hazardous situation where the tank could build up pressure and
possibly burst. 
Drain Valve - A child-resistant drain
valve is located near the bottom of the water heater. It provides a
way to drain the water from the heater for maintenance
purposes.