How water heaters are made

The inside of a water heater tank is coated in glass, and a sacrificial metal rod guards the spots the glass misses.

stations: 6
things upstream: 71
raw sources: 24
steps from the ground: 10

The short version

A flat steel sheet is rolled into a cylinder, its seam is welded, and top and bottom heads are welded on. Powdered glass is sprayed inside and fired so it fuses to the steel. Heating parts and a sacrificial anode rod are fitted, foam insulation is poured around the tank, and a steel jacket closes it.

Roll the shell

A flat steel sheet is rolled into a cylinder, and automated welders close its vertical seam in one pass.

Goes in
Sheet steel
Weld on the heads

Top and bottom heads are welded to the cylinder, and openings are made for the water inlet, outlet and fittings. The tank is pressure tested well above household pressure.
Line it with glass

Powdered glass called frit is sprayed inside the tank, then fired at high temperature so it bonds to itself and the steel. This glass is the barrier that keeps water from rusting the steel.

Goes in
Glass
Fit the heating parts

Electric models get heating elements screwed into the tank wall, plus a thermostat. A sacrificial anode rod hangs inside, corroding in place of the tank wherever the glass left small gaps.

Goes in
Magnesium
Pour in foam insulation

Liquid chemicals that expand into foam are poured into the space between the tank and a larger shell. They rise and harden around the tank.

Goes in
Polyurethane
Close the jacket

A steel jacket with a painted finish closes the unit.

Goes in
Sheet steel Paint

End of the lineWater heater

Worth repeating

Almost all water heaters have been made the same basic way for the past 80 years.
The glass lining goes on the same way as the enamel on ornate dinnerware: colored powder fired until it fuses.
Compact heaters can skip the side seam, using a one-piece body deep-drawn from a steel plate.