Glass-lined water heaters are supplied with sacrificial magnesium anode rods that protect the interior of the tank through cathodic action. As the rod works, it slowly dissolves over time and must be replaced when it is totally consumed.
ANODE ROD INSPECTION INSTRUCTIONS:
Locate the anode rod on top of the water heater. It is accessible through the hole closest to the center of the tank. There will be insulation covering it and there may be a plastic cap over the access hole. The anode rod has a 1 1/16th inch hex head.
Turn off the power if electric. If gas, turn valve from “on” to “pilot” on the water heater.
Turn off the cold/inlet water supply to the heater.
Drain off approximately 5 gallons of water from the unit.
Using a 1 1/16th socket wrench, remove the anode rod. (It could be well tightened from the factory.)
Inspect anode rod for consistent pitting. If pitting is consistent and thicker than 3/8” in diameter, reinstall rod. If rod is less than 3/8” in diameter, or bare inner core is exposed, other than at each end, replace anode rod.
Turn water supply back on and ensure the tank is completely filled up with water by opening all the hot water faucets in the house to release the air in plumbing system. Allow the water to run for three minutes before closing the faucets.
Turn the power back on, or turn the dial on the gas valve from “pilot” back to “on” depending upon your unit.