DEAR TIM: I have just replaced my hot water heater. It was installed just 4 years ago. This is the second time this has happened. Why is the tank corroding so quickly? Also, the plumber installed a funny looking small tank on the cold water line this time. Will this help extend the life of my new hot water heater? T. Q.

DEAR T. Q.: You are not alone in your misery. For a variety of reasons, many homeowners suffer from reduced water heater lives. There are some steps you can take to possibly extend the life of your hot water heater.

Hot water heater tanks are made from steel. We all know that when exposed to moisture and oxygen, steel will rapidly corrode. Water heater manufacturers, during the 1950's, began to apply a thin glass coating on the inside of water heaters. This coating isolates the steel from the water and dissolved oxygen within the tank. However, certain parts of the tank (inlet / outlet nipples, tank fittings, etc.) can not be easily coated with glass. These can and will corrode.

Water acidity, electrical conductivity, dissolved oxygen, and temperature contribute to water heater corrosion. The first three items are present in varying quantities depending upon the source of your water supply. The water temperature within the tank can also vary, depending upon how high you set the thermostat. As water temperature rises, so does the rate of corrosion.

All water supply systems have dissolved minerals within the water. These dissolved minerals enable the water to conduct electricity. Those people who have well water, or municipalities that draw water from deep wells can have high amounts of dissolved minerals within the water. As the mineral content of water rises, so does its ability to conduct electricity. This flow of electricity within the tank begins to attack the parts which do not have a glass lining.

To offset this corrosion process, manufacturers install a magnesium anode rod within each tank. This rod serves as a miniature lightning rod. The electrical current present in the water is attracted to this rod. The magnesium corrodes easier than the other parts. If your water conditions cause high electrical conductivity, this anode rod can be corroded in a short period of time. Once it is gone, the electricity begins to corrode the other tank parts. Fortunately, anode rods can be replaced. Have a qualified service technician check yours every year.

The little tank that the plumber installed will extend the life of your water heater, but for an entirely different reason. Water expands when it is heated. This expanded water used to simply push the water in your cold water line back into the city water main. However, many public water systems are requiring backflow prevention devices to be installed on residential water systems. These devices prevent water from your house being drawn back into the public water supply. Broken water mains, fire trucks, etc. can actually siphon water from your water pipes.

These backflow devices block the expanding water. The little tank is an expansion tank which acts as a temporary storage site for the expanded water. Without this tank, your hot water heater may fail because of the high internal pressures created by the expanding water. Some water heaters burn natural gas or propane. The internal flues which exhaust the combustion gases can collapse because of high internal pressures. A collapsed flue could cause carbon monoxide to concentrate within your house. Expansion tanks are good ideas. Consider installing one with your next hot water heater.