Steel water storage tanks usually have some type of lining to inhibit corrosion. A relatively new storage tank uses polybutylene, a high-temperature plastic, for a liner, but these tanks have fairly limited history and there’s little data available on any failure problems associated with them. Stainless tanks are typically more expensive than steel-lined tanks and have longer warranties and expected lifespans.
The most common solar storage tank is a modified electric water heater. Most are constructed of steel and lined with glass, which helps minimize corrosion and keep domestic hot water clear. Even though the tank is lined with glass, hairline fractures in the lining can lead to the tank’s eventual corrosion as the water reacts with the steel. The longevity of glass-lined tanks varies from 10 to 20 years depending on local water conditions. I have a storage tank in my basement installed in 1984 that is still holding water. This isn’t the norm, though; in most places, steel-lined tanks have a 10-year lifespan.
Slowing corrosion. Water with high levels of total dissolved solids, high acidity or alkalinity, high oxygen and dissolved carbon dioxide, high levels of salts or sulfates, or corrosion-related bacteria and electrochemical corrosion will more aggressively corrode a tank’s anode rod, which serves as a sacrificial metal, corroding faster than the steel in the tank. Anode rods made of magnesium or aluminum help protect water heaters and lined steel tanks against corrosion. It usually takes five to 10 years for an anode rod to sacrifice itself completely, but after it is gone, the steel tank will corrode next. Installing a new anode rod ($30) periodically can extend a steel tank’s life almost indefinitely.
Replacement anodes are available that hang under the tank’s hot water connection and also flexible ones are available that can be inserted into the tank in clearance-limited spaces. Make sure you buy the correct rod type for your local water conditions. Magnesium anode rods are used in most of the United States, but aluminum rods may installed where magnesium is a problem. A symptom of using a magnesium anode in areas where the water reacts negatively with this metal can be a sulfurous rotten egg smell and taste in the hot water.
Premium-priced tanks made with type 316 stainless steel have few problems with corrosion. They don’t need anode rods for corrosion protection and are compatible with copper and steel piping and components. Tanks made with the more common 304 stainless steel can encounter problems.
Near the end of their lives, all steel tanks start leaking at welds in the ports or seams. The leaks are small to start and are first noticeable as a wet spot on the floor that never dries. These leaks worsen over time, and the only solution is replacement.
Broken dip tube. The fine print on many water heater and storage tank warranties has a disclaimer for using more than 180°F water in the tank, since most tanks have some plastic components inside that may be affected by higher water temperatures. Newer water heaters have plastic “flappers” in the inlet and outlet pipe nipples that inhibit heat loss through thermosyphoning. Another common plastic part is the dip tube that carries incoming cold water to the tank bottom from the cold pipe connection on the tank top, where the thermostat and heating element or gas burner are located. This dip tube, when aged and subjected to high water temperatures, can become brittle and break off.
When a dip tube breaks, cold water can migrate directly to the hot water output since the two connections are only a few inches apart. The symptom alerting you to a broken tube is water delivered at the tap that is hot for only a few seconds and then turns increasingly tepid, or even cold. Dip tubes are most likely to break in backup water heaters that have been subjected to years of very hot water entering the dip tube from the preheat storage tank. Removing the cold water nipple on the tank’s top and pulling up the dip tube (or what’s left of it) with your little finger will allow inspection. A broken tube must be replaced (about $10).