Your home can have different types of water heater anode rods. When replacing your anode rod, the first decision will be what type of metal to purchase.
Not sure if this is your problem? See the articles 'Gas Water Heater Troubleshooting' and 'Fixing Electric Hot Water Heaters' for a complete listing of all the water heater topics.
Reason for Different Types
The anode rod keeps the tank from rusting. The metal that the anode rod is made of will corrode before the metal in your hot water tank. That is why they are sometimes called 'sacrificial' anode rods.
Rust is the main reason for a hot water tank failure. The anode rod will extend the life of the hot water tank. A normal anode rod will last abour five years. That is why there are a lot of water heaters out there with a six year warranty.
Water heaters with longer warranties will either use multiple (usually two different types) or a better grade of metal to achieve the warranty. The longer warranty is tied to how long the anode rods will take to corrode.
The other reason for different types is odor. Depending on the mineral content of the water in your home, bacterial can grow inside of the hot water tank. This bacteria can cause a foul odor. Certain metals for anode rods will reduce or eliminate the odor issue.
So, there are different types based on how they are installed and the type of metal that they are made of.
Types of Metal For Anode Rods
Aluminum Anode Rods
Most water heater come with aluminum anode rods from the factory. They are the least expensive and provide the lowest level of protection. An aluminum anode rod will last about five years.
In some cases, aluminum anode rods can cause odors, but this is more of a problem with magnesium. Aluminum rods can get a build up of corrosive materials on them. This will make them difficult to remove, since the diameter of the rod increases.
The main complaint about aluminum is having the aluminum introduced into your water supply. Aluminum has been linked to several health problems including Alzheimer disease.
Magnesium Anode Rods
It is thought that magnesium is a superior metal for anode rods. Magnesium produces a stronger current than aluminum and provides better insurance against rust.
Magnesium that is dissolved in water is thought to have health benefits in contrast to aluminum that may have health concerns. Magnesium corrodes at a faster rate than aluminum so they do not last as long.
Where water odor is a concern, magnesium should not be used, since it reacts with bacteria and can cause odor. Magnesium anode rods cost more than aluminum and are used less often.
Combination Anode Rods
Where foul odors in the water are present an anode rod made of aluminum/zinc/tin is recommended. These anode reduce the generation of bacteria, since zinc is a natural inhibitor.
This type still uses aluminum, so there is that health question. They are also the most expensive anode rods you can purchase.
Replacing Anode Rods
See the article 'Replacing an Anode Rod' for instructions on replacing yours. Additionally, you may want to review the article 'Purchasing an Anode Rod' before you buy one.