There are multiple anode rod installation types. The reason for the variation is the type of anode rod installed at the factory, the clearance you have to install a replacement rod and the type and conditioning of the water in your home.
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Anode Rod Installation Types
There are two types of anodes. One is a hex head type. If you look at the top of your tank you should be able to find one or two hex nuts that are exposed or under plastic covers. This type it the most common.
The other type is a combination anode that is in the hot water outlet nipple. Usually when a hot water tank has two anodes you will find one of each.
Hex head anode rods have a nut on them the threads directly into the top of the hot water tank. The tank has a threaded opening built in during the manufacturing process. It is possible to have two or more anode rods installed in your tank.
This style is the easiest to change, in terms of steps. However, the type of anode rod and the age of the tank, may make freeing up the connection difficult.
Hot Water Outlet Anode Rods
This option is sometimes used when a second anode rod is installed in the tank. It can also occur when the original anode rod cannot be removed and this style is used as a replacement.
As the name implies the anode rod is installed at the hot water outlet for the tank. It is hollow inside to allow water to pass through it. It is slightly harder to change because the pipe between the tank and the union needs to be removed to install it.
This style comes in all of the metal types and is available in a flexible format. This style costs more, nearly double in some instances, than the equivalent rod in a hex head format.
Flexible Anode Rods
Flexible anode rods are only needed for replacement purposes. The reason they are needed is clearance. A standard anode rod for a full height tank is around 40". That means you need 40" clear above the top of the tank to replace it.
You can cut the old one to get it out if you don't have the clearance. Cutting the new one, however, is not really an option. Flexible anode rods have two or three spots on them where they can bend.
Instead of forty inches clear, you may only need as little as 11". A flexible anode rod is often the only choice when the ceiling is less than full height above the water heater.
Flexible anode rods cost more than straight ones. Magnesium is the poplar choice for flexible anode rods, since it is a softer metal. They are available in aluminum and can even be found at home supply stores in theis format.
Aluminum/Zic/Tin anode rods that are flexible are not that common. If you have an odor problem, finding a suitable flexible rod may be a challenge.
Electric Anode Rods
This is the least common and most expensive option. Electric anode rods do not rely on the type of metal to work. Instead they introduce an electrical charge into the tank that stops the corrosion.
The unit is considerably more than a standard anode rod and would only be found on high end units or when it was installed as a replacement. The other drawback would be that an electrical connection is required.
Determining the Type
An hex nut on the top of your tank with no pipe attached to it is a give away for a hex type anode rod. It would still be possible that you also have a hot water outlet rod as well.
When there is no hex nut present, then the likely result would be a hot water outlet anode rod.
Do you have your owners manual? It should have the anode information in it. Can't find it? Try the manufacturer's website or go to Manuals Online to try and find it. You will need your model number.
Next, you are going to want to actually purchase a new anode rod. See the article 'Purchasing an Anode Rod' for more information. After that, you will need to install it. See the article 'Installing an Anode Rod'.