An anode rod is necessary because it prevents any corrosion of a water heater's metal lining. As long as a serviceable anode rod is installed, the metal of the rod will wear away instead of the water heater lining. The anode rod produces an electro-chemical reaction within the water, which protects the water heater.
Anode rods are attached to the top of the water heater with a ¾ inch hex head screw. Most rods are approximately 3 feet 8 inches in length: just a few inches shorter than the water heater itself. The core of an anode rod is a steel wire framed by one of three different metals: aluminum, magnesium or zinc. All anodes are made from one of these three metals, and each type of rod has a particular use.
- Aluminum. Aluminum rods are the best for hard water conditions, and most areas of the U.S. with hard water will have water heaters with aluminum rods. An easy way to check if your area has hard water is to check your anode rod. If the steel wire of the anode rod core is exposed, the rod is completely gone, or there is extensive passivation (see Anode Evaluation) this is indicative of hard water conditions.
A word of caution concerning aluminum rods. Today, many scientists believe that aluminum in the diet is linked to the development of Alzheimer's disease. Therefore, you should never drink or cook with water from a water heater tank which uses aluminum rods. If you're unsure whether your tank uses an aluminum anode rod, simply remove the rod (see anode removal and installation), and try to bend it with your hands. If it bends easily, it is probably made of aluminum.
- Magnesium. Magnesium rods are the most common type of anode rod. Magnesium works best in areas where the water is not hard. When replacing a magnesium rod, make sure the water heater lining is not corroded. If you install a new magnesium anode in a tank that is corroded, the subsequent electro-chemical reaction can cause a build up of hydrogen gas in that tank, which often leads to water leaks.
- Zinc. Zinc rods are simply aluminum rods with a portion of zinc mixed with the aluminum in a ratio of 1 to 10. The only reason for using an anode rod with zinc in it is to reduce any sulfur smell in the water. A new water heater almost never comes with a zinc rod already installed.
Now that you know what type of anode rod you have, you can inspect and evaluate your anode rod and install a new one, if necessary, by following these steps.
- Anode Check-ups. Your water heater can last for many years, even decades, if you check the anode rods regularly. Under normal water conditions, you should inspect the rods at least every four years. If you use softening agents such as rock salt or phosphates, the rate of corrosion can be up to three times faster than calcium carbonate—the actual cause of hard water. If you use other softening agents, you should inspect the anode rods at least every two years.
- Anode Evaluation. Often times the rod will be covered in calcium carbonate. Calcium carbonate is not corroded metal from the anode rod. Most times, you can remove it easily with a towel or simply brush it off with your hand. Once you’ve removed any calcium carbonate build-up, inspect the rod. If there is more sacrificial metal on the outside of the rod than exposed portions of the steel wire center, the rod is still good.
Anode rods corrode predictably, usually starting at the top or bottom of the rod. If the steel wire is exposed more than six inches from the top or bottom, the rod should be replaced. If the steel core wire in the middle of the rod is exposed, it should be replaced. Also, if the present diameter of the entire rod is less than half the original ¾ inch diameter (this is approximately ⅜ inch), it should be replaced.
Sometimes the calcium carbonate build-up on the rod can become hard. When this happens, it prevents the anode rod from doing its job and the lining of the tank will begin to corrode. This calcium carbonate build-up is called passivation. Unfortunately, if the anode rod is passivated, you will not be able to tell just by looking at it. Bend the rod by hand and check for flaking at the bend. If flaking does occur, replace the rod. Also, if the rod has split along its length or it is heavily pitted, you should replace it.
After the outer metal of the rod has worn away, the steel wire begins to corrode. After that the hex head (or the hot water outlet nipple in a combination anode) will begin to corrode. Then the tank lining will corrode. If you find the anode rod in any of the above conditions, the water heater may already be damaged, in which case it will need to be replaced.
- Access to Anode Rods. Easy access the anode rod is an important factor when looking for a new water heater. Sometimes, the location of the anode will be written on the water heater instructions. If the instructions for your water heater are not so considerate, however, you will usually find the anode rod attached to the top of the water heater tank by a hex head screw. Hex heads are threaded, watertight plugs approximately ¾ inches in diameter. The hex head will either be visible on top of the heater or hidden. If it is hidden, you should look for another water heater. If that is not an option, the hidden hex head will most likely be underneath a fiberglass, plastic or sheet metal top. The anode rod could also be connected to the hot water outlet (see Combination Anode). Commercial water heaters will almost always have a hidden hex head no matter the type of water heater.
- Hidden Hex Heads on Older Models. To access the anode rod with a hidden hex head, you will need to remove to the outer sheet metal top. Before removing the outer sheet, mark the placement of the top with a marker; this will make it easier to reassemble. Next, simply unscrew the screws holding the top in place. Once the top is removed, the hex head should be visible. Unfortunately, some models have foamed-in tops and cannot be removed. If this is the case, you will need to call a professional.
- Hidden Hex Head On Newer Models. Newer water heater models may have a fiberglass or plastic cover on the top. If this is the case you will be unable to remove the cover. PLEASE READ THE FOLLOWING CAREFULLY. IF YOU DO NOT FEEL COMFORTABLE DOING THIS YOURSELF, CALL A PROFESSIONAL.
In order to locate the hex head, you will need to drill a hole approximately ¼ of an inch through the plastic or fiberglass top. Do NOT drill into the water tank itself. Use a long flat-head screwdriver to probe underneath the top of the water heater to find the hex head.
On gas water heaters, the hex head will be the same distance from the flue as the hot and cold lines are from the flue. On electric water heaters, the anode will be off-center. You may need to drill more than one hole to locate the hex head. Once you have found the hex head, you should leave it permanently exposed. Use a hole-saw capable of cutting plastic or metal to carve a hole big enough to allow future access to the hex head. At this point you will need two people to unscrew the hex head--one to steady the tank, the other to use a breaker bar and a socket that fits the hex head. This could be anywhere from ¾ inch to 1-1/16 inches.
- Combination Anode. All water heaters have at least one anode rod usually attached to the top of the water heater with a ¾ inch hex head screw. Some residential water heaters may have two rods attached to the top with hex head screws. However, most water heaters that use two anode rods will have the second rod attached to the hot water outlet pipe nipple. This is known as a combination anode water heater. If your water heater has a longer warranty, it is most likely because it has a second combination anode rod.
If you are not sure you have a combination anode, you can check and find out. First, shut off your water! This should be the first thing you do whenever performing any kind of check or maintenance on your water heater. Next, use a pipe wrench to disconnect the hot water outlet at the top of the heater. Use a stiff wire to carefully explore the hole where the hot water nipple was. If the wire stops approximately 3 to 6 inches directly down, you have located the combination anode. If the wire meets no resistance, there is not an anode connected to the hot water outlet. You can remove the combination anode with a pipe wrench.
If you do not have a combination anode but wish to install one, then simply remove the hot water pipe nipple and replace it with a combination anode rod. Make sure that the nipple on the combination anode rod is thicker than the insulation on top of the water heater. This is usually between 2 and 6 inches.
- Anode Installation. To remove the old rod, pull it as far out as possible, bend it, then pull it the rest of the way out. Bend the new rod in the middle, insert it half way, straighten it against the opening, and install it the rest of the way. Screw in the anode rod at this time. If you are unable to screw it into place because it is too bent, pull it partially out and use the opening to straighten it further.
If the location of the water heater does not allow easy access to replace the rod, and you are unable to follow the instructions above, consider using a link-anode. A link-anode is an anode rod made up of many small links hooked together and looks similar to links of sausage. You can also try a zinc anode rod because the metal is more malleable than magnesium.
If none of these options work, you will have to drain the water heater, and then tip it over far enough to allow easy access for the anode.
- Impressed-Current Rod in Commercial Water Heaters. Some commercial water heaters use impressed-current rods rather than the sacrificial rods. Impressed-current rods require electricity because they do not create the self-generating currents that anode rods are capable of. Impressed-current rods do not need to be replaced; however, they should be cleaned periodically.
The location of the impressed-current rod will be listed in the instructions in plain view on the outside of the tank for most models. To clean them, simply wipe them off with a clean towel. If you discover rusting on the inside of water heater, call a plumber, the manufacturer, or install sacrificial anode rods.