Anode Evaluation
If the anode rod has more sacrificial metal than exposed steel rod, then it is still in good shape. However, if the entire surface becomes covered in calcium carbonate and this calcium carbonate becomes hard, this will prevent the anode from protecting the tank any longer. This is known as passivation. If the anode has passivated, it will not look so by sight alone. To test for passivation, you must bend the anode rod by hand. At the bend, observe for small amounts of flaking. The anode should be replaced if more areas of the rod are exposed wire than sacrificial metal. It should also be replaced if the top or bottom of the rod has deteriorated, exposing six or more inches of exposed wire. An anode should also be replaced if the anode is less than half of the rod's 3/4 in. diameter size. If the anode has passivated, split through its length, or has become heavily pitted, it could also be time for replacement. When all the sacrificial metal has worn away, then the steel rod will begin to wear away. After the steel rod wears away, the only thing left will be the hex head or the hot water outlet nipple if it is a combination anode. At this point, the tank will begin to corrode. If the anode is found in the above stated conditions, damage to the tank may have already occurred.
Hidden Hex Head On Newer Models
Hex heads are threaded watertight plugs about 3/4 inches in diameter. They are attached to anode rods at the top of water heaters. Some are easily seen from the top of the water heater. Other times it will be under fiberglass or under a piece of plastic. To locate the hex head, drill a shallow 1/4 inch hole through the plastic top of the water heater. Do not drill deep into the tank itself. Use a long flat-head screwdriver to probe underneath the top of the water heater to find the hex head. On gas heaters, the hex head will be the same distance from the flue as the hot and cold lines are. On electrics, the anode will be off center so as not to drop on the heating elements. A few holes may need to be dug in order to locate the hex head. Once the hex head is found, it should be 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. Use two people at this point to unscrew the hex head'one to steady the tank, the other to use a breaker bar and a socket that fits the head. Anywhere from 3/4 inch to 1-1/16 inch.
In the future, when buying a new water heater, purchase only those with already exposed hex heads.
Hidden Hex Head On Older Models
To find the hex head on older water heaters, simply unscrew the screws holding the top in place, mark the placement of the top and the water heater with a marker, then remove the top to find the hex head. Alas, many heaters found in today's buildings have foamed-in tops and can't be removed. Again, if the hex head is not exposed at the time of purchase, don't purchase that particular tank. Look for a tank with an already exposed hex head.
Anode Inspection
Anodes should be inspected at least every two years where softened water is used but at least every four years under normal water conditions. On occasion, the location of the anode is actually written on the water heater instructions.
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. To install the new one, bend the rod directly 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 there is not sufficent ceiling room to install the new anode rod, consider a link-anode. These anodes have many small links hooked together and look similar to links of sausages. You can also try zinc anodes because they bend far easier than magnesium ones. Another way to install an anode is to drain the water heater and tip it over enough to allow easy access for the anode.
Length
Anodes are typically 3 feet 8 inches. Anodes should be only a few inches shorter than the tank itself. Buy anodes that are a little too long instead of a little too short. This way you can cut the anode shorter if it is too tall.
Impressed-Current Rod
In relation to commercial water heaters, there are impressed current rods. These rods do not self-generate currents like sacrificial anodes. They derive power from an electrical power source. Many commercial heaters give the location of the impressed-current rod. They do not need to be replaced throughout their lifetime. They may need periodic cleaning. Simply wipe them off with a towel. If rust appears inside a water heater with an impressed-current rod, you should either call the manufacturer, call a plumber, or install sacrificial anodes.