Each commercial manufacturer of water heaters puts in their own amount of anodes inside a water heater. The amount is usually based on warranty duration, hence the more anodes in a water heater, the longer the warranty the manufacturer will provide. Most manufacturers intentionally provide shorter warranties unless specifically asked for a longer one. Also, most consumers don't know enough about water heaters (who would spend a lot of time learning about them unless you're in the business) to ask about warranties and anodes so they don't ask in the first place. Also, the fewer anodes installed, the more likely the water heater will malfunction and the more likely you will buy a new one.
Money is the reason to install a commercial water heater that is a tank-type heater. It costs less for the tank-type water heater, but installing multiple tankless water heaters, especially in establishments such as hotels and restaurants, could be extremely expensive. Going to a tankless water heater system initially costs more money. Households are slightly more likely to go to a tankless water heater system, but again the same problem occurs: money. Nonetheless in the next thirty to fifty years, virtually all water heaters will be tankless water heaters in the United States. It should be noted that most water heaters in Europe and Japan are already tankless water heaters. The tankless water heater industry has great momentum for new customers in the United States.
The average anode rod lasts ten years. Some places in the United States, anode rods won't last much longer than two to four years! Some places last twenty to thirty years or longer. This is all due to water quality in the area. If the water is too hard then people put salt in the water to soften it. This salt is more dangerous to an anode rod than the hard minerals it replaces. This will cause the anode rod to fail. If the water is too soft, which means that too much salt naturally appears in the water, then the anode rod will have the same problems. It's not a situation that you have any control over. Water has to be softened and anode rods need to be replaced. It's the only way.
The water heater industry can make commercial water heaters with up to 4 anode rods. These 4 rod water heaters are uncommon but they should come with an outstanding warranty. The more anode rods that appear inside a water heater tank, the more lengthy the warranty.
A plumber should replace anode rods since they are extremely difficult to change out and most times the water heater is stuffed in a utility closet with no overhead clearance from which to gain enough room to pull the water heater out and therefore the water heater must be partially disconnected to the plumbing and partially drained and tipped forward just to be able to remove it. What a pain filled experience it can be. Its best to hire a plumber to do it.
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.
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