Water heater anode rods are used to keep the water in the tank from corroding the metal lining of the heater. The anode is covered in a layer of sacrificial metal, so it draws the corrosion towards it, preserving the inside of the tank. Water heater anodes all serve the same basic purpose, but there are a few models on the market, so you will need to know what kind of anode rod your water heater uses before you attempt to replace them, or before you start maintaining the tank.
1. Aluminum Anode
This is the traditional metal rod, often found in older tanks and hot water systems, and was the reason mothers would not allow their children to drink from the hot water tap. As knowledge about the poisonous effects of aluminum has become more widespread, the use of these rods in hot water tanks has lessened, although they are still used in areas with hard water. In order to find out whether the rods in your hot water system are made of aluminum, take one in your hand, and bend it. If it is easily bent, then the metal is aluminum.
2. Magnesium Anode
Another popular metal for making anode rods is magnesium. This is probably the most widely used metal in modern water heaters, although it is not very good in places where the water is hard. If you live in a hard water area, and have magnesium anodes, you will have to check that the lining of the hot water tank is free from corrosion. Installing magnesium anodes in a corroded tank can lead to an electro-chemical reaction that produces hydrogen gas, leading to water leaks.
3. Zinc Anode
Zinc rods are made of a combination of aluminum and zinc, 1 proportion of zinc to 10 proportions of aluminum. Zinc serves to reduce the smell of sulfur which can affect some water tanks, although modern water heaters are not typically supplied with zinc anode rods installed at the factory.
4. Impressed Current Rods
These are anodes which use electricity to attract corrosion. These rods do not have a sacrificial metal, and lack the self-created currents which the anode rods are typically capable of. Impressed current rods don't need to be removed and replaced, as they don't corrode, but they do need to be infrequently cleaned in order to maintain the electrical charge. These anode rods are usually found in large commercial water heaters, where the risk of corrosion is higher than usual.
5. Combination Anode
The combination anode is an ordinary anode which is attached to an unusual place. Most anodes are at the top of the heater, attached by a hex head screw. Some water heaters have an anode attached to the hot water outlet. This anode is installed inside the pipe, and can be difficult to reach, often involving the removal and breakage of the pipe, and the unscrewing of the hot water pipe nipple section. You can add a combination anode by lowering a rod into the pipe after removing the nipple attachment, and replacing it with a rod.