Learn what a water heater anode rod is, how it works, and how to choose the right replacement for your water heater.

Understanding Your Water Heater Anode Rod

A lot of people take their water heater for granted, assuming they can install it and simply forget about it. For the most part, this is true. Tank style water heaters are fairly simple devices and do fine with a minimum of maintenance...so long as your anode rod is doing fine.

What Is an Anode Rod & How Does It Work?

What is an anode rod? The dictionary answer is "a sacrificial rod used mainly in water heaters. It helps protect the lining of the water heater and generally lengthens its life." But what does that really mean?

Plumbing involves metals and water. When these combine, you get this thing called galvanic corrosion. Galvanic corrosion is defined as "an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte." So your piping, which is one kind of metal, and your tank - which is another kind of metal - and the water together set the stage for some fantastic galvanic corrosion. This is not good.

To prevent the tank from rusting or your heater element from corroding, the brilliant plumbers and chemists of long ago created a sacrificial rod for the water heater tank. The idea was that the anode rod would corrode first, leaving the metal of the tank (and element if you have an electric water heater) alone - saving you from dealing with a rusty behemoth that randomly springs leaks. This works because the anode rod possesses a lower, more negative, electrochemical potential than that of the water heater's steel composition. The negatively charged electrons create a higher voltage to flow from the anode rod to the steel tank causing the anode rod to corrode instead of the steel water heater tank, or other exposed metals such as electric elements. The anode rod is "self-sacrificing" and will continue to corrode until eventually it must be replaced. When there's no sacrifical metal left on the anode, the tank can rust out - which is why it is so important to change yours regularly.

Pro Tip: When replacing your anode rod it's a good idea to also flush your water heater out to clean out any sediment, rust, or gel build-up accumulated at the bottom of the tank. You might also consider flushing your water heater out at least once a year as a normal maintenance procedure.

How Do I Check My Anode Rod?

Find out where the rod is located on the heater, and how to remove it - all of which should be included in your unit's documentation. If the manual is missing, it shouldn't be too difficult: most anode rods are labeled on top of the unit, and locked in place using a hex nut. Once loosened (with a crescent wrench, channellocks, a socket wrench, etc.) the rod should pull straight out.

If your water heater doesn't have a separate hole for your anode rod, it is most likely attached to the hot water outlet. Unscrew the flex supply to the hot water outlet, which should enable you to unscrew the anode rod and pull it out to check.

How Do I Know When To Change Mine?

Most anode rods that come pre-installed in water heaters are formed aluminum or magnesium around a stainless steel cable. When you check your anode rod, you'll probably see some pitting, or tiny holes; that's exactly what should be happening. However, to keep protecting your tank the anode rod must be replaced when a good chunk of the cable becomes visible. Waiting a long time is not a good idea: having a depleted anode rod will shorten the life of your water heater.

Another problem with waiting too long? The possibility of the old water heater anode rod breaking off and falling to the bottom of the water heater. Doesn't sound so bad, does it? Unfortunately, the problem then becomes the issue of the loose anode rod bouncing around inside the water heater. This is bad: it will cause cracks in the heater's glass lining, allowing the underlying metal to rust and drastically shortening the life of the unit.

Anode rods have a life expectancy of about five years, but as always it really depends on the quality of your water and how much of it travels through the heater. When sodium is added to the water (like when a water softener is used), anode rods can corrode more quickly: in as little as six months if the water is over-softened! Take care not to over-soften water, and make sure to check your anode rod more often if you have a water softener (at least every six months).

Change your anode rod right away if any of the following apply to your situation:

  • The water heater makes loud or multiple popping noises when heating up.
  • The water heater is more than 5 years old and you've never changed it before.
  • You notice a slimy gel substance when cleaning out faucet aerators.
    • As aluminum anode rods corrode they can produce an aluminum oxide deposit normally found at the bottom of water heaters, but occasionally make it into the main water supply and into faucet aerators. The aluminum oxide forms a virtually odorless "gel" substance that can vary in texture from a firm somewhat sticky curd to a thick running slurry. The gel can be either milky or clear and is often mixed with other water heater deposits such as scale, rust, or tiny sediment particles.
  • Your hot water starts smelling like "rotten egg" odor.
    • Unpleasant taste or odor coming from your hot water outlets can be caused by natural occurring iron bacteria in the water. This condition is usually more common in private or municipal well systems. According to the Minnesota Department of Health (Iron Bacteria in Well Water) iron bacteria are not known to cause disease, but can cause unpleasant odor, stains and tastes in the water. Iron bacteria does not produce hydrogen sulfide, the "rotten egg" smell, but can cultivate an environment where sulfur bacteria can grow and produce hydrogen sulfide (the rotten egg smell). Certain water conditions will cause a reaction with magnesium or aluminum anode rods to produce a "rotten egg" smell. By replacing the magnesium or aluminum anode rod with a combination aluminum/zinc/tin anode rod you may help decrease this smell, but it may not eliminate it completely.
  • Your existing anode rod looks similar to this
Example of corroded anode rod

How Do I Choose the Right Replacement Anode Rod?

There are several options for replacing your anode rod, with magnesium, aluminum, or a combination of aluminum, zinc and tin being the most common anode rod materials. (Most water heaters come with an aluminum rod as standard equipment.) What rod you choose will depend on three things: your water quality, the location of the anode rod, and how easy it is to access the installation point.

Unless your water is exceptionally iron-laden, you should be fine with an aluminum or magnesium rod. Aluminum is the longest-lasting and least expensive option. Magnesium will corrode slightly faster than aluminum, but there are health benefits to drinking water with dissolved magnesium. If you do have water with a lot of iron bacteria that causes a "rotten egg" smell, we recommend choosing an aluminum/zinc/tin rod as it can help to control the growth of the iron bacteria.

As mentioned above, you might have a water heater with a separate inlet for the anode rod or your anode rod might be installed on the hot water outlet. Since most water heaters use a standard 3/4" NPT connection, you simply need to choose the type of installation you have. If you have a separate inlet for the anode rod, but can't get the old one out or the connections are too corroded to install the new one, you can always install the new on the hot water outlet.

Pro Tip: Even if you install a new anode rod on the separate inlet, you can add another anode rod to your hot water outlet as a backup for even more protection.

If you have low ceiling clearance or difficult access points for installing your new rod, there are flexible rods available. Simply bend them along the designated sections to slide them into the tank, straightening them as you get them into the tank.

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Now that we've explained how marvelous anode rods are, why not get out there and check on yours? And remember: take care of your anode rod, and your anode rod will take care of your water heater (and you)!

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