Hot water heater anodes: what keeps water heaters from rusting out

What you'll find on this page: Most people have never heard of sacrificial anodes, even though they have been a key to water heater longevity for decades. This may be the single most important page on this site. It won't take you too long, so read these words even if you skip everything else!

A new anode, top, as compared to one that had been in use for seven years. Note the bare core wire.

The Hidden Ingredient

The single most important factor in whether a water heater lives or dies is the condition of its sacrificial anode. For more than 60 years, it has been used as a key part of the rust protection of a tank, although few people know it's there.

This is a rod made of magnesium or aluminum that's formed around a steel core wire and is screwed into the top of the tank. A six-year-warranty residential tank will have one, while a 12-year-warranty tank will have two, or an extra-large primary anode. Commercial tanks have from one to five. Special aluminum/zinc sacrificial anodes or powered anodes can be used to resolve odor problems caused by bacteria in some water. But if you have a vacation home where the water heater sits idle for long periods of time, using them may not be a solution. Click here if that is an issue.

When the tank is filled with water, an electrolytic process begins whereby sacrificial anodes are consumed to protect a small amount of exposed steel. Powered anodes replace that process with electricity and are not consumed.

Electrolysis simply means that when two metals are physically connected in water, one will corrode away to protect the other. Although few people have heard of this, the principle is used all over the place -- anywhere that someone wants to protect metal exposed to water. In marine applications, anodes are known as "zincs" and are usually made of that metal.

All metals fall somewhere on the galvanic scale of reactivity. When two are placed together in water, the "nobler" -- or less reactive -- one will remain intact while the more reactive one corrodes. When steel and copper are together, steel will be the one that corrodes. Indeed, steel is more likely to rust in the presence of copper than it would have been by itself. That's why dielectric separation is necessary on items like copper flex lines when they're connected to steel nipples.

Magnesium and aluminum are less noble than steel, which is why they're used for anode rods.

Promo for hex-head magnesium anode sales

Price: $41

Remember, the anode is screwed into the tank. That means it can be unscrewed and replaced.

A sacrificial anode's life depends on the quality of the water, the amount of use the tank gets, the water temperature, and the quality of the tank -- meaning how well it was constructed. When salt is added to the water (as in softened water), anodes corrode more quickly. Water softeners help reduce sediment, but anodes can corrode in as little as six months if the water is over-softened. Do not soften to zero. Leave 50-120 ppm of hardness.

People occasionally ask us if pipe-seal tape applied to the threads of the anode blocks the electrolytical reaction. Tanks we've serviced repeatedly usually have corroded anodes. We've tested with a multimeter and found current flowing between the anode and the tank, despite the tape.

While we generally advocate putting two anodes in a tank, that may not be a good idea if you have odor problems. Doubling the anode surface area may worsen odor even when special aluminum/zinc anodes are used that reduce or eliminate the odor.

If you have odor and soften, or for that matter, merely if you soften, consider getting a powered anode that replaces the sacrificial reaction with electric current and isn't consumed through use.

If you contemplate adding an anode to a new tank, make sure both rods are of the same metal. Otherwise, the magnesium rod will be consumed more rapidly in the presence of an aluminum one and you won't get as long a life. How do you tell them apart? In nearly all cases, an aluminum hex head will be flat on top, while a magnesium rod will have a bump, as in the photo at right. The size of the bump may vary, but bump, magnesium; no bump, probably aluminum.

The exception to that is Rheem and its sub-brands, Ruud/Richmond/General Electric. As far as we know, Rheem always uses magnesium even though there is no bump on top of its anodes. On the other hand, if the tank is five years old or more, having dissimilar metals is less of an issue, since the original one will likely be largely consumed.

Comparison of hex heads of aluminum, magnesium anode rods

And there is no way to tell metal type if a tank has a combo rod, but you're probably going to be replacing it anyway.

As to other brands, as far as we know, American/Whirlpool always uses aluminum anodes. Bradford White always uses magnesium, but many of their tanks have one combo rod in the hot port.

State/Reliance/Kenmore offers both combo rod tanks and tanks with a hex anode. As of March 2011, 9-year-warranty Reliances and Kenmores have magnesium, as does the Premier line. All the others use aluminum anodes. A.O. Smith makes both hex and combo rod tanks and uses both aluminum and magnesium, but only its high-end Conservationist model has magnesium. In both the Premier and Conservationist, there will probably be two anodes in the tank, so adding a third-party anode isn't an option except as a replacement. The 9-year-warranty Kenmore is sold through Sears and Orchard Supply Hardware. The Reliance brand is sometimes sold through Ace Hardware, True Value, True-Serve and Do-It-Best. One drawback is that they come with only one inch of insulation instead of the two we'd like to see.

There is one important exception to all this: all Smith, State and sub-brand ultra low-NOX heaters come with aluminum anodes.

If you decide to remove and check your anode, we'll tell you some of the possibilities and what they mean.

If there is rough, seemingly chewed-up metal all up and down the rod, that's normal. It's doing what it's supposed to do. If you can see six inches of the steel core wire, replace the rod. If all you have IS the steel core wire -- or less -- then extending the life of the tank by replacing the anode becomes more iffy. You might still get several more years out of the next anode. Or the tank might fail shortly after. It all depends on factors that exist where none of us can see them.

If the rod looks perfectly intact, with no sign of corrosion, then it has passivated. That means it has sort of gone to sleep. It might not be doing its job. It's a wild-card situation. We've seen tanks with passivated rods last a long time. We've seen them break. We're never sure what will happen next, and apparently none of the experts in this field whom you think might know are any more certain than we are. About all you can do is replace the rod and hope for the best. Or leave it and hope for the best. Wild-card situation.

A typical water heater top. Many have the hex-head anode exposed, as here. Others have it under a plastic cap, but in about the same position. A few have it under the sheetmetal. And on some, a combo anode is in the hot port, as in the position of the pink-topped nipple.

Combo and Hex-Head Anodes

There are two configurations of anodes. The first kind is called a hex-head anode and you can see a couple at the top of this page. They are found in their own port on top of the tank. With most brands, the hex head is exposed. On some, it may be under a plastic cap about halfway in toward the center from the edge. If there are caps on the edge, they were used to insert the foam insulation.

The other kind is called a combo or outlet anode, like the one in the picture at right. It is an anode/hot-water outlet/plastic-lined steel nipple and is used in the hot-water port. Often longer-warranty tanks have one hex-head and one combo rod, although a couple of manufacturers make tanks that have just one combo rod, with no place for a second one.

If you're adding a combo rod to a Rheem/Ruud/Richmond/GE tank made in 2005 or later, you'll need a special half-length one that is only available from a plumber affiliated with Rheem. It may be pricey. We were able to stock full-length ones for awhile, but our source dried up. Do not try to install a standard combo anode, or you may not have water pressure, due to a redesign of the tank's hot-water port in 2005.

The test, on older heaters, of whether you have a combo anode or not is to disconnect the hot plumbing and run a long screwdriver into the nipple. It should be blocked an inch or two down if there is an anode there. Newer heaters often have heat traps in the nipple that prevent that test. You may have to remove the nipple to determine if anything is beneath it.

Anode rods of all types can be purchased on our Product Page.

A Different Kind of Anode -- Powered

Water heaters always come with sacrificial anodes, and that is the most common type. But not the only type. A powered anode can be a permanent replacement for a sacrificial one. It replaces the sacrificial reaction by feeding electrical current into the tank by way of an electrode. The device plugs into a wall socket. However, it costs several times more than a sacrificial anode. We often recommend them for smelly water situations where sacrificial anodes may not help.

They are also a good choice if you're using a water softener but don't have odor problems because softeners can consume sacrificial anodes quite rapidly and if you don't keep a close eye on the latter, your tank may rust out before you know it. Having a powered anode means constant protection without the need for constant vigilance.

We still discourage their use in regular water and regular situations. In hard water, sediment can bury and burn out the lower elements of electric water heaters and, with the powered anode, cut off the bottom of the tank from the protection of the powered anode electrode. If that happens, your water heater may rust out. If you're curious, though, we now have a powered anode FAQ.

Why We Don't Like Aluminum Anodes

Water heaters typically come with magnesium or aluminum anodes. We prefer magnesium. We dislike aluminum for a bunch of reasons. Those are:

First off, aluminum, being lower on the Galvanic Scale than magnesium, produces less driving current between anode and cathode (in this case, the tank is the cathode). We think that means it doesn't do as good a job of protecting the tank, especially in softer waters.
Second, it produces about a thousand times its original volume in corrosion byproduct, most of which falls into the bottom of the tank as a sort of jelly, and adds to sediment buildup there.
Third, that gunk also occasionally floats out the hot-water port, appearing as a cottage cheese-like substance clogging aerators and filters.
Fourth, the rod actually expands as it corrodes so that it is hard, or maybe impossible to remove one a few months after installation because its diameter is bigger than when it was installed.
Fifth, along with that, it has a tendency to split off from the core wire, so that chunks fall into the bottom of the water heater, where they stop being anode and start being junk. That also means that if you try to take one out at that point, it may split away from the core wire and snag the underside of the top of the tank, like a fish hook.
Sixth, the build-up of sediment on the bottom of gas heaters encourages noisy operation, and some people can hear their water heaters loud and clear at night, which is not helpful for those wishing to sleep.
Seventh. There is a little booklet "The Danger of Food Contamination by Aluminum" by Dr. R.M. Le Hunte Cooper. It details the nasty things aluminum does to the body. The liver, brain, kidneys and spleen seem to be the main repositories, with nervous tissues holding the most by weight. It was written in 1932. Modern plumbing allows some water that came from the heater to be used as cold water. This doesn't matter if magnesium is used in the heater.
Eighth. If you find yourself ground zero in a disaster, and the water main is broken, about the last thing you want is to be drinking heavily-aluminum-laden water from the bottom of your water heater -- the source of last resort. It will make you sick in dreadful ways: trash your stomach and intestines, create instant arthritis in your joints. No fun!

But all that said, an aluminum/zinc anode is mostly aluminum, although it tends to corrode more slowly than pure aluminum. It is the most economical solution to odor problems if no water-softener is being used. Everything stated here applies to it. So we suggest that those using that anode, or who have bought a heater with a pure aluminum anode, especially if they have a single-control faucet, simply let the cold water run for a few seconds to purge the line of any cooled-off water from the water heater. That's all it takes.

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