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 (impressed-current) 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.
When the tank is filled with water, an electrochemical 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.
When two metals are physically connected in water, one will corrode away to protect the other. Sometimes that’s bad, but often it’s good. 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.
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. This may require some plumbing to add unsoftened water to softened water.
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 continuity 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. And all that said, the latest shipment of aluminum/zinc anodes we bought had hex heads with bumps! We thought we’d been sent the wrong stuff, but verified that it’s really aluminum/zinc.
The exception to that is Rheem and its sub-brands, Ruud/Richmond and General Electrics. 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. GE Geosprings may require a ratchet extension to remove the anode.
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/Maytag always uses aluminum anodes. Bradford White may use either aluminum or magnesium, and many of their tanks have one combo rod in the hot port. Giant and uses magnesium, as does Bock. Heaters made by American Standard can have either magnesium or aluminum anodes. Sometimes it’s mentioned in their spec sheets and sometimes not.
Older State/Reliance/Kenmores have had both combo rod tanks and tanks with a hex anode. More recently, they all seem to have hex anodes. Aluminum is usually used, but in the past, high-end models have come with magnesium. Today, some of those also offer powered anodes.
There is one important exception to the anode equation: all Smith, State and sub-brand ultra low-NOX heaters come with aluminum anodes.
It’s common for people to ask us to choose a water heater for them. We won’t do it; there are too many variables and many of them are buyer-specific. Occasionally, also, we’re asked to pick a tank that will permit the addition of the parts we sell that will extend life. Even that has gotten harder because the manufacturers are rolling out models that meet the April 2015 energy rules. We may be able to tell you more later, as we see what has changed.
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.
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 some brands, the hex head is exposed. On many, 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. On some older tanks, it may be hidden under the sheet-metal top.
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 unscrew the nipple and see if there is an anode connected to it.
Anode rods of all types can be purchased on our Product Page. Also you might want to consult our Sacrificial Anode Buying Guide.
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. To learn more about them, go to the Powered Anode Page.