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The cause of rotten egg odor

The most common cause of smelly water is anaerobic bacteria that exist in some water and react with the magnesium and aluminum sacrificial anodes that come with most water heaters to produce hydrogen sulfide gas, making the classic rotten egg odor. The problem is most common in well systems, either private or municipal. Softening can make smelly water much worse.


What not to do to get rid of smelly water

We've heard of plumbers and handymen advising people to remove the sacrificial anodes from their water heaters as a solution to smelly water. It's a solution all right, but one that will ensure your water heater rusts out in record time. There is a reason why removing an anode voids the warranty.

Smelly water at just one sink

If you have smelly water at one sink, but not all of them, dump hydrogen peroxide down the basin overflow. Sometimes bacteria can build up there, too.

The complete fix, in most cases...

Very often, replacing the standard magnesium or aluminum anode rod with an aluminum/zinc alloy anode will solve the problem. The zinc is a key ingredient, since pure aluminum anodes will also reek to high heaven.

For most folks, an aluminum/zinc anode is the cheapest fix for this problem and we urge you to try it first before considering the alternatives -- unless you soften your water. More on that in a moment.

Contrary to our usual advice, we do not think you should put two anodes in your tank, even aluminum/zinc ones, as it may worsen the odor.

These anodes come in four flavors: standard hex-head, flexible hex-head, standard combo, flexible combo.


Those terms, doubtless, mean nothing to you, but they're important if you're to choose the right anode.

The photo at right shows hex and combo anodes. The latter is also called an outlet anode.

Hex-heads go in their own hole on top of the water heater. In most cases, you'll be able to see the hex head. If you can't, the anode is either hidden under the sheetmetal, or possibly under a plastic cap, or your tank has a combo anode.

Combo anodes share the hot-water-outlet port. If you're not sure if there is an anode in there, try to run a long screwdriver down it. If there is an anode, the screwdriver won't go more than a few inches. This used to be easier, when nobody used heat-trap nipples. If you can't do this, unscrew the nipple to see if there is an anode beneath it.

Some water heaters have two anodes. Not only is it important to put an aluminum/zinc anode into the heater; it's also important to remove all previous anodes or the hot water will still smell.

Hex-head and combo magnesium anode rods
Standard hex-head anodes need 44 inches of overhead clearance. Standard combos need 48. Flexible anodes are loose links connected with flexible wire. They are good down to 12 inches overhead clearance, and can be cut shorter if they are too long for the tank.

... But, If You Have Softened Water

A few people buy an aluminum/zinc anode and the odor doesn't go away. That's vexing for them and us. The cases involved softened water. Softening can speed up anode consumption by increasing the conductivity of the water. That can increase the amount of hydrogen sulfide gas produced.

These are cases for powered anode rods. A sacrificial anode creates an electrical reaction inside a water heater as it corrodes. A powered anode does the same by feeding electricity into the tank. Since there is no magnesium or aluminum, there's no smell. We don't recommend them for everybody, though, because they're several times more expensive than sacrificial anodes. But they are permanent: they aren't sacrificial, so they don't need replacement.

One more thing: There are several configurations of residential water heaters. Most have a hex-head anode in its own port somewhere on the top of the tank. A few do not. Some of Bradford White's, A.O. Smith's and State's residential tanks employ a combo anode/hot-water outlet/nipple in the hot port. A powered anode can be used with those tanks by adding a galvanized tee to the hot port. The bottom port of the tee will connect to the tank; the plumbing to the house will go out the side port; the powered anode will screw into the top port with the element hanging down inside the tank.

How do you tell if an anode is hex or combo? It's a fair question because some heaters do have a hex anode but it's hidden under sheetmetal, or perhaps under a plastic plug in the top. The easy test though, for a combo, is to disconnect the hot-side plumbing and unscrew the nipple to see if it's just a nipple or has an anode attached to it. It's worth doing this test even if your tank has a hex anode because if there is a standard anode anywhere in the water heater, you'll have rotten eggs, no matter what anode you employ elsewhere.

It might be that toward the end of the life of a water heater, there was too little anode left to make much hydrogen sulfide gas. Or it might be the water supply changed in some way.

There's something few realize: water is a chemical and one that is constantly changing. The water that flows out of the tap this evening may be different from that from this morning, either because of what's in the ground or because water companies have changed their sources of supply or added something new to it..