Hot water smells like rotten eggs

Why hot water smells like rotten eggs

Every tank water heater has an anode rod, a long metal rod that hangs down from the top of the tank. It's there on purpose to corrode, so the steel tank doesn't. Most tanks come with a magnesium rod. In some water, especially well water or softened water, harmless sulfate-reducing bacteria live in the warm tank and react with the magnesium, producing hydrogen sulfide gas. That's the rotten egg smell, and because the gas forms in the tank, you notice it only from the hot taps, strongest after the water has sat overnight or after you've been away.

There are two ways to stop it. You can knock the bacteria back by flushing and disinfecting the tank, which often works for weeks or months. Or you can change the conditions so the reaction doesn't happen: an aluminum-zinc anode rod, or a powered anode that uses a small electric current instead of a sacrificial rod, usually solves it long term. Removing the anode rod altogether stops the smell but voids most warranties and lets the tank rust out early, so don't do that.

Why the hot water smells

Let's fix it

1. Rule out gas and find the sourceSmell the air near the water heater, furnace and stove. A rotten egg smell in the air, not in a glass of water, can mean a gas leak: leave, don't use switches or phones inside, and call your gas company from outside. If the air is fine, fill one glass from a cold tap and one from a hot tap, let them sit about 1 minute and smell each away from the sink. If only the hot glass smells, the water heater is the source, and you can keep going. If the cold water smells too, the source is your water supply. For a well, have the water tested and talk to a well or water treatment pro; for city water, call the water utility.
   Done

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2. Turn off the heater and the waterFor an electric heater, turn off its breaker and confirm it's off. For a gas heater, turn the gas control knob to Pilot or Vacation, which keeps the burner from firing while the tank is low, and leave the gas line alone. Close the cold water shutoff valve on the pipe going into the top of the tank. Open a hot water tap somewhere in the house to let air in so the tank can drain. If you can, wait a few hours or run hot water at a tub until it's no longer hot, so you're not handling scalding water.
   Done

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3. Flush the tankAttach a garden hose to the drain valve near the bottom of the tank and run it to a floor drain, a utility sink or outdoors below the tank level. Open the drain valve and let the tank empty. Then open the cold shutoff for a minute or two at a time to stir up and push out sediment, until the water runs clear. Close the drain valve. Flushing alone sometimes reduces the smell for a while, and it removes sediment that bacteria live in. Don't turn the heater back on with the tank empty; an electric element can burn out in seconds without water around it.
   Done

   Something went wrong?

   • The drain valve barely trickles Sediment is clogging it. Open and close the cold supply in short bursts to push it through, or have a plumber replace the valve.
   • The drain valve drips after closing Snug it gently, or put a hose cap on it. Plastic drain valves crack if forced.

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4. Disinfect the tank if your manufacturer allows itMany water heater makers describe a hydrogen peroxide treatment for odor. Check your manual or the maker's website first, since some don't recommend it and the amount matters. A common instruction is about 1 to 2 pints of 3% hydrogen peroxide per 40 gallons. It's poured into the tank through the anode or hot outlet opening with the water off, then refilled and left for a couple of hours before flushing. Don't use bleach unless the manufacturer says to, and never mix chemicals. Refill the tank completely, with a hot tap open until water flows steadily and the air is out, before restoring power or turning the gas control back on.
   Done

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5. Replace the anode rodThis is the long-term fix. With the tank off and partly drained (a few gallons below the top), find the anode's hex head on top of the tank, sometimes under a plastic cap or combined with the hot outlet. Loosen it counterclockwise with a 1 1/16 inch socket and a long breaker bar; it's usually very tight, so have a helper hold the tank steady. Lift the old rod out. Wrap the new rod's threads with thread sealant tape and thread it in clockwise, then tighten firmly. Choose an aluminum-zinc rod or a powered anode, which most makers offer for odor problems. If there isn't enough overhead space, use a flexible link-style rod. Check your warranty terms first.
   Done

   Something went wrong?

   • It won't budge Don't strain the tank or connections. A brief burst from an impact wrench often frees it; otherwise, have a plumber do it.
   • Not enough overhead clearance Buy a segmented or flexible anode that bends as you insert it.

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6. Refill, restart and checkOpen the cold shutoff with a hot tap open in the house until water flows steadily with no sputtering, so the tank is full. Check the anode fitting and drain valve for drips. Then turn the breaker back on, or turn the gas control back to its setting and relight the pilot if your heater needs it, following the label on the heater. Keep the thermostat at about 120°F, the usual safe setting; higher settings increase scald risk, so use anti-scald valves if you go higher. Run the hot water the next morning: the smell should be gone or much weaker, and it may take a few days to fade completely.
   Done

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Watch it done

Water Heater Anode Rod Replacement (Stop Rusty, Smelly Hot Water!)By 1 Tom Plumber on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it comes back

If the smell returns after a flush and peroxide treatment, the anode rod is the real cause, so replace it with an aluminum-zinc or powered anode. If you already did, the bacteria may be coming in with the water supply, which a water treatment specialist can test and address.

If the hot water runs out fast or the tank rumbles, sediment is building up. See the guide for a noisy water heater and the guide for hot water that runs out fast.

If you notice water around the base of the tank, see the water heater leak guide before you do anything else.