Hot water smells like rotten eggs

Best for

Homes where the cold water at every tap smells clean and only the hot water smells of sulfur, on a well or city water.

Wrong fit

If the cold water smells too, the gas is coming from the well or the plumbing, so read the main causes page and test the water first.

Tradeoff

An anode swap is cheap but not guaranteed, because bacteria in the tank can keep the smell going until the tank is disinfected.

Why only the hot water smells

Most tank water heaters contain a sacrificial anode rod. It corrodes so the steel tank doesn’t. The standard rod is magnesium, and Penn State Extension explains that a magnesium rod can chemically reduce sulfate in the water to hydrogen sulfide, the rotten-egg gas. UGA Extension adds that this is why, when the heater is the only source, only the hot water smells.

Bacteria can be part of it too. Sulfate-reducing bacteria like the warm, low-oxygen inside of a tank. UGA’s odor circular says heaters are more likely to smell when hot water goes unused for a while, when the heater has been off, or when the thermostat is set low. That’s why the smell often shows up after a vacation, or in a guest bathroom.

Two things have to be present: sulfate in the water and a reaction in the tank. Sulfate is common and harmless at typical levels, so you don’t need to remove it. You need to stop the reaction.

Confirm it’s the heater

Do this before you buy anything.

1. Let the water sit for a few hours, ideally overnight.
2. Smell the cold water at two or three taps, including one far from the heater.
3. Smell the hot water at the same taps.

The Minnesota Department of Health says that if the cold water has no smell, the water heater is the likely source. If the cold water smells too, even faintly, the gas is at least partly coming from the well or the plumbing. In that case go to why water smells like rotten eggs and work through the other causes first.

One more check: Virginia Cooperative Extension notes that H2S from the well can smell stronger in hot water, because heat drives the gas out. So “much worse in hot” isn’t the same as “only in hot”. Clean cold water is the test.

Fix 1: replace the magnesium anode

This is the usual fix, and it is cheap.

• What to use. Penn State recommends an aluminum rod and says it should stop the odor while still protecting the tank. UGA Extension, the Connecticut Department of Public Health and the Wisconsin DNR list aluminum or zinc (sold as aluminum-zinc rods). UGA and Connecticut add one rule: if you keep the water above 150°F, use only aluminum.
• Don’t just remove it. Removing the rod often stops the smell, but Penn State and Minnesota warn it can shorten the heater’s life, and Penn State says it will likely void the warranty.
• Check the warranty first. Virginia Cooperative Extension says an aluminum rod may help but advises checking the heater maker’s warranty terms before you swap.
• Size. Minnesota describes the rod as 1/2 to 3/4 inch in diameter and 30 to 40 inches long, under a plug on top of the heater. Some heaters have the rod built into the hot outlet fitting instead. Check your heater’s manual for the right replacement.

Getting an old rod out can take a long breaker bar, and there may be little headroom above the heater. A segmented (flexible) rod helps where the ceiling is low. If you aren’t comfortable with this, Minnesota advises hiring a plumber, and so does Wisconsin.

A new rod doesn’t always solve it. Virginia Cooperative Extension cautions that if microorganisms in the heater are causing the smell, replacing the rod alone won’t fix it. That’s why the disinfecting flush below is often done at the same time.

Fix 2: a powered anode

A powered anode (also called an impressed-current anode) protects the tank with a small electric current from a wall outlet instead of a sacrificial metal. With no magnesium in the tank, the rod reaction is gone. It’s worth considering when an aluminum rod didn’t work, or in a home with a softener, since UGA Extension notes softened water speeds up how fast a magnesium rod dissolves.

Before you buy one, check three things: that it fits your heater’s anode port, that the heater maker allows it under the warranty, and that you have an outlet that is always on. A powered anode only protects the tank while it has power.

Fix 3: disinfect and flush the tank

If bacteria are living in the tank, kill them and flush them out. State health departments describe two methods.

Chlorine bleach. Minnesota and Vermont say disinfecting and flushing the heater with a chlorine bleach solution can kill sulfur bacteria. Both warn the smell can return within a few weeks if not all the bacteria are killed. Alberta Agriculture also lists hydrogen peroxide for disinfecting the tank, and suggests trying disinfection before more drastic steps.

Heat. Minnesota says raising the heater to 160°F (71°C) for several hours destroys sulfur bacteria, followed by a flush. This is dangerous. Minnesota and Vermont say to check that the pressure relief valve works first, to warn everyone in the house about scalding water, and to turn the thermostat back down afterward. Wisconsin’s DNR says not to try it unless the heater has a pressure relief valve, and that it reduces but doesn’t eliminate sulfur bacteria. Unless you know water heaters well, Minnesota advises hiring a plumber.

Simple flush. For a heater that smells after sitting unused or running with a low thermostat setting, UGA Extension recommends flushing the system. Follow the drain-and-flush steps in your heater’s manual.

Keeping it from coming back

Once the smell is gone, a few habits make it less likely to return:

• Don’t let the heater sit. UGA Extension links heater odors to hot water going unused for a while and to heaters that were turned off. If you leave for weeks, run the hot water at every tap when you get back. For a heater in a seldom-used space, such as a cabin or guest suite, plan on a flush after long idle spells.
• Don’t set the thermostat too low. UGA lists a low thermostat setting as a cause. Balance that against scald risk, especially with children or older adults in the house, and don’t leave the heater at the 160°F disinfection temperature.
• Check the anode on schedule. Even an aluminum rod wears away. Your heater’s manual will say how often to inspect it.
• Watch for a cold-water smell. If the cold side starts to smell later, the cause has moved upstream to the well or pipes, and the fix changes.

If you have a softener, expect rods to wear faster. UGA notes softened water speeds up how quickly a magnesium rod dissolves, which gives sulfate-reducing bacteria more to work with.

Why a whole-house sulfur filter is the wrong fix

A sulfur filter treats water on its way into the house. If the cold water is already odor-free, the water entering the heater is already clean. The gas is being made inside the tank, downstream of any filter. An air-injection system costing thousands of dollars won’t change what happens in your heater, while a new anode rod costs a small fraction of that.

The same logic applies to a shower filter or a carbon cartridge on the hot line. It might mask some smell for a while, but it leaves the reaction in place.

There’s one exception. If testing shows H2S in the cold water as well, you have two problems: gas from the well and gas from the heater. Treat the well water first, then deal with the heater if the hot side still smells. The treatment-by-level table covers the well side.

On city water

This happens on city water too. A utility can deliver water with ordinary sulfate levels, and the heater does the rest. If the cold water is clean, the fixes above apply. If the cold water smells, call the utility. See sulfur smell in city water.

When to call a plumber

• The anode won’t budge, or there’s no room to pull it.
• You want to try the high-temperature method and aren’t sure the relief valve works.
• The heater is old and leaking, rusty at the base, or nearly at the end of its life. Replacing it may make more sense than fixing it.
• The smell comes back after a new rod and a disinfecting flush. That can mean sulfur bacteria are also in the well or pipes, and the Wisconsin DNR lists periodic shock chlorination as the fix for sulfur bacteria. See shock chlorination.