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Hot water smells like rotten eggs: causes and how to fix it

A rotten-egg or sulfur smell that shows up only in your hot water almost always starts inside the water heater tank. It's usually caused by hydrogen sulfide gas made by bacteria reacting with the tank's anode rod. It's unpleasant but common, and in most cases you can fix it without replacing the heater.

Safety first. If you smell rotten eggs in the air near a gas water heater, furnace or stove, rather than in the water, treat it as a possible gas leak. Don't light anything or flip switches, leave the house, and call your gas utility or 911 from outside.

Step 1: Is it the heater or the water supply?

Run cold water and hot water separately from the same faucet, and smell each one.

Only the hot water smells: the problem is in the water heater. Keep reading.
Both hot and cold smell: the source is likely your water supply. On a private well, sulfur bacteria or naturally occurring hydrogen sulfide in the groundwater are common causes; get the well water tested and look at treatment options. On city water, call your water utility.
Only one faucet or drain smells: it may be the drain or P-trap, not the water. Fill a glass, step away from the sink and smell the water itself.

Why water heaters make the smell

Most tank water heaters have a sacrificial anode rod, usually magnesium or aluminum, that corrodes so the steel tank doesn't. Sulfate-reducing bacteria, which are harmless in small numbers and common in well water and softened water, can react with the anode and produce hydrogen sulfide. Warm water in the tank is a good place for them to grow, especially if the thermostat is set low or the heater sits unused for a while, like after a vacation or in a rental between tenants.

How to fix it

1. Flush the tank

Turn off the power (or set a gas heater to PILOT or VACATION), shut the cold water supply, attach a hose to the drain valve, open a hot faucet in the house, and drain the tank. Flushing removes sediment that bacteria live in. On its own it may only help for a while, but it's the first step for the other fixes.

2. Disinfect the tank

Many manufacturers describe a way to disinfect the tank, often with household bleach or hydrogen peroxide, and then flush it thoroughly. Follow the instructions in your heater's manual or on the manufacturer's website, because the method and amounts vary, and some chemicals can harm the tank or its parts. Make sure everyone in the house knows not to use the water during the treatment.

3. Replace the anode rod

If the smell keeps coming back, swapping the standard magnesium anode for a different type is the usual long-term fix. Common options are an aluminum/zinc anode, which tends to reduce the reaction, or a powered (impressed current) anode, which doesn't get used up the same way. Removing the anode completely does stop the smell, but it leaves the tank unprotected and can void the warranty, so don't do that.

Anode rods are often very tight and need a large socket and breaker bar, plus enough clearance above the heater to pull the rod out (flexible, segmented rods exist for low ceilings). Check your manufacturer's recommended replacement anode so you don't affect the warranty.

4. Check the temperature setting

A very low thermostat setting can let bacteria grow more easily. Many manufacturers ship heaters set around 120°F to reduce scald risk. Raising the temperature temporarily is one approach some manufacturers mention, but higher temperatures increase scalding risk, so follow your manual and use caution with kids and older adults in the house.

5. Consider your water softener

Softened water can make anode corrosion faster and the smell worse. You don't need to give up the softener, but a different anode type is often the answer in softened homes.

When to call a plumber

The smell returns soon after flushing and disinfecting.
You can't break the anode rod loose, or there isn't room to pull it out.
The water is also discolored or rusty, which can mean a corroding tank. See how long water heaters last.
Both hot and cold water smell on a well system, and you want help with treatment.
You smell gas in the air, not just in the water. Leave and call the gas utility first.

Tip: Before you call, note whether the smell is hot-only, whether you're on a well or city water, whether you have a softener, and the heater's brand, model and age from the rating label. That tells the plumber which anode to bring.