Hot Water Smells Like Rotten Eggs

Is it the heater, the well or the softener?

Run a quick sniff test. Vermont's health department suggests smelling the hot and cold taps after the water has sat for a few hours, since the hot side is usually stronger. Their guide to reading the result:

If both hot and cold smell, this page won't fix it. Go to sulfur smell in well water instead.

Why the tank makes the smell

Most tank water heaters have an anode rod, a sacrificial metal rod that corrodes so the steel tank doesn't. In water with sulfate, the anode's reaction can give off hydrogen sulfide, and sulfur bacteria can thrive in the warm tank. Warm water, a tank that sits unused and a well with no chlorine all make it worse. That's why the smell often shows up after a vacation or in a guest bathroom.

On its own, the smell usually doesn't mean your water is unsafe. Vermont does note that in rare cases hydrogen sulfide comes from sewage or other pollution, so it recommends testing a well for total coliform bacteria and nitrate too. See coliform bacteria in well water if you haven't tested in the last year.

Fixes that work, in order

Vermont recommends hiring a plumber unless you know your way around a water heater. If you do, here's the order we'd try.

1. 1

   Cause 1: Replace the anode rod with a different metal

   This is the fix that usually sticks. Vermont suggests replacing the anode with one made of another material, such as aluminum, which may still protect the tank without producing the gas. A.O. Smith also recommends an aluminum/zinc composite anode for this problem.

   Don't just pull the anode and leave it out. Vermont warns that removing it can shorten the heater's life a lot. Anode rods are often seized tight on old tanks, and you need clearance above the heater to slide a new one in. A flexible, segmented rod helps when the ceiling is low. More on that in water heater anode rod.

2. 2

   Cause 2: Disinfect and flush the tank

   Killing the bacteria knocks the smell back. A.O. Smith suggests flushing the tank with a mix of water and hydrogen peroxide. Vermont describes disinfecting and flushing with a chlorine bleach solution, and warns the smell may return within a few weeks if some bacteria survive. That's why a flush alone tends to be temporary unless you also change the anode.

   Our water heater flush guide covers the drain and refill steps. Follow your heater's manual for anything you put into the tank.

3. 3

   Cause 3: Heat the tank to kill the bacteria

   Vermont describes raising the tank to 160°F for several hours, then flushing out the dead bacteria. A.O. Smith's version is a brief raise to 140°F before returning to a safer setting, with mixing valves to prevent scalding. Many standard thermostats won't go that high, and it's the riskiest option here. If you do it, nobody uses hot water during the treatment, and the dial goes back down afterward. A.O. Smith calls 120°F the right setting for most homes.

Tankless and RV heaters

Tankless heaters have no anode rod and little standing water, so they rarely make this smell themselves. If a tankless home has sulfur-smelling hot water, look at the well, the softener or a storage tank upstream.

RV water heaters often have an anode and sit unused for weeks, which is the perfect setup for the smell. The same fixes apply: drain, disinfect per the RV maker's instructions and check the anode.

When to call a plumber, and what it costs

Call a plumber if the anode rod won't break loose, if you don't have room overhead to change it, or if you don't want to work with high tank temperatures. A pro also has the impact tools to get a seized rod out without twisting the tank fitting.

HomeGuide's numbers:

A.O. Smith says anode rods usually last about 5 years, depending on your water. If yours has never been changed, it's due anyway.