Why Does My Hot Water Smell Bad?

Hot water that smells, most often of rotten eggs or sulfur, is one of those problems where a single test narrows the cause almost immediately. Before assuming anything about the water heater, it is worth establishing whether the heater is involved at all.

The test that tells you where to look

Run the cold water and smell it. Then run the hot and smell that. Do it at more than one fixture, and fill a glass and step away from the sink before smelling it, because drain odor rising from the sink is easy to mistake for odor in the water.

Hot only. The water heater is almost certainly involved. That is the subject of most of this article.

Both hot and cold. The smell is arriving in the supply rather than being created in the tank. That is a question for your water provider, or for the well if you are on one.

Only at one fixture. Suspect the drain rather than the water. A smelly trap produces odor that is very easy to attribute to the faucet running above it.

Why hot water smells and cold water does not

The rotten egg smell is hydrogen sulfide gas dissolved in the water. In a household water heater it is usually produced inside the tank rather than arriving in it.

Two things combine to make a storage tank a good place for this to happen. A tank holds a large volume of water standing still at a warm temperature, which suits certain sulfur reducing bacteria. And the tank contains a sacrificial anode rod, usually magnesium, which corrodes by design and in doing so releases what those bacteria use.

The result is a smell that appears in hot water only, is often strongest first thing in the morning or after the house has been empty, and fades slightly once water has been running for a while.

That timing pattern is a good confirmation. Water that has sat in the tank longest smells worst.

When it starts after the house has been empty

A very common trigger is a period of no use. A vacation, a vacant rental, a guest bathroom nobody touches.

Water sitting undisturbed at a warm temperature for days gives the process time to develop. Households often notice the smell on returning home and assume something has failed while they were away, when what has actually happened is simply that nothing moved.

The same applies to a heater set to a low temperature to save energy. Lower temperatures are more hospitable to the bacteria involved, which is one of the trade offs of turning a tank down a long way.

Is it harmful?

At the concentrations that produce a noticeable smell in household hot water, this is generally treated as an aesthetic problem rather than a safety one. It is unpleasant rather than dangerous.

That said, water quality questions are properly answered by the people responsible for your water. If the cold water smells as well, contact your water provider. If you are on a private well, testing is the sensible route, and your local health department can normally point you to how that is done in your area.

A new or sudden change in how your water smells, tastes or looks is always worth reporting to your provider rather than assuming.

Other smells and what they suggest

Musty or earthy. More often related to the supply than the heater, particularly if the cold water shares it.

Plastic or chemical. Sometimes noticed with new plumbing or a new heater and usually easing over the first weeks of use. A persistent chemical smell is worth raising with your water provider.

Burning or hot metal near the unit. Not a water smell at all. On a gas heater that warrants attention promptly, and if you smell gas rather than burning, leave the building, avoid operating switches on the way out, and call your gas utility from outside.

What usually resolves it

Changing the anode rod. Because a magnesium rod is often central to the reaction, swapping it for a different material, commonly an aluminum zinc alloy, is the usual answer. This keeps the tank protected while removing the ingredient driving the smell. Removing the rod altogether is not the answer, since it leaves the tank unprotected and shortens its life considerably.

Flushing and disinfecting the tank. Draining, cleaning and treating the tank clears the existing population. On its own it often works for a while and then the smell returns, because the conditions that produced it are unchanged. Combined with an anode change it tends to hold.

Raising the temperature. Running the tank hotter suppresses the bacteria. It also raises the scalding risk at every faucet in the house, so it is a decision to make deliberately rather than casually, and mixing valves exist precisely for this trade off.

Anode work involves draining the tank and breaking into a fitting that is often seized after years in place, so it is more involved than it sounds on paper.

When to get someone involved

Reasonable triggers are a smell that returns after flushing, an anode rod that has never been checked, a heater old enough that the rod is likely spent, or a smell appearing alongside other symptoms.

Age is relevant context here, and our article on how long a water heater usually lasts covers how to find out how old yours is. If the unit is also rumbling, water heater noises explains what that indicates.

See water heater repair for servicing.

Frequently asked questions

Why does only my hot water smell?

Because the smell is being produced inside the tank rather than arriving in the supply. A warm standing volume of water and a corroding anode rod together create the conditions. Cold water never passes through that environment.

Will flushing the water heater fix the smell?

Often temporarily. Flushing clears what is in the tank now, but if the anode and the operating temperature are unchanged, the conditions that produced it remain. A lasting fix usually involves the anode rod.

Why is the smell worse after a vacation?

Because the water sat undisturbed at a warm temperature for days. Water that has been standing longest smells worst, which is why it is often most noticeable on the first shower after returning and eases over the next day of normal use.

For plumbing service in Houston, call (877) 563-1074 to connect with an independent plumbing service provider.