In my experience, this issue almost never comes from the municipal supply or the well itself. It usually starts closer to home.

Why hot water makes smells more noticeable

Heat accelerates chemical reactions and releases gases more quickly. That means anything dissolved in water—or growing inside a system—becomes easier to smell once the water is heated. Cold water can mask problems that hot water exposes immediately.

I’ve had homeowners insist their water heater couldn’t be involved because the smell was “in the water.” Once we isolated the hot line, the answer became obvious.

The most common culprit: the water heater

By far, the most frequent cause of hot-water-only odor is the water heater itself. Inside many tanks is a sacrificial anode rod designed to prevent corrosion. Under certain conditions, especially in water with naturally occurring sulfur or specific bacteria, that rod can trigger a rotten egg or musty smell.

I once worked on a home where the smell appeared only in the master bathroom. The water heater was fine, but low usage in that branch allowed odor-causing reactions to intensify. Flushing the heater and addressing the anode resolved it completely.

When the smell is sulfur-like

If the odor resembles rotten eggs, sulfur-reducing bacteria are often involved. These bacteria aren’t usually harmful, but they can thrive in warm, low-oxygen environments like water heaters.

This doesn’t mean your well is contaminated. I’ve seen perfectly clean well water develop sulfur odor only after sitting in a hot tank overnight.

Metallic or “electrical” smells

Some people describe hot water odor as metallic, sharp, or slightly electrical. This often points to aging heater components or reactions between minerals and the tank lining. I’ve encountered this in heaters that were still technically functional but nearing the end of their useful life.

Ignoring this smell doesn’t usually cause immediate failure, but it’s often an early warning sign.

Why cold water usually smells fine

Cold water moves faster through plumbing and doesn’t sit in a heated environment. That limits bacterial growth and chemical reactions. When homeowners test only cold water and assume the source is clean, they’re not wrong—but they’re also not seeing the full picture.

Common mistakes homeowners make

The biggest mistake is treating the whole house when the issue is localized. I’ve seen people install expensive filtration systems to fix a smell that lived entirely inside the water heater.

Another mistake is masking the smell with scented cleaners or shower products instead of addressing the cause. That usually makes the odor feel worse over time, not better.

I’ve also seen people delay action because the smell comes and goes. In reality, intermittent odor often means conditions inside the heater are fluctuating, not improving.

What usually fixes the problem

In many cases, flushing the water heater and addressing the anode rod solves the issue. Sometimes a temperature adjustment helps. In other cases, targeted treatment is needed if bacteria are persistent.

What rarely helps is ignoring it. Odors don’t resolve themselves, and they tend to grow more noticeable as conditions stabilize in favor of whatever’s causing them.

Knowing where to focus saves time and money

After years of diagnosing hot-water odor complaints, I’ve learned that the smell itself is a clue. If it only appears when water is hot, the problem is usually not the water source—it’s the system heating it.

Once homeowners stop chasing the wrong fix and focus on the heater and nearby plumbing, the solution becomes clearer. When that happens, the smell disappears just as quietly as it arrived, and hot water goes back to being something you don’t think about at all.