Quick Answer:
If you smell a musty odor only when you run hot water, start by testing multiple fixtures and flushing the hot side. Often the cause is stagnant hot-water lines, a tank with sediment or biofilm, or debris dislodged after work on the plumbing. Drawing hot water from several taps will help isolate whether the problem is in the water heater, a single branch, or one fixture.
Why This Happens
Warm water encourages the growth of harmless but smelly bacterial films and can also hold more dislodged particles and sediment than cold water. If a hot branch doesn’t get regular flow, the water can become stagnant and develop a musty smell. The water heater itself can also trap organic material or sediment where bacteria can multiply, producing that odor when hot water is used.
If the smell appears after any service or repair, small amounts of debris or trapped water in a branch can cause odors until the system is flushed. You may also notice tiny bits or discoloration when you run the hot taps — if so, check Particles only in hot water for related steps to isolate particles versus odor sources.
Step-by-Step What to Do
1. Confirm it’s only hot water
Run hot and cold from the same fixture. If cold water is fine and the musty smell appears only with hot, that narrows the issue to the water heater or hot plumbing branch, not the supply source.
2. Draw hot water from multiple fixtures to isolate branch
Open hot taps at these points, one at a time, and run them for several minutes:
- Kitchen sink
- Bathroom sink(s)
- Shower(s)
- Laundary or utility sink
If the smell appears from every hot tap after running for a while, the water heater or the main hot piping is likely the source. If it only appears at one or two fixtures, the problem is probably the branch or the fixture itself.
3. Flush the water heater
Flushing can remove sediment and reduce bacterial buildup. Turn off power or gas to the heater per the manufacturer’s instructions, attach a hose to the drain valve, and run hot water until it clears at the open drain. Then restore power/gas and test again. If you aren’t comfortable with this, skip to calling a professional.
4. Clean low-use fixtures and aerators
Remove and rinse aerators and showerheads on affected fixtures. Soak them in a vinegar solution if mineral deposits are present, then flush the fixture. If the smell began after recent plumbing work, look into Hot water contamination after repair for guidance on how repairs can introduce odors and what to check.
5. Increase flow or schedule regular flushing for low-use lines
Run hot water through seldom-used fixtures weekly to prevent stagnation. For long dead-ends or seldom-used branches, a more permanent fix may be needed from a plumber.
6. Retest and document
After each action, retest by running hot water at multiple fixtures. Note where and when the smell appears — this helps a professional diagnose the problem if the odor returns.
What Not to Do
- Do not ignore musty odors in hot lines.
- Do not pour disinfectants or strong chemicals into the water heater or plumbing without professional advice.
- Do not disassemble gas or electric water heater components unless you are qualified; turn off power first and call a pro if unsure.
When to Call a Professional
- The smell persists after flushing and cleaning fixtures or returns quickly.
- You suspect contamination after a repair and the source is unclear.
- You’re unsure how to safely flush your specific water heater model or you have a gas heater and need the pilot/controls checked.
- There are signs of corrosion, persistent discoloration, or visible biofilm inside fixtures.
Safety Notes
- Hot water can scald. Run a gentle flow when testing and avoid directing hot streams at skin.
- Turn off electricity or gas to the heater before doing any maintenance that involves the tank or plumbing connected to it.
- Collect drained water safely and dispose of it where it won’t damage landscaping or flooring.
Common Homeowner Questions
- Why does only the hot water smell musty? Hot water and stagnant lines encourage bacterial films and trap debris that cold mains don’t, so the odor shows only on the hot side.
- Will flushing the heater fix it? Often yes—flushing removes sediment and microbial buildup—but not always if the issue is in a branch or a fixture.
- Is it safe to keep using the water? Short-term use is usually safe, but avoid drinking hot water until the cause is resolved and consider using cold water for drinking and cooking.