Quick Answer:
If you pour a glass and notice a musty odor, it’s often caused by organic compounds or biofilm in the plumbing, the water heater, or the source water. Start by checking several cold taps to see whether the smell is isolated or system-wide. Treat the water as potentially contaminated until you identify the cause.
Why This Happens
- Organic compounds like geosmin and methylisoborneol (produced by algae or decaying plant matter) can give water an earthy or musty smell.
- Bacterial biofilm can grow inside pipes, faucet aerators, or the hot water tank; some bacteria produce noticeable odors even if they aren’t immediately harmful.
- Hot water heaters can collect sediment or bacterial growth that causes odors in only the hot supply.
- Private wells and seasonal changes at surface water sources are more likely to introduce organic material into the supply.
- Fixtures and aerators can trap debris and foster odors at a single faucet or set of fixtures.
Step-by-Step What to Do
1. Compare several cold taps
Open several cold-water faucets (kitchen, bathroom, outdoor hose) and smell them. If only one tap smells, the problem is usually local to that fixture. If all cold taps smell, the issue is in the house plumbing or incoming supply.
2. Check hot water separately
Run a hot-water tap. If the smell appears only with hot water, suspect the water heater (sediment or bacterial growth). If both hot and cold smell, the source is upstream of the heater.
3. Flush cold taps
Run cold water for 3–5 minutes at each tap to see if the smell clears. Sometimes stagnant water in a seldom-used line causes odors; flushing replaces it with fresh supply.
4. Inspect and clean aerators and screens
Unscrew aerators from faucets and check for gunk or discoloration. Clean them with warm soapy water and a small brush, then reattach and retest the water.
5. Check the water heater
Look for rust or sediment around the drain valve and consider draining a few gallons to remove buildup. If the heater smells when you open the hot tap, it may need a full flush or treatment. If you’re unsure how to flush a tank safely, see the section on calling a professional below.
6. Consider your water source
If you have a private well, test for bacteria and organic contaminants. If you’re on municipal water, contact the supplier to ask about recent treatment changes or source issues. Seasonal algae blooms or treatment changes can cause temporary odors.
7. Collect a sample and test
If the smell persists after these checks, collect a sample in a clean container and have it tested for bacteria and organic compounds. Testing will show whether the smell is from harmless organic material or a contamination that needs treatment.
8. Narrow by symptom and location
If the odor is strongest at shower heads or only on the upper floors, those patterns help narrow the cause. For more focused scenarios, see Earthy odor from shower and Smell only in upstairs fixtures.
What Not to Do
- Do not drink musty water until cause is identified.
- Don’t ignore a persistent whole-house odor — it could indicate contamination in the supply or a failing component in your system.
- Avoid pouring chemicals into a well or plumbing without professional guidance; improper treatment can make conditions worse or produce unsafe byproducts.
When to Call a Professional
- The smell does not clear after flushing and cleaning aerators.
- The odor is only in hot water and draining the water heater is beyond your comfort level.
- Testing shows bacteria or concerning contaminants, or you have a private well and suspect contamination.
- You notice other signs of contamination: discoloration, oily sheen, or illness after using the water.
Safety Notes
- Treat musty-smelling water as potentially unsafe for drinking and cooking until you know the cause.
- Boiling usually does not remove earthy or musty odors and can concentrate some contaminants; rely on testing and proper treatment instead.
- If anyone in the household is immunocompromised, take extra precautions: use bottled water for drinking and food preparation until the issue is resolved.
Common Homeowner Questions
- Is a musty smell in water dangerous? — Often it’s caused by organic compounds or nonpathogenic bacteria, but you should avoid drinking the water until testing confirms it’s safe.
- Will boiling get rid of the odor? — No; boiling typically won’t remove earthy or musty smells. A carbon filter or targeted treatment is more effective if testing shows organic contaminants.
- How long should I run faucets before the smell disappears? — Try 3–5 minutes at each tap; if the smell persists after flushing, further inspection and testing are needed.
For more related articles, see the Musty, Earthy, or Moldy Water Smell hub.