Symptoms
- Hot water has a musty, earthy, or moldy smell
- Cold water from the same tap smells normal
- Household may have a water softener or filtration system
- Smell may be mild or only noticeable after the tank sits
Likely causes
- CommonBacterial or biofilm growth inside the tankWarm, stagnant conditions in a rarely used tank can support bacterial growth that produces musty odors.
- PossibleWater softener resin or filter needing serviceA neglected softener or filter upstream of the water heater can introduce musty-tasting or smelling water.
- PossibleAnode rod reaction producing an odor different from the classic rotten-egg smellSome anode rod reactions can present as more musty than sulfurous depending on water chemistry.
What to do
- Confirm the smell is limited to hot water and not present in cold water from the same source.
- Check whether your home has a water softener or filtration system and when it was last serviced or had media replaced.
- Flush the water heater tank thoroughly to remove standing water that may be harboring bacteria.
- Raise the tank temperature to around 140°F briefly to help reduce bacterial growth, then return it to a safe setting like 120°F.
- If a water softener or filter is due for service, address that as it may be the actual source rather than the tank.
- If the smell persists after flushing and checking treatment equipment, consider having your water tested.
Tools and parts
- Garden hose (for flushing)
When to call a pro
If flushing and checking any water treatment equipment don't resolve the smell, a plumber or water treatment specialist can test the water and recommend next steps.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.