Quick Answer:
If you see dark particles only when running hot water, the most likely causes are inside the water heater or a component on the hot side (corroding anode, tank lining particles, or loose material in hot-only piping or fixtures). The quickest way to check is to run only hot water into a clear container and look for flakes that settle.
Why This Happens
Hot water spends time inside the water heater where metal parts, anode rods, rubber components and tank liners sit in warm water. That environment can break down some materials faster than cold plumbing does. Tiny black flakes are often:
- Pieces of degraded rubber (valves, gaskets, dip tubes).
- Corrosion products from the sacrificial anode or internal fittings.
- Particles from a failing tank lining or from sediment that has built up and loosened.
Other possibilities include mineral deposits that have reacted with iron or manganese, or material coming from a single fixture or appliance connected only to the hot line.
Step-by-Step What to Do
1. Do the hot-only container test
Run only hot water into a clear container (glass or clear plastic) for 30–60 seconds, then stop and let it sit. Look for dark flakes that float or settle to the bottom. This confirms the problem is coming from the hot side rather than the supply overall.
2. Compare with cold water
Run cold water into a separate clean container. If the cold sample is clear while the hot sample has specks or flakes, that narrows the issue to the water heater or hot-only piping.
3. Check fixtures and removable screens
Remove aerators, showerheads and inline filters on hot lines and inspect for trapped particles. Sometimes debris accumulates at a single outlet and looks like a bigger problem.
4. Test appliances and laundry
Check whether your washing machine or dishwasher gets the same black particles. If you see matching material in laundry or appliance filters, that helps isolate which hot line is affected. Also compare to known issues like Grit in washing machine water to see if symptoms match.
5. Try a post-curb-stop check
If you or a utility recently worked near your supply, do a short run of hot water after the main has been used. Increased particles after that activity can indicate disturbed sediment in the supply or the meter area — compare to guidance about Sediment after curb stop use.
6. Flush a small amount from the heater
If you can safely follow the heater manual, drain a few gallons from the tank drain valve into a bucket and observe the water. If heavy particles appear in that drain water, that indicates sediment or material is coming from the tank. If you’re not comfortable doing this, skip it and call a pro.
7. Monitor and note changes
Keep notes on when specks appear (continuous vs. occasional), which taps show them, and whether hot water clears after a brief run. This information helps a technician diagnose the issue faster.
What Not to Do
- Do not replace the water heater without first confirming the particles come only from the hot side and from the tank. Replacing a heater may not fix a problem that is in a single fixture or a piping section.
- Do not attempt gas or electrical repairs on the heater yourself if you are not qualified.
- Do not use strong chemical cleaners in the tank or hot lines without professional advice — improper chemicals can damage components or create unsafe conditions.
When to Call a Professional
- If the hot water contains lots of flakes or the particles are large and abrasive.
- If flushing the tank produces persistent black sediment or the water is discolored after a flush.
- If you suspect the anode rod, tank lining or internal corrosion, or if the heater is older and showing other signs of failure (leaks, noises, loss of capacity).
- If you’re uncomfortable performing the flushing or inspection steps yourself.
Safety Notes
- Hot water can cause serious burns. Use caution when running or collecting hot water; let it cool before handling.
- Turn off power (electric) or gas to the heater before attempting any work on the tank. If unsure, hire a technician.
- Wear gloves and eye protection when inspecting or handling sediment. Dispose of collected water and debris responsibly.
Common Homeowner Questions
- Why does the problem only show up with hot water? — Because hot water components and the tank create conditions where certain materials break down or sediment accumulates.
- Will flushing the tank always fix it? — Flushing can remove loose sediment but won’t fix parts that are actively corroding or a failing liner; a professional check may be needed.
- Is this dangerous for health? — Small particles are usually a nuisance rather than an acute health risk, but persistent contamination, strange odors, or discoloration should be evaluated by a pro.
For more related articles, see the Black Specks, Sediment, or Grit in Water hub.