Quick Answer:
If your sink only backs up or stops draining when you run hot water, the most common causes are material that loosens with heat (grease, soap scum, or mineral scale) moving into a partial blockage, or a venting/drainage behavior that changes with temperature. First, test hot vs cold to confirm the pattern; then follow a careful cleaning and inspection process before calling a pro.
Why This Happens
- Hot water can soften and dislodge grease, soap scum, or mineral deposits that are stuck in the piping. Once loosened they can shift and block the trap or a downstream joint.
- Temperature changes can change flow behavior and pressure in a partially blocked drain, making a problem show up only when water is hot.
- Blocked or restricted vents can change how air moves through the drain when hot water and steam are present, which makes gurgling or slow drainage more likely. See Sink drains then gurgles if you notice bubbling noises.
- In rare cases the issue can come from the faucet mixing valve, the disposal, or a failing component that reacts differently to hot water.
Step-by-Step What to Do
1. Test hot vs. cold and note what happens
- Run cold water only for 30–60 seconds and watch. Then run hot water only for the same time. Repeat once or twice.
- Note when the sink slows or backs up: during hot-only, after switching, or only when both run. Write down what you see—this helps a plumber if you call one.
2. Try a controlled flush
- With the drain stopper removed, run cold water, then gradually add hot until the problem appears. This helps confirm whether temperature or volume triggers the stoppage.
3. Check and clean the stopper and strainer
- Remove any screen or stopper and clear hair, soap, and visible debris. Some blockages are right at the sink opening.
4. Inspect and clean the P‑trap
- Place a bucket under the trap, loosen the slip nuts, and remove the P‑trap. Empty and clean it—hot-water loosened grease often collects here.
- Check the tailpiece and trap inlet for debris. Reassemble securely and test again.
5. Use a hand auger or drain snake if needed
- If the trap was clean but the sink still stops with hot water, feed a small hand snake down the tailpiece into the drain to clear farther-down deposits. Don’t force it; back it out and clean as needed.
6. Check nearby fixtures and vent behavior
- Run other fixtures and listen for gurgles or pressure changes. If other drains react the same way, the problem may be farther down the line or with the vent. If you notice a pressure sensation, you may want to read about Sink drains then pressure builds.
7. Retest and monitor
- After cleaning, repeat the hot/cold tests to confirm the issue is resolved. If the problem returns quickly after a hot flush, the underlying cause is probably beyond the trap or needs professional attention.
What Not to Do
- Do not keep using only cold water to avoid the hot-only stoppage.
- Do not pour boiling water repeatedly into plastic pipes—this can soften fittings or seals.
- Avoid chemical drain cleaners when the pipes are hot; they can react unpredictably and damage finishes or seals.
When to Call a Professional
- If cleaning the trap and using a hand snake doesn’t stop the backups.
- If multiple fixtures are affected, or you smell sewer gas.
- If you suspect a venting problem, a collapsed pipe, or something deeper than you can reach safely.
- If you prefer not to work under the sink or are unsure about reassembling the trap.
Safety Notes
- Turn off power to a garbage disposal before working near it.
- Wear gloves and eye protection when removing traps or using tools.
- Do not mix chemical drain cleaners with other cleaners, and avoid them if the drain is hot. They can splash or produce fumes that are dangerous.
- Use a bucket to catch water when removing the trap to avoid slips and contamination.
Common Homeowner Questions
- Why does hot water cause blockages if cold water does not? Hot water can soften grease and free trapped debris that then moves and lodges in a narrow spot.
- Will pouring boiling water clear it? Boiling water may help with grease in metal pipes, but it can harm plastic pipes and fittings—use caution.
- Are chemical drain cleaners safe for this? Generally no—especially with hot water present. They can harm pipes and are hazardous to use; mechanical cleaning is safer.