Quick Answer:
If a drain clogs only when you run hot water, the likely causes are grease or soap scum that softens with heat and moves or re-forms, a partial trap blockage that shifts when warmed, or mineral deposits that react to temperature. Start by reproducing the problem with controlled tests, then clear and inspect the trap and nearby piping before trying any stronger fixes.
Why This Happens
- Grease and soap: Hot water melts fats and certain soap residues, allowing them to travel farther down the drain where they can cool and stick again.
- Soap scum and minerals: Hard water combines with soap to form a waxy residue that behaves differently with hot water. See Soap buildup from hard water for more on that issue.
- P-trap or loose fittings: Heat can change how parts sit and let debris shift into a blocking position only when hot water flows.
- Appliance timing: Running the dishwasher or garbage disposal along with hot water can push extra grease or food into the drain at the same time.
Step-by-Step What to Do
1. Reproduce the problem and compare
- Run cold only for several minutes and note if any slowing or gurgling happens.
- Run hot only for the same length of time and watch for the clog. Run cold vs hot cycles to compare clog timing.
- Do the test a couple of times to make sure it’s consistent and to rule out one-off debris.
2. Remove and inspect the P-trap
- Place a bucket under the trap, loosen the slip nuts, and remove the trap.
- Check for hair, grease, soap, or small objects and clean the trap thoroughly.
- Reassemble and test with both cold and hot water again.
3. Use gentle mechanical cleaning
- Try a plunger first on the affected drain with an overflow covered (if applicable).
- If that doesn’t work, use a hand-held drain snake (3/8″–1/2″ cable) to reach past the trap into the drain line.
- A bathtub-style snake or a plumber’s auger may be needed for deeper clogs.
4. Avoid harsh chemicals; consider household options
- A mixture of baking soda and vinegar can help loosen some organic buildups; follow with hot (not boiling) water to flush.
- Do not mix multiple commercial cleaners and avoid concentrated acids—these can damage pipes and are hazardous.
5. Monitor and repeat as needed
- After cleaning, run several hot and cold cycles to confirm the clog is gone.
- If the clog reappears, note the timing, what appliances were running, and whether it’s only one fixture or multiple fixtures.
What Not to Do
- Do not mask hot-water-only patterns. If you cover up the symptom (for example, by always running cold first), you lose diagnostic information.
- Do not pour boiling water into PVC drains repeatedly—extremely hot water can soften or warp some plastics.
- Do not use strong chemical drain cleaners as a first fix; they can burn skin, release fumes, and damage pipes.
- Do not force fixtures or fittings without proper tools—stripped nuts and broken components make repair harder and costlier.
When to Call a Professional
- The clog returns repeatedly after you’ve cleared the trap and snaked the line.
- Multiple drains slow or back up at the same time—this can indicate a deeper line or sewer issue.
- There’s a persistent odor, sewage backup, visible corrosion, or a leak you can’t fix safely.
- If the sink is new and keeps clogging despite basic cleaning, consider expert help—this can be related to installation issues or recurring buildup like Recurring buildup in brand-new sink.
Safety Notes
- Wear gloves and eye protection when removing the trap or using a snake.
- Use a bucket to catch water and avoid spills that can create slip hazards.
- If you used any chemical cleaners, ventilate the area and avoid mixing cleaners with other products.
- Turn off appliance power if you need to work near electrical outlets or garbage disposals.
Common Homeowner Questions
- Why does it only clog with hot water? Hot water can melt grease or soften soap/mineral residues so they move and then re-solidify further down the line.
- Can I fix it myself? Often yes—cleaning the P-trap and using a hand snake solves many hot-water-only clogs.
- Are chemical drain cleaners safe? No—avoid them as a first choice; they can be dangerous and damage pipes.