Quick Answer:
If hot water backs up while cold drains normally, the most common cause is a soap, grease, or body-oil residue that softens with heat and sticks to the pipe or trap. Start by removing the stopper and checking for visible buildup, then clear or clean the trap. If the problem continues, inspect the tailpiece and drain line for grease or other residues.
Why This Happens
Hot water can change how residues behave. Soap scum, shaving cream, hair oils and small amounts of grease can soften or liquefy when heated, then redistribute and cling to pipe walls farther down the drain. Cold water keeps those residues harder and they can let water pass more easily.
Left unchecked, this can lead to recurring slow drainage or other issues closely related to sink odor or flow problems — see Bathroom Sink Smells Like Sewer Gas After Sitting for related signs to watch for.
Step-by-Step What to Do
1. Clear the area and remove the stopper
- Put a bucket under the P-trap to catch water. Remove any items from under the sink.
- Remove the pop-up stopper or lift assembly and check for hair and soap buildup. Wipe or pick out any debris you find.
2. Check for visible soap buildup
- Look down the drain and into the tailpiece. A white or light-colored film is often soap scum.
- Use an old toothbrush or a small bottle brush with warm soapy water to scrub the tailpiece and stopper parts.
3. Remove and inspect the trap, then flush it
- Loosen the slip nuts on the P-trap and remove it into the bucket. Inspect for gunk, hair, or greasy residue.
- If the trap is metal, you can flush it with boiling water. A safer option for PVC or unknown materials is to rinse it with very hot tap water or pour boiling water into the trap after it is removed (outside or into a suitable container) to dissolve residues, then brush and reassemble.
- Reinstall the trap and test with both hot and cold water.
4. Inspect for grease residue farther down
- If the trap was clear but hot water still backs up, the residue may be farther in the drain line. Try running very hot water and a plunger on the sink to dislodge softened deposits.
- For recurring issues, mechanical cleaning with a hand auger or removing the tailpiece and snaking the drain may be needed.
5. Test and observe
- Run cold water, then hot water, and watch the flow. If hot backs up but cold is fine, you’ve likely addressed the building residue. If the problem persists or gets worse, proceed to the next steps or call a pro.
- If you see odors or other fixtures are affected, check the linked guidance on related problems such as Bathroom Sink Fills Then Drains Slowly.
What Not to Do
- Do not use lye-based cleaners. They can damage pipes, harm finishes, and are dangerous to handle.
- Don’t pour boiling water into PVC drain piping without verifying the material can handle it — remove the trap first if you plan to use boiling water.
- Avoid mixing chemical drain cleaners with other products; chemical reactions can create hazardous fumes or splashes.
When to Call a Professional
- Multiple fixtures are slow or backing up — this can indicate a main line or venting problem.
- Hot water backups return after cleaning, or you find hardened grease or scale beyond the trap.
- There are strong sewer odors, visible sewage, or repeated blockages despite basic cleaning.
Safety Notes
- Wear gloves and eye protection when working on drains. Drain water can be dirty and may contain chemicals.
- Use tools carefully: overtightening trap fittings can crack plastic; use adjustable wrenches and hand tools gently.
- Never mix household chemicals. If you’ve used one product, flush and ventilate thoroughly before trying another method.
Common Homeowner Questions
- Why does hot water make it worse? Hot water softens soap and grease, allowing it to move and stick deeper in the pipe.
- Can I fix this without removing the trap? You can try removing the stopper and cleaning the tailpiece first, but removing the trap is often the most effective way to clear trapped gunk.
- Will vinegar and baking soda help? They can help loosen light buildup but are not a cure for heavy grease or hardened soap scum.