Quick Answer:
If the sink only drains slowly when you run hot water, the temperature is usually changing how a partial clog behaves rather than damaging the pipes. Common causes are grease or soap that melts and sticks, hair and soap scum shifting, or a restricted trap that hot water makes more obvious. Start by comparing hot and cold flow and then work through simple checks before using harsh chemicals or calling a plumber.
Why This Happens
- Grease and fatty residues melt with hot water and can smear onto the pipe walls or move and re-seat on a blockage, making the drain slower only with hot water.
- Soap scum and mineral scale soften when warm and can change shape, briefly narrowing the passage.
- Air or venting issues are less common but can change how water flows under different temperatures.
- Normal household hot water temperatures are not likely to harm pipes; the issue is usually the material inside the drain reacting to heat.
Step-by-Step What to Do
1. Run hot vs cold to compare behavior
Turn the cold tap on full for 30–60 seconds, then run the hot tap for the same time. Note whether the slow drainage only happens with hot, only with cold, or both. This quick test confirms whether temperature is linked to the problem.
2. Remove and clean the strainer or stopper
Take out the sink strainer or pop-up stopper and clean any hair, bits of food, or gunk. Small visible blockages often cause temperature-dependent symptoms.
3. Use a plunger
With water level low but covering the drain, plunge firmly for 10–20 seconds, then check drainage with hot and cold. Repeat a couple of times if needed.
4. Flush with safe, controlled hot water
If your pipes are metal, pour hot (not boiling) water down the drain in stages to see if the flow improves. If pipes are plastic (PVC), avoid very hot or boiling water—use warm water instead.
5. Clean the P-trap if comfortable
Place a bucket under the trap, loosen the slip nuts, and remove the trap to clear debris. Reassemble carefully and test again. If you’re unsure, stop and call a pro.
6. Try a gentle biological or enzyme cleaner
If the clog seems organic (grease, hair, soap), a biological enzyme cleaner used according to label instructions can help over a few hours or overnight. Do not mix cleaners.
7. Check related situations
- If you recently worked on the faucet or plumbing, compare installation issues against **Slow drain after installing new faucet**.
- If the drain is slow only with cold water instead, read **Slow drain only on cold water** for steps tailored to that pattern.
What Not to Do
- Do not use only hot water to mask issue. That hides the symptom and may make the clog worse.
- Do not pour boiling water into PVC or plastic piping—use warm water instead.
- Do not mix household drain chemicals or use them immediately after biological cleaners; chemical reactions can be dangerous.
- Do not force metal tools deep into the drain without knowing what’s down there—you can damage the trap or finish.
When to Call a Professional
- Repeated slow drainage after basic cleaning and plunging.
- Multiple fixtures are slow or there are backups elsewhere in the home.
- Strong sewer smell, sewage backup, or signs of water damage under the sink.
- You find corroded or damaged pipes, or you’re uncomfortable removing the P-trap.
Safety Notes
- Wear gloves and eye protection when cleaning drains or removing the P-trap.
- Turn off water at the faucet or supply valves if you need to disassemble plumbing under the sink.
- Avoid mixing chemicals. If you’ve used a chemical drain opener, wait and ventilate before doing anything else and consider calling a pro.
- Do not use boiling water on plastic pipes. Boiling water can soften or warp PVC fittings.
Common Homeowner Questions
- Q: Can hot water damage my pipes? A: Not at normal household temperatures; hot water usually affects clog material rather than the pipes themselves.
- Q: Will boiling water clear a grease clog? A: Boiling water can melt grease, but it can also push grease further into a clog and can damage PVC—use warm water or professional help instead.
- Q: Is this a venting problem? A: Unlikely if only one sink is affected; vent issues usually show up as slow drains across multiple fixtures or gurgling sounds.