Quick Answer:
If your drain runs normally with cold water but slows when you run hot water, the most common cause is grease or soap build-up that softens and shifts with heat. Compare how the sink behaves with cold versus hot water and try simple traps and clog checks first. If those checks don’t help, call a plumber.
Why This Happens
- Grease and fat: Solid grease can sit in the pipe near the trap or the drain line. Cold water may flow around it, but hot water softens or melts the grease so it moves and partially blocks the opening.
- Soap scum and hair: Soap mixed with hair can form a sticky plug that changes shape with temperature, letting cold water slip by but trapping hotter flow.
- Trap or vent issues: A partially blocked P-trap or a slow vent can react differently under the pressure and flow changes caused by hot water.
- Recent work on fixtures: If this began after changing a fixture, it could be a connection or alignment issue. See Slow drain after installing new faucet for related checks. If you added a disposal, check the unit and the connection — see Slow drain after garbage disposal install.
Step-by-Step What to Do
Step 1 — Compare hot and cold water drainage
Fill the sink partially with cold water and let it drain while you watch and listen. Repeat with hot water and note any differences in speed, bubbling, gurgling, or backflow. This direct comparison helps confirm the problem reacts to temperature and narrows down likely causes.
Step 2 — Clear the strainer and visible debris
- Remove the sink stopper or strainer and clear any hair, soap, or food bits you can see.
- Rinse the removed parts in a bucket and inspect for grease buildup.
Step 3 — Plunge the drain
- Use a sink plunger (seal the overflow if present) and plunge several times with cold water first, then try hot water to see if the blockage shifts or clears.
Step 4 — Check and clean the P-trap
- Put a bucket under the trap, loosen the slip nuts, and remove the trap. Clean out any sludge or grease and inspect the pipe for buildup further down.
- Reassemble carefully and test with cold and hot water.
Step 5 — Use a hand auger if needed
- If the P-trap is clean but the drain still slows with hot water, use a hand drain snake to reach further into the line. Avoid force; work gently to pull out hair and greasy plugs.
Step 6 — Monitor and test
- After cleaning, run both cold and hot water for a minute and watch drainage. If the slow behavior persists only with hot water, you likely have grease or a soft blockage that needs more aggressive removal or professional cleaning.
What Not to Do
- Do not force grease deeper with hot water.
- Don’t pour harsh chemical drain cleaners into a hot-water-reactive clog — they can be dangerous, may not reach the blockage, and can damage pipes or finishes.
- Avoid using excessive force with tools that can damage the trap or fittings.
When to Call a Professional
- The clog returns quickly after cleaning or only responds briefly to DIY fixes.
- Multiple drains in the house are slow or toilets gurgle when you run hot water — this suggests a deeper sewer or vent issue.
- You find signs of corrosion, leaking connections, or you’re uncomfortable removing the trap or snaking the line yourself.
Safety Notes
- Wear gloves and eye protection when working under the sink or handling clogged material.
- If you use any chemical or enzymatic cleaner (use sparingly), follow the manufacturer’s label and never mix products.
- Turn off power before working near a garbage disposal and avoid putting your hand into the disposal opening.
Common Homeowner Questions
- Q: Will pouring boiling water fix it? A: Boiling water can sometimes soften grease, but it may also push the grease farther down where it re-solidifies. Use caution.
- Q: Can I use store-bought drain cleaners? A: Enzyme-based cleaners can help organic buildup; avoid caustic chemicals if you plan to work on the trap or have older pipes.
- Q: Is this an emergency? A: Not usually. If water backs up into other fixtures or you have sewage smell or continued leaking, call a plumber promptly.