Quick Answer:
If a sink starts slowly and then suddenly speeds up, it usually means a partial blockage, air pocket, or shifting debris in the trap or drain line. Run water steadily and watch for changes to help locate the behavior, then follow simple checks to clear or contain the problem.
Why This Happens
Drain flow can change as debris, grease, or soap scum moves in the pipe. A partial clog can let water pool, then break free in a lump that rushes down and makes the flow look normal until the next buildup. Other causes include a blocked P-trap, poor venting that traps air, or problems higher in the line where the blockage shifts.
If the issue started after a recent fix or service, consider that tools or materials may have left a residue—see Slow drain after plumbing repair for similar scenarios to watch for.
Step-by-Step What to Do
Step 1: Observe the behavior
- Run water steadily at a moderate flow for 30–60 seconds and watch how the drain reacts. Note whether it starts slow and then speeds up, or fluctuates with bursts.
- Listen for gurgling noises in the sink or nearby fixtures—this can suggest an air pocket or venting issue.
Step 2: Clear the strainer and visible debris
- Remove and clean the sink strainer or stopper. Hair, food particles, and grease often sit on top and cause partial slowdowns.
- Run water again to see if the problem improves.
Step 3: Use a plunger
- With the sink partially full, use a cup plunger over the drain and give several firm pumps to dislodge trapped material.
- Run water steadily and watch for change.
Step 4: Inspect and clean the P-trap
- Place a bucket under the trap, loosen the slip nuts, and remove the P-trap to clear trapped debris. Reassemble carefully and check for leaks.
- Run water steadily again to confirm the flow.
Step 5: Try a hand auger for deeper clogs
- If clearing the trap doesn’t fix it, feed a short hand auger (plumber’s snake) into the drain to reach clogs further down.
- After pulling material out, run steady water and watch for consistent flow.
Step 6: Check other fixtures and vents
- Run nearby sinks or the toilet to see if multiple drains are affected—multiple slow drains suggest a main line or vent issue.
- If the problem follows a city water interruption or pressure surge, consider guidance for outages and pressure changes described in Kitchen sink slow after city outage.
What Not to Do
- Do not ignore fluctuating drain behavior.
- Do not pour strong chemical drain cleaners down the sink—these can damage pipes and create dangerous fumes, especially if mixed accidentally with other cleaners.
- Do not force fittings apart without knowing what you’re doing; cross-threading or overtightening can cause leaks.
When to Call a Professional
- Persistent slow drains after you’ve cleared the trap and tried a plunger or hand auger.
- Backups in multiple fixtures or sewage odors—these indicate a main-line issue.
- Leaks you can’t stop, corroded pipes, or repeated clogs—these warrant a pro inspection and possible camera diagnosis.
- If you’re uncomfortable removing the trap, working under the sink, or if the home has complex plumbing, call a licensed plumber.
Safety Notes
- Wear gloves and eye protection when working under the sink or handling debris.
- If there’s a garbage disposal, always disconnect power before working on the drain or trap.
- Do not mix household chemicals; if chemical cleaners have been used, ventilate the area and avoid further chemical use.
- Use a bucket to catch water when removing the P-trap and check all slip nuts for a watertight reassembly.
Common Homeowner Questions
- Q: Will pouring boiling water fix this? A: Boiling water can help dissolve grease near the top of the drain, but it won’t clear deeper partial clogs.
- Q: Is a fluctuating drain an emergency? A: Not usually, but don’t ignore it—if it worsens or you get backups, call a plumber.
- Q: Can I use a chemical drain cleaner safely? A: It’s better to avoid them; they can damage pipes and are hazardous if misused.