Quick Answer:
If the sink only slows when you dump a lot of water, the new discharge path is likely restricted. First run hot water for 30 seconds, then drain a full sink of water to confirm it only happens at high volume. If it does, check the trap arm for kinks or any uphill section and confirm the disposal outlet isn’t partially blocked by a mis-seated gasket before calling a plumber.
Why This Happens
When a disposal is added the drain geometry changes: the disposal outlet, the tailpiece, the trap and the trap arm must line up so water can flow freely. Small misalignments, a partially seated gasket, or a narrowed connector will work for small flows but slow down when you pour a lot of water at once. Flexible or accordion-style connectors tend to collect grease and debris and create a choke point. Occasionally the disposal outlet sits higher or lower than the old tailpiece and forces the trap arm into an uphill run that restricts flow. If you also hear odd sounds nearby, see Noise started after install for related checks.
Step-by-Step What to Do
1. Reproduce the problem the right way
- Run hot water from the tap for 30 seconds to warm and thin any grease in the line.
- Fill the sink and then remove the stopper so a full sink of water drains at once. Note whether the slow condition only appears with that high volume.
2. Turn off power and prepare
- Switch off the disposal at the circuit breaker before putting hands or tools near the unit.
- Place a bucket under the trap to catch water if you need to remove it.
3. Inspect the trap and trap arm
- Remove the P-trap and look for blockages. Clean any sludge or debris.
- Check the trap arm (the horizontal pipe to the wall): make sure it runs slightly downhill toward the wall. Look for kinks, sharp bends, or any uphill rise that would restrict flow.
4. Check the disposal outlet and gasket
- Look at the point where the disposal meets the tailpiece or flange. Confirm the gasket is sitting flush and not pushed into the outlet where it could partially block flow.
- If access is hard, loosen the mounting or tailpiece enough to see the gasket seating; do not power the unit while you do this.
5. Examine the connector type
- If there’s a corrugated or accordion-style connector, consider replacing it with a smooth-walled trap adapter. Corrugated tubing traps gunk and often causes slow drainage under heavy flow.
6. Reassemble and retest
- Put the trap back together, restore power, and repeat the hot-water + full-sink test. If the problem is corrected, the source was likely a localized restriction you cleared or corrected.
What Not to Do
- Don’t install an accordion connector to ‘make it reach’—they trap gunk and slow drains.
- If the disposal outlet height is wrong for the existing drain, don’t try to jury-rig a solution yourself; a plumber is appropriate for rerouting or adjusting the drain height.
- Don’t stick your hand into the disposal or run it while you’re working on fittings—always kill the power first.
When to Call a Professional
- The slow drain continues after you’ve cleared blockages and corrected alignment, especially under high-volume drain tests.
- If the plumbing needs the trap arm rerouted or the disposal outlet height adjusted to match existing drain lines.
- If you encounter electrical issues or the unit trips the breaker; see Disposal trips breaker after install for what an electrician or plumber will check.
Safety Notes
- Always turn off the disposal at the breaker before working on it. Do not rely on the wall switch alone.
- Use tools and gloves—avoid putting your hand into the disposal or the trap opening.
- Avoid pouring harsh chemical drain openers into the disposal; they can damage seals and are dangerous if you later put your hand inside the plumbing.
Common Homeowner Questions
- Why is the sink fine with regular use but slow when I dump a lot of water? A partial restriction will pass small flows but choke under high volume; that’s why the full-sink test is useful.
- Can I fix a mis-seated gasket myself? Often yes if you can access the joint and power is off; reseat or replace the gasket and retest.
- Is it OK to use a flexible connector? Smooth-walled connectors are better; avoid accordion-style connectors because they trap debris and cause slow drains.