Sink fills only on hot cycles

Quick Answer:

If the sink backs up only when hot water runs, a developing clog that softens or moves with heat — often grease — is the most likely cause. Compare how the sink behaves on cold and normal mixed-temperature cycles, then test the sink with hot water and inspect the drain path for grease-sensitive restrictions. Small clearings can sometimes be done by a homeowner, but recurring or large backups deserve a professional.

Why This Happens

Grease, soap scum, and food solids can stick to pipe walls. Hot water can soften grease and either let it slough loose or temporarily expand a partially blocked section. That change can be enough to make the drain overflow only on hot cycles. Other causes include a kitchen drain tied to appliance discharge (for example, a misrouted dishwasher) or a trap/pipe that expands slightly with heat and loses its normal flow pattern.

If the dishwasher is nearby, check the connection — a shared line or high discharge can trigger backups when an appliance uses hot water. Also consider recent work on the drain; a minor change or partial reassembly can let hot water reveal a weakness. If you’ve had recent service, watch for a Backup after plumbing repair.

Step-by-Step What to Do

1. Compare hot vs normal cycles

• Run a normal mixed-temperature cycle (warm tap, typical sink use) and note whether water drains smoothly.
• Then run a pure hot-water cycle and watch for filling, slow drainage, gurgling, or overflow.
• Record which cycle causes trouble — this helps isolate heat-sensitive blockages.

2. Test sink flow with hot water

• Carefully run a sustained stream of hot tap water into the sink (not boiling). Watch how fast water leaves the sink and whether any solids come back up.
• If the sink backs up during this test but not with cold water, you very likely have a grease-sensitive restriction that softens or shifts with heat.

3. Inspect the visible drain parts

• Remove and check the strainer and clean out any trapped debris.
• Place a bucket under the P-trap, loosen the slip nuts, and inspect the trap for grease or blockages. Clean it if you can do so safely.
• Check the dishwasher connection and air gap. If your appliance discharges into the same sink line, confirm the hose is installed correctly — sometimes the appliance creates hot-water backpressure. If you suspect a connection issue, look for signs that the Dishwasher drains into sink.

4. Try simple clearing methods

• Use a plunger on the sink (block the other basin if you have a double sink) and make several firm plunges.
• If that doesn’t work, a hand-crank drain snake for the sink tailpiece or the trap arm can catch and remove soft grease or hair clumps. Do not force threaded or glued joints.

5. Monitor and repeat tests

• After cleaning, repeat the hot vs mixed tests to confirm improvement.
• If clearing yields only temporary relief or the problem reappears on hot cycles, the restriction is likely deeper in the drain and needs professional attention.

What Not to Do

• Do not assume hot-only backups are harmless; heat-triggered issues often mean a developing clog.
• Do not pour boiling water or strong chemical drain cleaners into a pipe that may have older seals, plastic fittings, or a recent repair — those can be damaged and cause leaks or worse clogs.
• Do not force an auger or other tool past a tight spot if you feel resistance; you can damage pipes or joints.
• Do not ignore recurring backups. Small problems that show up only with hot water often grow into full clogs that affect all cycles.

When to Call a Professional

• You’re getting repeated backups after cleaning, or the same hot-only symptom returns within a few days.
• Several fixtures (sink, dishwasher, washing machine) are slowing or backing up together.
• There is foul-smelling sewage, visible leaks, or you see signs of pipe damage around fittings.
• If the issue began or worsened after work was done on your plumbing, bring in a plumber to diagnose a possible installation or partial blockage problem like a Backup after plumbing repair.

Safety Notes

• Let hot water cool slightly before you test by hand to avoid scalding. Use gloves when working in a drain.
• If you open the P-trap, set a bucket underneath and wipe up spills promptly to avoid slip hazards and water damage.
• Avoid mixing chemical drain cleaners with other methods — residue can splash or react. If you’ve used chemicals, warn the plumber; they can be hazardous.
• Don’t climb into cabinets or under sinks with wet electrical devices nearby. Turn off power to nearby garbage disposals before working on the sink.

Common Homeowner Questions

• Why does hot water cause backups when cold does not?
  Hot water can soften grease or shift a partial clog, allowing material to move and temporarily block flow.
• Can I fix this myself?
  Yes for small, accessible traps and soft grease — use a plunger, remove and clean the P-trap, or a hand snake; stop and call a pro if it’s beyond reach or returns quickly.
• Will this damage my pipes if ignored?
  Over time, recurring heat-triggered backups often lead to larger clogs, leaks, or odor problems, so it’s best to address them promptly.