Quick Answer:
If the sink drains normally with cold water but slows, bubbles, or backs up when you run hot water, it usually points to a localized clog, a connection to a dishwasher or disposal, or a venting issue. This is rarely an immediate catastrophic failure. Run the short diagnostics below to isolate whether the problem is the faucet, the trap, a connected appliance, or the venting before calling a plumber.
Why This Happens
Hot water changes how grease, soap, and residues behave. It can melt and move a greasy clog, reveal air-locks in venting, or expand seals and fittings enough to shift misaligned parts. Common homeowner-level causes are:
- Grease or soap softening and moving into a narrow spot farther down the line.
- Backflow or discharge from a dishwasher or garbage disposal that kicks in when hot water runs through the shared drain.
- P-trap or slip-joint seals that expand or loosen with heat, changing the drain path.
- Partial vent blockage that allows gurgling or slow draining when warm water creates stronger pressure differences.
Step-by-Step What to Do
Step 1 — Reproduce and observe safely
- Run cold water, then run only hot water, and note the exact behavior: gurgling, slow drain, backflow into sink, or gushing from other fixtures.
- Do not touch hot metal parts or try to remove fittings while pipes are hot. Let water cool before handling.
Step 2 — Isolate connected appliances
- Turn off and unplug the garbage disposal, then run hot water. If the problem stops, the disposal or its connection may be the cause.
- Run the dishwasher on a drain cycle or watch its drain path while you run hot water; a failing check valve or a blocked air gap can let dishwasher water flow back when hot water changes pressure.
Step 3 — Check the faucet and aerator
- Remove and inspect the aerator for debris or altered spray patterns after hot water use; buildup can act differently with heat.
- If the spray looks off or redirects when hot water runs, consult the guidance in Kitchen Sink Sprays Sideways From Aerator.
Step 4 — Inspect the trap and slip joints
- Place a bucket under the trap, loosen the slip nuts carefully, and inspect the P-trap for grease, hair, or a deformity that might shift with heat.
- Hand-tighten slip nuts to test; avoid over-tightening. If the trap moves or feels unstable when hot water runs, see Kitchen Sink Trap Keeps Twisting Loose for more on common causes and fixes.
Step 5 — Run venting and house-wide checks
- Listen for gurgling in other drains while running hot water in the kitchen. Gurgling across multiple fixtures often points to a vent or main drain issue.
- Do not climb on the roof to clear vents yourself unless you have the right safety gear and experience. Instead, use the other checks to determine if a professional is needed.
Step 6 — Try a safe, basic drain clear test
- After removing obvious debris, run very hot (but not boiling) water for a few minutes to see if flow improves. This can move grease without harsh chemicals.
- If flow improves temporarily, a deeper grease buildup is likely and may need mechanical cleaning.
What Not to Do
- Avoid forcing fittings or twisting pipes with excessive force — that can crack plastic or strip threads.
- Do not pour caustic drain chemicals into the sink. They can damage pipes, harm seals, and create hazardous conditions if mixed with other cleaners.
- Do not ignore early warning signs like smell, gurgling, or intermittent slow draining — small issues often become bigger and more expensive to fix.
When to Call a Professional
- Persistent backups or repeated slow drains after clearing the trap and checking appliances.
- Gurgling in multiple fixtures, sewage smell, or signs of a main-line or vent-stack blockage.
- Visible leaks, damaged fittings, or if you’re uncomfortable removing traps and disconnecting appliance lines.
- If a plumber is needed, describe the exact symptoms you found during the diagnostic steps — it will save time and cost.
Safety Notes
- Never work on plumbing while fixtures are hot enough to burn. Let hot water cool before touching pipes or fittings.
- Wear gloves and eye protection when handling debris from the trap or working near wastewater.
- Avoid mixing chemical cleaners. If someone has used a chemical product already, ventilate the area and do not add other cleaners.
Common Homeowner Questions
- Why does hot water make a clog worse?
Hot water can melt grease or loosen soap scum, letting it shift into a tighter spot and temporarily make the drain worse. - Can I fix this myself?
Often you can isolate and fix simple issues by checking the trap, disposal, and aerator. Call a pro if the problem is beyond basic checks or if multiple fixtures are affected. - Will vinegar and baking soda help?
They can help with mild residue but are not effective for heavy grease clogs. They are safer than caustic chemicals but may only provide a temporary improvement.