Overflow only when hot water used

Kitchen sink backing up only with hot water running from the faucet.

Quick Answer:

If the sink only backs up when you run hot water, the most likely cause is softened grease or fat shifting in the drain and temporarily blocking flow. Run a quick comparison test with hot then cold water, check the trap for greasy sludge, and avoid pouring boiling water into PVC. If hot water consistently triggers the rise, call a plumber.

Why This Happens

Grease, cooking fat and soap scum can build up as a semi-solid coating inside pipes. Cold water keeps that buildup firm and partly open; hot water softens or melts it so pieces shift and can close off the path. A partial clog that only closes when warmed is a common reason a sink overflows with hot water but drains fine with cold.

Other contributors include a full or faulty P-trap that collects greasy sludge, or a downstream restriction where softened material moves and wedges. Venting problems are less common for this symptom but still possible.

If the situation started after a guest visit or after changing fixtures, check related issues like Sink backs up when guests use water or, if you recently added equipment, compare with Overflow after installing filter.

Step-by-Step What to Do

1. Run a controlled hot-then-cold test

  • Start with a measured flow: run hot water into the sink for 30–60 seconds and note how quickly it drains and whether the water level rises.
  • Immediately switch to cold water at the same flow rate and compare. If the water rises only with hot flow, that points to grease softening and shifting.

2. Use cold water for follow-up testing

  • If hot water causes the rise, repeat testing using only cold water to confirm the sink will remain clear when the line is cool.
  • Keep running cold water if you need to use the sink until the clog is cleared or a pro evaluates the line.

3. Check and clear the P-trap

  • Place a bucket under the trap, loosen the slip nuts and remove the trap. Inspect for greasy sludge, solidified fat or debris and wipe it out.
  • Clean the trap by hand (wear gloves) and reassemble. Many grease problems are visible and removable at this point.

4. Try simple clearing methods if the trap is clean

  • Use a plunger on the sink (block overflow if present) and push a few times to dislodge soft blockages.
  • If the plunger doesn’t work and the trap was clean, a short hand snake from the sink side can reach soft material. Don’t force a long rigid snake into plastic fittings.

5. Observe and repeat tests

  • After cleaning, run the same hot-then-cold test. If the drain still backs up only when hot, the blockage is likely farther down or re-forming from residual grease.
  • If cleaning helps only temporarily, avoid using hot water for heavy rinsing until a permanent fix is made.

What Not to Do

  • Don’t pour boiling water into PVC to melt grease—this can damage joints.
  • If hot water reliably triggers overflow, a plumber is appropriate.
  • Avoid mixing chemical drain cleaners with other chemicals or trying strong acids—those can harm pipes and your health and rarely fix grease blockages long-term.

When to Call a Professional

Call a plumber if any of the following apply:

  • Hot water reliably triggers overflow even after you clean the trap and try a plunger.
  • There is a strong sewage smell, repeated backups, or water backing into other fixtures.
  • You cannot access the clog or the plumbing uses complex or glued joints you don’t want to risk damaging.

A pro can inspect farther down the line, use a drain camera to locate greasy deposits, and recommend cleaning or pipe work that prevents recurrence.

Safety Notes

  • Wear protective gloves and eye protection when opening traps or using tools.
  • Turn off water if you disconnect fittings, and have a bucket to catch spills.
  • Do not mix chemicals. If you’ve already used one drain product, wait and flush with plenty of water before attempting another method or touching pipes.
  • Beware of scalding when working with hot water—test temperature carefully.

Common Homeowner Questions

  • Why does the sink only back up with hot water? — Grease or fat softens with heat and shifts to block the pipe.
  • Will pouring boiling water fix it? — No; boiling water can damage PVC and may only temporarily move the grease without removing it.
  • Can I clear this myself? — You can check and clean the trap and try a plunger or short snake, but call a plumber if the problem returns or hot water consistently causes overflow.

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