• Drain Is Slow Only With Hot Water

    Drain Is Slow Only With Hot Water

    Quick Answer:

    If hot water flows slowly while cold water drains normally, the likely cause is grease or soap that liquefies with heat, travels a short distance, then re-solidifies where the pipe cools—often in the trap or near a branch line. Start by comparing flow rates, inspect and clean the trap and the drain walls, then perform a controlled hot flush to clear softened residue. If the problem is in a kitchen with a disposal or a shared branch, evaluate those components too.

    Why This Happens

    Grease, fatty foods, and some heavy soaps behave differently when warm. Hot water can melt or loosen these materials so they move with the flow, but as the water cools in the pipe the grease can congeal again and stick to the trap or pipe walls. Over time this creates a restriction that affects hot water more than cold. The same pattern can occur where branch lines meet, or around a disposal outlet.

    If you also notice persistent odors, check the links below for related problems: Drain Smells Like Moldy Towels and Drain Smells Like Sewage After Rain.

    Step-by-Step What to Do

    1. Compare hot vs cold flow

    • Run cold water and note how quickly it goes down for 20–30 seconds.
    • Run hot water and note the difference. Do this a few times to confirm the pattern.
    • If cold is fine and hot is slow, that points toward grease or soap that responds to temperature.

    2. Check for grease congeal in the trap

    • Place a bucket under the P-trap (sink) to catch water.
    • Loosen the slip nuts by hand or with pliers, remove the trap and inspect the inside for congealed grease or gunk.
    • If you find a sticky, greasy film or a solid plug, that’s likely the cause.

    3. Clean the trap and drain walls

    • Empty the trap contents into the bucket and wipe it clean with paper towels; seal and discard those towels in the trash (not down the drain).
    • Use a bottle brush or old toothbrush and a mild detergent to scrub the trap and the first few inches of the drain opening reachable by the brush.
    • For deeper cleaning, use a hand auger (plumber’s snake) to pull out sticky residue from the branch line; avoid forcing tools past fittings.

    4. Run a controlled hot flush after cleaning

    • Heat water in a kettle or use the hot tap (not boiling if the drain contains PVC piping). Boiling water can warp plastic.
    • Pour hot water slowly and steadily in 1–2 liter (quart) increments to gently melt remaining grease and carry it away. Repeat several times.
    • Follow with a cold-water rinse to verify improved flow.

    5. Evaluate disposal/branch line if kitchen

    • If you have a garbage disposal, check that it’s clear and that the knockout plug was removed when installed; run the disposal with cold water while testing flow.
    • If multiple fixtures on the same branch show similar symptoms, the clog may be in the shared branch line and will need a deeper auger or professional attention.

    What Not to Do

    • Do not pour fats, oils, or grease down the drain. They may flow when hot but will solidify and form blockages.
    • Avoid using lye-based (caustic) drain cleaners in older plumbing systems. These chemicals can damage older pipes and create hot reactions that make blockages worse or hazardous.

    When to Call a Professional

    • Repeated slow drains after cleaning, or a problem that returns quickly.
    • Multiple fixtures are slow or there is sewage backing up into drains or floor drains.
    • If you cannot access the clog, or if the pipes are old and you worry about damage from DIY methods.

    Safety Notes

    • Wear gloves and eye protection when handling trap contents or using tools.
    • Catch and dispose of greasy waste in sealed bags in the trash—never rinse it into the sewer system.
    • Do not mix chemical cleaners; if one has been used, avoid adding another. If chemicals were used recently, wait and ventilate before working on the drain.
    • Check pipe material before using very hot or boiling water—avoid boiling water on PVC and other plastics.

    Common Homeowner Questions

    • Why does cold water drain fine but hot is slow?
      Grease or soap melts with heat, moves a short way, then re-solidifies where the pipe cools—creating a hot-specific restriction.
    • Will pouring boiling water fix it?
      A controlled hot flush can help after cleaning, but boiling water can damage plastic pipes; use caution and pour slowly.
    • Can I prevent this from happening again?
      Keep fats and oils out of the drain, run cold water while using a disposal, and clean traps occasionally to remove buildup.