Quick Answer:

If cold water drains noticeably slower than hot, the most common cause is grease or fats that have solidified in the pipe or trap when the water is cool. Other possibilities include a partial clog near the fixture, a clogged vent, or a problem farther down the line. Start by comparing cold and hot drain rates and then work through simple checks you can do safely before calling a pro.

Why This Happens

Grease, oil and some soaps are liquid when warm but solidify as they cool. If grease has built up inside the drain or the P-trap, cold water will move more slowly because the pathway narrows. Hot water will pass more easily because it melts or thins the blockage while it runs. Other causes include hair and debris trapped at the stopper, mineral buildup, or a venting problem that affects flow under certain temperature conditions.

Step-by-Step What to Do

1. Confirm the difference (compare cold and hot drain rates)

Run a measured amount of cold water (for example, fill a 1‑gallon container) and time how long it takes to drain. Repeat with hot water and compare. Doing this comparison helps confirm the issue and rules out occasional slow drains from other causes.

2. Clear the visible stopper and screen

Remove the pop-up stopper, strainer or aerator and clear any hair or debris. Small obstructions at the top of the drain can cause big differences between hot and cold flow.

3. Use a plunger

Block the overflow (if present) and plunge the sink with a standard sink plunger. This can dislodge loosened grease or debris and restore normal flow without chemicals.

4. Remove and inspect the P-trap

Place a bucket under the trap, loosen the slip nuts, and remove the trap. Clean out any grease, gunk or hair. If the trap is full of greasy residue that hardens when cool, that confirms the cause.

5. Flush with hot water and dish soap

After removing gross debris, flush the drain with very hot (not boiling) water and a squirt of grease-cutting dish soap. For metal pipes hot water is fine; with plastic pipes, avoid boiling water to prevent warping. The soap helps dissolve grease and carry it down the line.

6. Check other fixtures and the vent

Run water in other nearby fixtures. If multiple fixtures are slow or you smell sewer gas, the problem may be farther down the line or a vent blockage. If the issue only affects the one fixture, it’s likely localized to that drain or trap. If you recently had service interruption or pressure issues, consult **Slow drain after city outage** for related checks.

7. Reassemble and retest

Put the trap and stopper back in place, then repeat the cold vs. hot drain test. If the cold still lags, the clog may be deeper and require a drain snake or professional help. If the problem is a sink, see **Bathroom sink draining slowly** for sink-specific steps.

What Not to Do

• Do not pour grease to test cold flow. Never add grease to your drain for any reason; it will create or worsen clogs.
• Do not pour boiling water into plastic pipes — it can soften or deform PVC. Use very hot tap water instead.
• Do not mix chemical drain cleaners with other chemicals or tools — they can cause dangerous reactions and damage pipes.
• Do not use a wire coat hanger to poke deep into drains — you can push clogs farther in or scratch pipes.

When to Call a Professional

• After you remove the trap and clear what you can but the drain is still slow.
• If multiple fixtures back up or you smell sewer gas — this can indicate a blockage in the main line or a vent issue.
• If you suspect a broken or misaligned pipe, or if you can’t access the blockage safely.
• If you’re uncomfortable performing the trap removal or using a drain snake yourself.

Safety Notes

• Wear gloves and eye protection when handling dirty traps or using chemicals.
• Ventilate the area if you use any chemical products; follow the product instructions exactly.
• Do not mix different chemical drain cleaners or add bleach to products containing acids or bases.
• Use tools designed for plumbing (plunger, bucket, slip-joint pliers, hand auger) and avoid improvised tools that can damage pipes.

Common Homeowner Questions

Why does cold water slow down but hot is fine? Cold causes grease and fats to solidify in the drain, narrowing the flow; hot water can temporarily melt or loosen that buildup.

Can I fix this myself? Often yes — clearing the stopper, plunging, and cleaning the P-trap plus flushing with hot water and dish soap will solve many cases. Stop and call a plumber if the clog is deep or multiple fixtures are affected.

Will pouring boiling water clear the clog? Boiling water may help in metal pipes but can damage plastic piping. Use very hot tap water with dish soap instead, unless you know your pipes are all metal.