Quick Answer:
If running hot water in one basin makes water rise or flow into the other basin, the likely causes are a partial blockage in the shared drain, a problem with the venting, or an air pocket that behaves differently with hot water. This is usually not the water heater affecting drainage—start by comparing how hot and cold water behave and follow basic checks before calling a plumber.
Why This Happens
- Many double-basin sinks share the same tailpiece, trap and drain line. When the hot side moves faster or creates different air pressure, water can be pushed into the other bowl.
- Hot water can loosen grease or expand trapped air, temporarily changing flow and letting water pass sideways instead of down.
- Restricted vents or partial clogs down the line make the system sensitive to temperature and flow rate, leading to symptoms like Opposite bowl gurgles.
Step-by-Step What to Do
1. Compare hot and cold flow
- Run only cold water in the affected basin for 30–60 seconds. Watch the other bowl for any movement or backing up.
- Run only hot water for the same time and compare. Note if the opposite bowl behaves differently with hot than with cold.
- Record what you see—this tells you if the problem is temperature-related or simply a general clog.
2. Try simple clearing with the stopper removed
- Remove stoppers and any strainers. Cover the other basin’s drain with a wet cloth to direct pressure toward the problem side.
- Use a sink plunger on the affected side only, keeping a tight seal. Plunge gently and then test both basins.
3. Check the P-trap and tailpieces
- Put a bucket under the trap, loosen the slip nuts and remove the P-trap. Clean out hair, grease and debris.
- Inspect the tailpieces and the connection between basins for buildup or misalignment, then reassemble and test.
4. Look farther down the drain
- If the trap is clear but the issue persists, use a hand auger (drain snake) gently into the sink drain to 4–6 feet. If you reach resistance, try to break it up and pull debris out.
- After snaking, run cold and then hot water to compare behavior again.
5. Check vents and other fixtures
- Try running water in other drains (tub, washer) to see if they affect sink behavior. A shared, partial clog or blocked vent can cause cross-flow when flow or temperature changes.
- If you see water moving back into the sink when other fixtures run, that suggests a deeper line or vent issue and may require a professional.
6. Test for recurring issues
- After cleaning, test with a steady hot flow for several minutes and watch the opposite bowl. If the problem returns only with hot water, note timing and any noises.
- If the situation shows signs similar to a full backup—such as water rising into a normally dry basin—treat it like a more serious blockage or venting failure, similar to a Sink backs up into clean bowl scenario.
What Not to Do
- Do not adjust water heater settings to fix drainage.
- Do not pour large amounts of chemical drain cleaner into the sink when a basin is backing into another bowl—these chemicals can damage pipes and make the problem worse.
- Do not remove plumbing parts beyond your comfort level; avoid cutting or forcing metal or plastic fittings without the right tools.
- Do not ignore repeated backups—small problems can become sewage backups if left unchecked.
When to Call a Professional
- When plunging, snaking and trap cleaning don’t stop the cross-flow or backups.
- If the issue comes back quickly after clearing, indicating a deep blockage or venting problem.
- If you smell sewer gas, have slow drainage throughout the house, or see sewage backing toward fixtures—call a plumber right away.
Safety Notes
- Shut off the water and place a bucket under the trap before disassembling to avoid spills.
- Wear gloves and eye protection when handling drain debris or using a hand auger.
- If you suspect corrosive chemicals were previously used, ventilate the area and avoid mixing chemicals—call a pro if you’re unsure.
Common Homeowner Questions
- Why does hot water cause the other bowl to fill?
Hot water can loosen grease and change air pressure in the drain, letting water move into the shared path instead of downward. - Can I fix this with a store-bought snake?
A hand auger often helps for short clogs; if you can’t reach or clear the blockage, contact a plumber. - Is this the same as a vent problem?
It can be. Blocked vents change how air and water move in the system and can make temperature differences show up as cross-flow.