Q: Why does water back up from one sink into the other?
A: In most dual-sink setups, both drains connect to the same horizontal branch before reaching the main line. If that section is partially clogged, the water from one basin pushes air and debris into the other. That’s why you see water rise evenly in both sinks — it’s pressure balancing through the clog.
Q: Why didn’t the auger or sewer jetter fix it?
A: Kitchen clogs often form from a mix of grease, food residue, and detergent buildup. These create sticky deposits that are hard to cut through with a hand auger or small jetter hose. If your line has multiple elbows or long runs before reaching the stack, the tools can’t reach the main obstruction.
Q: Can an air admittance valve (AAV or ACC) cause drainage problems?
A: Yes, if it’s installed incorrectly or is failing. The valve allows air into the system to prevent vacuum lock. If it’s clogged or missing a proper vent path, your sinks may gurgle and drain slowly. Always mount it above the P-trap and ensure it opens freely when draining water.
Q: What should I try before calling a plumber?
A: Start with these steps:
- Check that your horizontal drain pipe has a slight downward slope — no dips or traps in the line.
- Remove the P-trap and inspect for solid buildup. Clean it thoroughly with hot water and detergent.
- Use a longer drain snake from the basement clean-out instead of under the sink — it may reach deeper clogs.
- Flush the line with near-boiling water mixed with dish soap after snaking to remove leftover grease.
Q: What if the blockage is still there?
A: Then it’s likely the clog is deeper — often where the kitchen branch ties into the main sewer. This is where professionals use motorized drain machines or camera scopes to find the obstruction. It’s an extra cost, but it prevents future backups and reveals issues like collapsed pipes or heavy grease buildup.
Q: How can I prevent this in the future?
A: Run hot water for 30 seconds after each use, avoid dumping grease down the sink, and occasionally pour a mix of vinegar and baking soda to keep pipes clear. Installing a proper vent and maintaining clean-outs also helps reduce recurring clogs.
For a detailed step-by-step on clearing stubborn kitchen clogs, check out the Family Handyman kitchen sink guide.
I’m Chris Mayer, writing for Plumbing 101.