Quick Answer:
This odd symptom—one fixture is weak but suddenly improves when another tap runs—usually points to a sticking cartridge or check valve at the affected fixture, or to how a pressure‑balancing device is reacting. It can also indicate a cross‑connection inside a single‑handle faucet. Try simple tests (partially operate the shutoffs, run other taps to observe behavior) and clean or replace the cartridge/check if needed.
Why This Happens
Most modern single‑handle faucets and some shower valves use cartridges, check valves, or pressure‑balance mechanisms. If a check or the cartridge sticks, flow at that fixture may be restricted until the system pressure changes. Running another fixture can briefly shift pressure or force the stuck part to move, so the weak tap looks better while the other is open. A similar result can occur if a cross‑connection inside the faucet lets water flow back or equalize when other fixtures run.
Step-by-Step What to Do
1. Confirm the symptom
- Run the weak fixture and note how low the flow is.
- Have someone open a different tap (cold or hot) and watch the weak fixture for improvement.
- Try both hot and cold to see if the problem is one side only.
2. Check the shutoff stops under the fixture
- Locate the hot and cold stops under the sink or behind the access panel.
- Open them fully, then partially close one side to see if flow changes—this helps identify if a valve or the cartridge is reacting to pressure changes.
- If stops are stiff, operate them carefully; don’t force them.
3. Inspect and test the cartridge/check valve
- Turn off the fixture’s local stops (or the main if needed) and relieve pressure by opening the faucet.
- Remove the handle and trim to expose the cartridge or ceramic disc assembly.
- Look for debris, mineral buildup, or a part that moves inconsistently. Manually move the cartridge to feel for sticking.
- With the cartridge out, briefly turn water back on (catch water in a bucket) to check if flow improves—this helps confirm the cartridge as the source.
4. Clean or replace the cartridge/check
- Soak removable parts in a mild vinegar solution to dissolve mineral deposits, rinse thoroughly, and reassemble to test.
- If cleaning doesn’t fix it, replace the cartridge or check valve with the correct part for your faucet model.
- Reinstall carefully, test both hot and cold, and ensure stops are fully open.
5. Verify there’s no cross‑connection through the faucet
- Cross‑connections inside a single‑handle faucet can allow pressure from another line to change flow or introduce contamination.
- If the fixture improves only when another tap runs, check the cartridge seals and internal check valves; replace them if worn.
- Don’t ignore signs that water from one side is mixing into the other—this can be a health risk.
6. Test again and monitor
- With everything back together, test the fixture alone and while other taps run.
- Monitor over a few days to confirm the issue is resolved and doesn’t return when the system pressure changes.
What Not to Do
- Don’t ignore cross‑connection hints—if ‘improves when others run,’ test cartridges/checks before it becomes a contamination issue.
- Don’t force corroded fittings or use excessive heat or impact tools on valves; that can break parts or create leaks.
- Don’t rely on random chemical cleaners to fix a mechanical sticking cartridge—cleaning or replacement is usually needed.
When to Call a Professional
- If the cartridge or valve is behind a wall or hard to access.
- If you suspect a true cross‑connection or backflow that could contaminate potable water.
- If replacing the cartridge doesn’t stop the problem or if multiple fixtures behave oddly at once.
- When you’re not comfortable turning off the main supply or working with plumbing connections.
Safety Notes
- Always shut off the correct shutoff stops or the main water before disassembling a faucet, and open the handle to relieve pressure.
- Use hand protection and eye protection when working with old parts or cleaning solutions.
- If you must remove a cartridge and turn the water on for a check, catch water in a bucket and keep electrical devices away from the work area.
If your situation varies depending on the time of day, also consider checking **Pressure fine in morning but poor at night**. If this began suddenly with a kitchen fixture, review **Kitchen sink pressure suddenly dropped** for related checks.
Common Homeowner Questions
- Why does running another tap fix it temporarily? A second tap changes system pressure or flow paths and can unstick a valve briefly, revealing a failing cartridge/check.
- Can I just leave it until it gets worse? No. Signs of cross‑connection or sticking parts can lead to contamination or a sudden failure, so test and repair soon.
- Is cartridge replacement difficult? For many sink faucets it’s a straightforward DIY task if you have basic tools and the correct replacement part; stop and call a pro if access is limited or you’re unsure.
For more related articles, see the Low Pressure at Single Fixture hub.