Quick Answer:
If the toilet only runs while another fixture (like a sink or shower) is used, the most common causes are a marginal flapper seal or a fill valve that reacts to brief pressure changes. Run a faucet test while watching the tank to confirm whether pressure swings trigger the running; likely fixes are cleaning or replacing the flapper, adjusting or replacing the fill valve, or addressing a nearby appliance or smart device if recently installed.
Why This Happens
When you use a sink or shower the water demand briefly lowers pressure in the supply line. That change can do two things inside the toilet tank:
- Expose a borderline flapper seal: a flapper that doesn’t sit perfectly can lift a little when pressure changes, letting water leak out and causing the fill valve to open.
- Trigger a sensitive fill valve diaphragm: some valves react to short pressure dips and open briefly, making the toilet run until the pressure stabilizes.
Other triggers include recent hardware changes (for example, a new smart shutoff or a replaced fill valve). If something changed right before the problem started, check that part first—if you recently added a smart shutoff, see Toilet runs after installing smart shutoff.
Step-by-Step What to Do
1. Watch the tank while running another fixture
Run a sink faucet while watching the tank: if pressure changes trigger running, inspect for borderline flapper seal or a sensitive fill valve diaphragm. Use a slow-to-medium flow so you mimic normal use.
2. Check the flapper seal
- Turn off the toilet’s water supply at the shutoff valve and flush to empty most of the tank.
- Inspect the flapper for warping, mineral build-up, or a brittle edge. Clean the seating surface and the flapper with a soft cloth; if it’s damaged or old, replace it.
- After reinstalling or replacing, run the faucet test again to see if the running stops.
3. Observe the fill valve and refill tube
- With the tank full, trigger the sink faucet test again and watch the fill valve. If it opens briefly with pressure changes, the valve diaphragm may be too sensitive or worn.
- Check the refill tube height and position so it doesn’t siphon water from the overflow tube.
- If you recently replaced the fill valve, compare behavior to before—if the issue began after replacement, consult Toilet runs after replacing fill valve.
4. Try simple adjustments
- Adjust the float height or tension per the manufacturer’s instructions to change when the fill valve reacts.
- Tighten or reseat any loose supply-line fittings that could change how the valve senses pressure.
- Repeat the faucet test after each change.
5. Replace parts if needed
- If cleaning and adjustments don’t stop the behavior, replace the flapper first (it’s inexpensive and often fixes the issue).
- If the valve still reacts oddly, replace the fill valve assembly. Shut off the water, drain the tank, and follow the kit instructions or call a pro if unsure.
What Not to Do
- Don’t ignore fixture-triggered running—pressure changes can expose weak seals; if repeatable, a plumber is appropriate.
- Don’t force or overtighten plastic fittings; that can crack parts and make leaks worse.
- Don’t pour chemical drain cleaners into the toilet to try to stop running; that won’t fix internal tank parts and can be hazardous.
When to Call a Professional
- The running is repeatable and you cannot find a leaky flapper, faulty fill valve, or installation-related cause.
- Multiple fixtures behave oddly or water pressure problems affect the whole house—this could indicate a supply issue beyond the toilet.
- You’re not comfortable shutting off water and replacing tank parts, or a recent device (like a smart shutoff) may need professional setup.
Safety Notes
- Turn off the toilet shutoff valve before working inside the tank to avoid spills and unexpected flow.
- If the home has any electrical devices near plumbing (smart shutoffs, electronic valves), turn power off or follow manufacturer safety steps before touching them.
- Use basic hand tools and avoid excessive force; if a part won’t come loose, stop and get help to avoid breakage.
Common Homeowner Questions
- Why does it only run when I use the sink or shower? Pressure dips from other fixtures can lift a marginal flapper or briefly open a sensitive fill valve.
- Can I fix this myself? Often yes—cleaning or replacing the flapper and adjusting or replacing the fill valve usually solves it.
- Will this waste a lot of water? It can—intermittent running adds up over time, so test and fix it promptly.
For more related articles, see the nan hub.