Quick Answer:
If your toilet starts running after replacing the main shutoff, it’s often sediment or a disturbed fill valve part. First flush sediment from tubs and sinks, then inspect the toilet tank’s fill valve screen. Clean the screen; if the diaphragm or internal parts are damaged, replace the fill valve.
Why This Happens
- Work at the main shutoff stirs sediment and mineral deposits in the supply lines. That grit can reach toilet fill valves and clog or damage small screens and diaphragms.
- A pressure change or brief surge while restoring supply can shift a float, deform a diaphragm, or dislodge debris into the valve.
- Modern plastic fill valves have small parts that don’t tolerate sand or grit; once a diaphragm is nicked the valve may run continuously.
Step-by-Step What to Do
1. Restore water and flush nearby fixtures
- Turn the main supply back on slowly to avoid a pressure surge.
- Open tubs, showers and all sink faucets. Let them run for a minute or two to flush out any sediment stirred up by the work.
- Run the cold tap until the water appears clear.
2. Check the toilet tank
- Remove the tank lid and set it somewhere safe.
- Watch the fill cycle. If the water constantly trickles or runs into the overflow tube, note where the flow starts.
3. Inspect and clean the fill valve screen
- Shut off the toilet’s shutoff valve (if available) or work quickly with the main on after flushing lines.
- Some fill valves have a small mesh or plastic screen at the inlet where the supply attaches. Gently remove and rinse it under running water to clear grit.
- If the screen looks damaged or clogged beyond cleaning, plan to replace the valve.
4. Check the diaphragm and internal parts
- With the tank water lowered, inspect the fill valve diaphragm (or piston) for tears, warping, or mineral buildup.
- A cracked or deformed diaphragm will not seal properly. If you see damage, replacement is the reliable fix — repair kits don’t always restore proper function.
5. Replace the fill valve if needed, then test
- Replacing the entire fill valve is often faster and more reliable than repairing a damaged diaphragm. Follow the valve manufacturer’s instructions or the toilet’s repair guide.
- After replacement, turn the supply back on slowly, let the tank fill, and watch one or two cycles to confirm the running has stopped.
- If it still runs or keeps refilling, check the flapper/flush valve seal as well. For additional troubleshooting, see Toilet keeps running after flush and Toilet won’t stop filling.
What Not to Do
- Don’t skip flushing after main shutoff work — sediment can ruin new fill valves.
- Don’t assume the fill valve is fine without looking at the inlet screen and diaphragm; small damage is easy to miss.
- Don’t try complex tank repairs without a basic understanding — forcing parts or using inappropriate sealants can make the problem worse. If running continues, a plumber is appropriate.
When to Call a Professional
- If the toilet still runs after you’ve flushed sediment and replaced a damaged fill valve, call a plumber — persistent running can mean a more complex supply or valve issue.
- If you detect leaks at the supply connection, cracks in the tank, or you’re not comfortable replacing valves, hire a licensed plumber to avoid breaks or water damage.
- If multiple fixtures are showing sediment or low pressure issues after the shutoff work, a pro can test and clear the house supply safely.
Safety Notes
- Work with the water off when possible, and be ready to stop and shut the main off again if you find a leak during repairs.
- Use gentle force on plastic parts. They can crack if overtightened or pried roughly.
- Keep the tank lid stable and avoid placing it where it can fall and break — a broken lid can cause injury or additional mess.
Common Homeowner Questions
- Why did this start after the main shutoff work? Sediment and pressure changes from the job commonly dislodge grit that harms delicate fill valve parts.
- Can I clean the diaphragm instead of replacing the valve? You can try cleaning, but a torn or warped diaphragm should be replaced—replacement valves are inexpensive and more reliable.
- How long should testing take after repairs? Watch two or three fill cycles (about 10 minutes) to confirm the running has stopped and no leaks appear.
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