Quick Answer:
If a toilet keeps refilling after you replaced the fill valve, the most likely problem is a worn flapper or a failing flush-canister gasket. Start by inspecting and replacing the flapper or flush seal rather than buying another fill valve. After replacing the seal, run a few tests and watch the tank for several minutes to confirm the leak is fixed.
Why This Happens
When a toilet refills at random, water is entering the tank because it is leaking out of the bowl or overflowing into the refill tube. Common causes include:
- A flapper or flush-canister gasket that no longer seals properly due to wear or mineral buildup.
- A chain that is too tight or snagged, preventing the flapper from seating.
- A cracked seat or warped canister that lets water bypass the seal.
- Changes in house water pressure or plumbing work elsewhere that cause occasional triggers — if refills line up with weather or other work, see Phantom flush during storms.
Step-by-Step What to Do
1. Observe and confirm the pattern
- Watch the tank after a flush. Note whether the refill is continuous, intermittent, or only at certain times (e.g., after other fixtures run).
- Put a few drops of food coloring into the tank, wait 10–20 minutes without flushing, and see if colored water appears in the bowl. If it does, water is leaking past the flapper/seat.
2. Shut off the water and drain the tank
- Turn the shutoff valve clockwise at the toilet base and flush to empty the tank.
- If the valve is stuck or the tank won’t drain fully, place a towel beneath the tank to catch any spills.
3. Inspect and replace the flapper or flush seal
- Remove the flapper or canister top and inspect the rubber seal and seat for cracks, hardening, or mineral deposits.
- Clean the seat with a soft cloth to remove deposits; if the seal still shows wear or doesn’t seat evenly, replace it. This is usually a simple, inexpensive part and should be tried before replacing the fill valve again.
- When fitting a new flapper or flush seal, make sure it matches the type of flush valve (flat flapper, tapered, or canister-style gasket).
4. Adjust chain and water level
- Set the chain so there is a small slack when the flapper is closed—no tugging, no excess slack that can get under the seal.
- Set the water level to the marked line on the overflow tube or about 1 inch below the top of the tube so water won’t spill into it during normal operation.
5. Recheck other parts and recent changes
- Confirm the overflow tube isn’t cracked or misaligned and that the fill valve is installed correctly.
- If the problem started after other plumbing work or a new appliance, consider that wider pressure changes may be involved — if it began after work on your water heater, also check Random refills after water heater install.
6. Test and monitor
- Turn the water back on, let the tank fill, and flush several times. Observe for any refill without flushing and check with the food coloring test again if needed.
- Monitor the toilet for a day to ensure the fix is lasting.
What Not to Do
- Don’t keep replacing fill valves blindly. If the flapper or flush seal is the leak source, another fill valve won’t help.
- Don’t use random sealants or glues on the flapper or seat—these can make future repairs harder and may not be safe for plumbing rubber parts.
- Don’t overtighten plastic fittings; you can crack parts or warp sealing surfaces.
- Don’t ignore the problem if the leak path is unclear—delay can mean higher water bills and more damage. Call a plumber if the leak path is unclear.
When to Call a Professional
- If you replace the flapper or flush seal and the toilet still refills randomly.
- If the tank or bowl has visible cracks, or if parts are corroded or damaged beyond simple replacement.
- If the water shutoff valve won’t work or you suspect a hidden leak in the wall or floor.
- If you’re uncomfortable working inside the tank or diagnosing intermittent pressure problems.
Safety Notes
- Turn off the water supply before working inside the tank. Open the valve slowly when testing to avoid sudden pressure surges.
- Use simple hand tools only. Avoid metal prying tools that can scratch or nick plastic and rubber parts.
- Wear gloves if you’re handling old parts with mineral buildup. Clean the area afterwards.
- Avoid chemical sealers or adhesives inside the tank; they can damage rubber parts and contaminate water.
Common Homeowner Questions
- Q: Why did the problem start after I replaced the fill valve?
A: The fill valve may have been fine; the real leak was likely a worn flapper or seat that went unnoticed until you worked on the tank. - Q: Can I just bend the chain shorter to stop leaks?
A: Chain length matters—shortening it too much can keep the flapper from closing; adjust so there is light slack when closed. - Q: How long will a new flapper last?
A: A good rubber flapper typically lasts several years; water quality and cleaners can shorten its life.