Quick Answer:
If the toilet keeps running after you replaced the flapper, first check the flapper alignment and the chain slack (about 1/2 inch). Make sure the new flapper sits flat on the flush valve seat and that you bought the correct flapper model for your toilet. Simple adjustments to the chain or a quick clean of the seat often fixes it.
Why This Happens
Common causes after a flapper swap are fit and position problems, not plumbing mysteries. A flapper that is too large, too small, or the wrong shape won’t seal properly. If the chain is too tight the flapper is held slightly open; if it’s too long the flapper can get caught under the seat. Debris or mineral deposits on the seat can stop a good flapper from making a flat seal.
If the running is intermittent or only at certain times, the issue can point to your refill or overflow arrangement — see Toilet runs only at night and Toilet runs randomly for those patterns.
Step-by-Step What to Do
1. Watch a test flush
- Lift the tank lid and flush. Watch the flapper as the tank refills.
- Note whether the flapper falls straight and seats fully, or whether it rocks to one side or sits crooked.
2. Check flapper alignment and chain slack
- When the flapper drops, you want about 1/2 inch of slack in the chain so the flapper can settle flat. Adjust the chain hook or clip on the flush lever arm to get that slack.
- Too tight: flapper never fully closes. Too loose: chain can get under the flapper or snag and prevent a seal.
3. Confirm the flapper seals flat on the seat
- With the flapper closed, run a finger (wear a glove) around the perimeter to feel for gaps. Look for mineral crust or residue on the seat.
- Clean the seat area with a rag and vinegar if there is visible buildup, then dry and test again.
4. Verify you used the correct flapper model and size
- Compare the new flapper’s shape and diameter to the old one or to the valve opening. Universal-looking parts still come in different sizes or lift types (3-inch vs 2-inch; flat vs beveled).
- If the flapper doesn’t match the seat profile, it won’t seal even if everything else looks right.
5. Check the refill tube and overflow tube
- Make sure the refill tube clips to the rim of the overflow tube and isn’t pushed below the water line. A submerged refill tube can siphon water and cause continuous flow.
- Confirm the overflow tube height hasn’t changed; if the tank water level sits above the tube top, water will run into the bowl continuously.
6. Test again and observe
- After adjustments, flush and watch for a few minutes. If it stops running, leave the lid off and test again later to be sure it doesn’t resume.
- If the flapper still won’t seat, note whether it’s a fit issue or something else like a warped seat or damaged flush valve.
What Not to Do
- Don’t keep swapping flappers blindly—wrong size or shape is a common reason the problem persists.
- Don’t force the chain or try to shorten it by tying knots that can slip or fray; use the clip/hook positions provided.
- Don’t use bolt-on adhesives or sealants on the valve seat unless a part specifically calls for it—this can make future repairs harder.
- If you can’t match parts or the seat looks damaged, a plumber is appropriate rather than trying every part on the shelf.
When to Call a Professional
- The flapper fits correctly and you still have a leak or continuous running after cleaning and adjusting chain slack.
- The flush valve seat is warped, cracked, or corroded and won’t accept a new flapper.
- You’re unable to identify the correct replacement part or the toilet has an uncommon valve design.
- Repeated repairs don’t fix water loss — better to call a pro than risk higher water bills or damage.
Safety Notes
- Turn off the water supply when replacing parts if you feel unsure; this prevents spills. The shutoff valve is usually at the wall behind the toilet.
- Wear gloves when working inside the tank to avoid contact with residue or cleaning chemicals.
- Handle the tank lid carefully; porcelain lids are heavy and can break, creating sharp edges.
Common Homeowner Questions
- Why does it still run after I replaced the flapper? Usually the flapper isn’t seating properly because of chain tension, misalignment, or the wrong flapper model.
- How much chain slack is correct? About 1/2 inch of slack so the flapper can fully settle on the seat.
- Can I fix a bad seat myself? Minor buildup can be cleaned, but a warped or cracked seat usually needs part replacement or a plumber.
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