A running toilet does not always need a new flapper. First find out where the water is going. On a toilet with an accessible tank, you can distinguish water leaking past the flush seal from water continuously entering the overflow tube. The EPA WaterSense dye test is a useful first check.
- Put dye in the tank
- Wait 10 minutes; do not flush
- Look for colour in the bowl
- Check seal or overflow path
Run the test without flushing
Set the tank lid down safely. Add a few drops of food colouring or a dye tablet to the tank, then wait 10 minutes without flushing. If colour appears in the bowl, water is passing from tank to bowl. Flush immediately after the test to prevent staining.
Colour shows that tank water is reaching the bowl, but does not identify its route. Check the overflow tube before attributing the flow to the flush seal. Look for debris on the seal and a chain that is so tight it holds the flapper open. Match any replacement part to the toilet model.
Check the overflow separately from the dye result
Look inside the tank for water continuously spilling into the overflow tube. Continuous overflow suggests a fill-valve or water-level issue, regardless of the dye result. Compare the level with the maker's marked line and follow the model's adjustment instructions; call a plumber if the flow continues.
Water on the floor or wall is a different problem. Wipe it up, close the toilet supply valve if safe, and arrange prompt repair. This tank test does not fit concealed-cistern, sensor-flush or flushometer toilets without an accessible tank.
A worked example
Suppose you hear a brief refill at night but the floor is dry. Colour in the bowl without water entering the overflow tube directs attention to the seal and chain. Water pouring into the overflow tube directs attention to the fill valve and level, regardless of colour. A photo of the model label helps when seeking the right part or professional advice.
Common questions
Can I use detergent instead of dye?
Use food colouring or a purpose-made dye tablet so the result is visible, then flush promptly.
Does a clear bowl rule out every leak?
No. Overflow and leaks outside the tank need separate checks.
Should a tenant replace the part?
Follow your lease and building rules. Record the test result and contact the responsible manager.
Official sources checked
This is general information; individual circumstances and product or agency instructions may differ.