Szeto Plumbing · San Gabriel Valley
Running toilet: flapper or fill valve?
A toilet that keeps running or refills by itself wastes water every hour.
Mon–Sat, 8:30 AM–7 PM
Call or text (310) 628-9399
Kitchens, baths, heaters, main lines
Updated · 3 min read
Short answer
If water spills into the overflow tube, the fill valve is set too high or failing. If the tank refills on its own with no overflow, the flapper leaks. A few drops of food coloring in the tank confirm a leaky flapper within minutes.
On this page
Look at the overflow tube
Lift the lid and watch after a flush. When the tank is full, water should stop below the top of the overflow tube.
If it keeps trickling into the tube, adjust the float lower. If it will not shut off at any setting, the fill valve needs replacing.
The dye test
Put a few drops of food coloring in the tank, do not flush, and wait.
EPA's Fix a Leak Week page recommends this same test and notes leaky toilets are a common household leak.
Replacing the flapper
Close the stop valve, flush to drain, unhook the chain and pull the flapper off its pegs. Fit the new one, set the chain with a little slack, and refill.
In SGV homes with very hard water, wipe mineral crust off the flush valve seat before fitting the new flapper. A rough seat lets even a new flapper leak.
Replacing the fill valve
Unscrew the nut under the tank with water off, lift out the old valve, and install a new one adjusted to the tank's water line. Connect the supply line and test.
Common questions
Why does my toilet run for a second every few minutes?
That is a slow flapper leak. The tank drops below the float point and refills briefly.
Can a running toilet raise my water bill a lot?
Yes. A constantly running toilet can waste a large amount of water over a billing period.
Should I replace both parts at once?
If the toilet is old, replacing both avoids a second visit soon after.