Fixing a Toilet That Won't Stop Filling

Understanding the Cycle

A toilet that runs continuously is wasting water by constantly refilling the tank as water leaks into the bowl. This phantom cycling can waste 200 gallons or more per day. The water enters the bowl either through a leaking flapper or by overflowing the overflow tube. Place a few drops of food coloring in the tank and wait 15 minutes without flushing — if color appears in the bowl, the flapper is leaking. If the water level in the tank is at or above the overflow tube, the fill valve or float is the problem.

Flapper Replacement

Flappers deteriorate due to age, water chemistry, and cleaning chemicals (especially chlorine drop-in tablets, which accelerate flapper degradation). Turn off the water supply, flush to empty the tank, and unhook the old flapper. Clean the flush valve seat (the smooth ring the flapper sits on) with a Scotch-Brite pad to remove any buildup that prevents a tight seal. Install a new flapper matched to your toilet model and adjust the chain length so there is about half an inch of slack when the flapper is closed.

Overflow Tube and Fill Valve

If water is flowing into the overflow tube, the water level is too high. Adjust the float to lower the water level to about 1 inch below the overflow tube. If adjusting the float does not stop the overfill, the fill valve diaphragm may be worn and not shutting off properly. A quick test: lift the float to its highest position manually — if water still flows, the fill valve needs replacement. Also check that the refill tube from the fill valve is clipped to the outside of the overflow tube and not inserted inside it, as this can create a siphon that continuously adds water.

Overflow Tube Height

The overflow tube in the tank should be at least 1 inch taller than the desired water level. If someone has cut the overflow tube too short or if it has deteriorated, water may overflow into the bowl at normal fill levels. The overflow tube is part of the flush valve assembly, which bolts through the bottom of the tank. While replacing the entire flush valve is a bigger job than replacing just the flapper or fill valve, it provides a complete refresh of the tank's internal components and is worthwhile if the flush valve is old or damaged.