A toilet that runs water periodically, without being flushed, has a leak. Usually the seal between the tank and the bowl is leaking, but the problem could come from other sources. No matter where the problem is, you are wasting valuable water on a problem that is easily corrected.
Leaks at the Tank
If water is dripping from the tank onto the floor, this indicates that the toilet is leaking. Check the water connections at the back of the toilet and the tank bolts. You may need to clean the connections and replace the washers.
Float Arm Problems
Another common problem is a float arm that is set too high. Remove the tank lid and find the float at the top of the valve assembly. If possible, check the float arm while the water is running. If water is draining through the overflow and the tank is still running, adjust the float arm until the water level stops about an inch below the top of the overflow tube.
Flapper Valve Problems
When the toilet refills regularly without flushing, the problem is usually a leaky flapper valve. The flapper valve sits at the bottom of the tank, controlling the water flow into the toilet bowl. Sometimes there is an obstruction or the valve is out of alignment and not seating properly. Valves that are old and brittle need to be replaced.
Fill Valve Problems
If the above repairs do not fix the problem, you may need to replace the fill valve. You will need to turn off the water supply and drain the tank. Replace the fill valve assembly at the bottom of the tank. Turn on the water and adjust the water fill levels so that the water level stops below the overflow tube.
A Different Leak Problem
Though it does not cause sporadic flushing, a defective wax ring is a common problem that causes water leaks at the base of the toilet. If the toilet has not been used in a while, such as in a vacation home that has been closed, wait 24 hours and see if the problem clears up on its own. Sometimes the wax dries out and causes leaks. If this doesn't solve the problem, look for cracks in the base of the toilet. A cracked toilet base will have to be replaced. If the toilet is intact, replace the wax seal.