A toilet's flapper valve sits over the large drain opening in the bottom of the toilet's tank, regulating when and how much water drains out of the tank and into the bowl. When the flapper goes bad, the toilet will either leak water or the toilet's flushing power will diminish.
Once you remove a new flapper valve from its packaging and install it into your toilet's tank, you have exposed the flapper to the air as well as the water. Just by simply using the flapper valve, it will begin its progression toward going bad, although that progression may take years to complete, which means that sooner or later you will need to replace your toilet's flapper no matter how you use the toilet.
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A flapper valve is supposed to float in the water inside the tank until a certain amount has drained out. Once the water level reaches this point, the flapper then floats back down and rests over the drain opening again, allowing the tank to refill with water in preparation for the next flush. A flapper may begin to take on extra water, becoming water logged, so that it does not float in the water as long as it did when it was new. When this occurs, you must flush a toilet more than once to empty the toilet's bowl since not as much water exits the tank each time you flush.
Since flapper valves are made either completely out of rubber, or at least have some rubber on them to create a seal around the tank's drain opening, the presence of hot water in the tank will wear out the flapper valve. Rubber will crack and deteriorate faster when exposed to heat, meaning the flapper will no longer flex and cover the drain opening as it did before. Pouring hot water into the toilet's tank, or improper plumbing that feeds hot or even warm water into the tank, will lead to the flapper going bad prematurely.
The drop-in tablets that go into a toilet's tank will cause the flapper, as well as other parts inside the toilet's tank, to wear out prematurely. These tablets provide a powerfully concentrated mixture, releasing cleaners into the water each time they exit into the bowl. Over time the cleaners eat away at the flapper valve until it no longer seals the drain opening in the bottom of the tank. Instead of using tank tablets, use a cleaner that attaches to the side of the bowl where it will not harm the flapper valve.