A new replacement fill valve inside a toilet tank.

Fill Valve

Originally contributed by Jeff Butler • last updated 12/28/2020

While fill valve mechanisms inside a toilet have existed for over 100 years, its more modern concentric-float design has been around since the late 50s and is still growing in popularity because of its reliability.


The fill valve, which is a tower-like fixture inside the toilet tank, is responsible for filling and maintaining a predetermined amount of water inside the tank. There are two common types of fill valves, a side float design and a concentric float design. The side float design, commonly referred to as a ballcock, is easily identified by the float ball that is connected to the fill valve by a long (typically metal) arm. The concentric float design is identified by a float cup that travels up and down the shaft of the fill valve. With either type, the float rises as the water level increases to a set height/volume, closing the valve and preventing more water from entering the tank. There is also a less common floatless fill valve that uses a pressure sensitive diaphragm to sense the water level from the bottom of the tank.

To Replace Or Repair? That Is The Question

If you’re hoping to avoid a costly plumber's bill, you’ll be happy to know that diagnosing problems with and replacing a fill valve is something most people can do themselves.

  • Fill valves are responsible for some of the most common toilet problems, including a toilet that won’t stop running.
  • If you have an older type of fill-valve, it’s usually easier and cheaper to replace it with a newer design than it is to try and repair it.