A very common type of toilet repair is adjustment or replacement of the fixture's fill valve. This part is often known by the generic term ballcock, although strictly speaking, a ballcock is an older form of this valve that is rarely seen anymore. You may, however, still have one of these older ballcocks in your toilet; but when you replace it, it's likely you'll want to install one of the simpler modern designs.
There are many types of fill valves available, and dozens if not... hundreds of manufacturers who make them. Most are quite interchangeable and can fit the standard tank openings on any toilet, but you do need to make sure to pay attention to sizes, especially the length of the valve stem. Modern toilets may have a rather small tank, and you'll need to make sure the new fill valve stem is short enough. Some fill valves are adjustable to fit different tank sizes. Also, some toilet tanks may have flush valves with oversized overflow tubes that require a specialized fill valve. The easiest way to buy a suitable replacement is to bring the old fill valve with you to the store, where the clerk should be able to direct you to several acceptable replacements.
Here is a quick breakdown of some of the fill valves you might see in your toilet tank.
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Old Style Ballcock or Plunger Type
This is the oldest style of fill valve, but its simple design is so dependable that it's possible you still have one in your toilet. They are identified by their brass construction and by the presence of a float ball attached to a long arm connected to the fill valve. The ball itself is usually plastic, but in older models, it may be a copper ball.
Mechanically, this type operates with a simple plunger valve in which a washer at the tip of a valve stem seals against an inlet opening inside... the valve. When the toilet flushes, the stem lifts away from the water inlet, allowing fresh water to enter the tank. As the water rises, so does the float ball. When the ball rises high enough, the valve stem is levered down against the water inlet, stopping the flow of water.
This type can often be repaired simply by installing a new rubber washer on the tip of the valve stem, provided you can get the parts disassembled to service them. Eventually, though, it's likely you will replace a plunger-type valve with a more modern design.
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Diaphragm-Style Fill Valve
This is a modification of the older brass style ballcock, but instead of a plunger-type valve, the float ball controls a diaphragm seal inside the valve. You can identify this type by the fact it has a float ball, but the stem of the fill valve will be plastic rather than bronze. At the top of the valve is a rounded body secured with screws, housing the rubber diaphragm inside.
When the diaphragm becomes worn, it is theoretically possible to replace it. But parts may be difficult to find, and... many people find it easier to simply replace a diaphragm fill valve with a float-cup type.
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Float-Cup Fill Valve
This is now the most modern type of fill valve, found in most toilets you see. It is easy to install, very dependable, and universal since most styles can be easily adjusted for heights anywhere between 7” and 13”, depending on which brand you buy.
Instead of a float ball attached to an arm, this type uses a float cup attached to the stem of the fill valve. As the float cup moves up and down with changing water levels inside the toilet tank, an attached lever opens or closes the fill valve.
This... type is fairly easy to repair by replacing the inner seal on the fill valve.
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Floatless Fill Valve
This type of fill valve sits at the bottom tank, underwater, and you may not notice it at all when you first open the tank. It uses a diaphragm to sense the water level from the bottom of the tank. These are well-suited for low profile toilets because of their small size, but the older models are no longer allowed, due to the possibility of siphoning water back into the fresh water supply. If you have one of these older models of floatless fill valve, there is a good chance it is no longer... allowed by Code and should be replaced. Newer types, however, have anti-siphon features that remove the health danger. You should make sure, though, that your local Building Code allows this style to be used—some communities do not allow the use of floatless fill valves of any type.
This type also has the easiest water level adjustment; it is done by just turning the screw on top of the fill valve.