How A Toilet Fill Valve Works?

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Last Updated on September 1, 2022 by acechapman

This is a simple and harmonious system that allows you to flush the toilet, then when you return, you can flush it again. The same amount of water will always be used to flush the toilet. There are many interconnected parts inside the toilet tank, including a fill valve for the toilet that controls how much water gets in after flushing the toilet. Read further to learn how a toilet fill valve works.

There are three main types of toilet fill valves: float cups, old-style pistons, floatless, and plastic/brass dial diaphragm ballcocks. They all serve the same purpose, keeping the tank full, but they have different designs and operating mechanisms.

It is essential to understand how each type of toilet works in order to solve any issues such as the fill valve you may encounter when using it. Below are the details for each type, as well as a review of the top fill valves.

What is a toilet fill valve and how it works?

The toilet fill valve, which is made up of several interconnected parts within the tank, controls the water flow to refill the tank.

The toilet flushes itself, so it doesn’t play any part. Both are interrelated, however. Flushing the toilet activates the fill valve to allow water in the tank.

It consists of a mechanism that measures the water level. In most cases, it is a floating sensor that moves with the water level. The fill valve opens to allow water in until the maximum level has been attained.

Others use a pressure sensor to determine if water is present in the tank.

how a toilet fill valve works

Types Of Toilet Fill Valves 

The mechanism that controls the water level inside a tank is what fill valves are called. You can broadly classify toilet fill valves into modern, float cup, and floatless fill valves.

Float Cup Fill Valve  

This fill valve does not have a floating ball but instead uses a plastic cup that moves along a shaft to indicate the water level in the tank.

This fill valve is one of the most reliable because it has very few moving parts and is easy to set up.

The floating cup falls to the bottom of the shaft when the toilet is flushed. This opens the water inlet. The water that flows into the toilet tank raises the cup up until the inlet is again closed.

Some float cup valves have a plastic screw mechanism that you can use to raise or lower your float.

The majority of float cup valves have a plastic screw mechanism that allows you to adjust the water level in the tank. To raise or lower the floating level, you just need to turn the screw.

Internal Float Fill Valve

This is a more recent version of the float cup style with an internal and smaller concealed float lever within the head of the valve. Internal floats are smaller than external float cups and have a more precise shut-off.

Because the water inlet is located inside the fill valve, the valve is very quiet. Some manufacturers call this QuietFill.

To adjust an internal floating, turn the top of the fill valve counterclockwise. Move Y in the desired direction, then lock it by turning clockwise.

Floatless Fill Valve

This type of fill valve doesn’t have a ball or float cup, as its name implies. They use a pressure-sensing dial at the bottom of the tank. The valve is submerged and closes when the pressure reaches its maximum.

To serve low-flow toilets, floatless valves were designed. They are usually made from plastic and are highly reliable.

Adjust a floating valve by turning the adjustment screw at the top. This will raise or lower the water level.

Plunger/Piston Type Fill Valve

Piston/piston fill valves use a piston or plunger system to open or close the water inlet into a toilet tank. The bottom-fill type inlet means that water is injected from the bottom into the tank, rather than from the top.

A piston, or plunger, is attached to an end of a brass rod. This is then attached to a floating ball at its end. The floating ball is always on top of the water surface.

The ball falls to its lowest point when the toilet flushes. It does this by moving the brass rod along with it. The brass rod then moves the piston that opens the water inlet.

When the water enters the toilet tank, the rod also raises. When the tank is full, the rode lifts the piston and closes it slowly until it stops.

You can adjust the amount of water that flows into your toilet tank by bending the brass rod upwards or downwards. You should maintain the water level at a certain maximum level.



The diaphragm fill valves are very similar to the piston types. They also have a brass rod with a float ball at one end. It also floats with the water level.

Instead of changing the setting of a piston with a brass rod, it has a diaphragm sealing inside a round valve head. As there is a plastic version, the brass part refers only to the material from which the diaphragm was made.

The floater falls down when the toilet is flushed and the brass rod follows it. The brass rod is pressed on top of the diaphragm, which opens the water inlet to the tank.

The water fills up and the ball moves upwards as it does. The brass rod is released by the pressure created when the ball moves the brass tube. The diaphragm gradually closes the water outlet until it is closed completely.

You can adjust the water volume in the toilet tank by turning the brass rod that connects the floating ball and diaphragm upwards or downwards.


This type of toilet fill valve works in the same way as the brass diaphragm.

The ball opens the diaphragm by moving downwards, and the ball closes the diaphragm by moving upwards.

The diaphragm, which is made from plastic material instead of brass, is the main difference.

Instead of twisting the brass rod to adjust the water level, turn the floating rod adjustment screws at the top.

The water level is decreased by turning clockwise, while the tank’s water level is increased by turning counterclockwise.


The water level of the toilet tank will be determined by the fill valve and the toilet float. The tank’s water level will be either too low or too high if the fill valve isn’t properly adjusted. The toilet tank water level should be approximately 1 inch below the overflow tube.

Low water levels can cause a toilet to not flush as hard, and you may need to flush twice.

The water in the tank will rise to excessive levels and flow to the bowl through the overflow tube. This will result in a continuously refilling and running toilet.

It is simple to adjust a toilet fill valve. A screwdriver is all you need to adjust a toilet fill valve.

  • If you have a floating ball, check where the fill valve connects to the float arm. There will be a screw. Use the screwdriver to turn the screw counterclockwise to raise or lower the water level.
  • Look for a long, plastic screw near the fill valve if you have a floating ball. To lower the tank’s water level, turn the screw counterclockwise.
how a toilet fill valve works

How to know if the toilet fill valve is bad?

The toilet is making a hissing/whistling sound.

If the fill valve is blocked or not fully opening, water will force its way through it. When you flush your toilet, it will make a hissing/whistling sound when it fills up.

The toilet is running continuously.

The toilet fill valve seal can wear out and the valve will not close even if the tank is full. The tank will continue to fill. It will continuously fill the tank with water.

Supply line coupling that leaks.

If water is leaking from the coupling connecting the supply line and the fill valve it could be a sign that the fill valve has been blocked mainly by mineral deposits. It is possible that the fill valve takes too long to refill or does not work at all.


It is possible to repair the toilet fill valve but most often, it is best to replace it. Fill valves are inexpensive and simple to replace.

Sometimes, it is a good idea to fix a fill valve. You can fix a fill valve that is relatively new if it has mineral deposits or any other debris.

  • Turn the valve clockwise to turn off the water supply to the toilet.
  • Flush the toilet.
  • With one hand, hold the fill valve shank and the cap at its top with the other. This is how you remove the cap.
  • Verify that there is no dirt or oil on the seal.
  • Place a cup upside down on top of your fill valve.
  • Turn on the water.
  • The water pressure will force out all dirt, while the cup will stop water from splashing everywhere.
  • Once you unblock the valve completely, put it back.

Factors When Choosing The Right Fill Valve Type

Here are some things to think about when buying a replacement kit for your fill valve.


Modern fill valves can be used in all toilet types and designs, but not all of them will work. Some universal fill valves will not work with dual flush toilets, for example.

However, most modern designs will not work with older ballcock designs. Before you buy a replacement kit, make sure to verify the model and type of toilet you have.


Quality plastic and rubber parts fill valves last longer than those made from metals that are easily rusty.

Ease Of Installation 

You can easily do these replacements. It is not necessary to hire a professional plumber. You can choose kits that are quick to install and have easy-to-follow instructions.


This is especially important for households and places that have many people.

Pressure (Low/High)

Low-pressure toilets can be used with some fill valves, such as floatless fill valves.


It should be simple and quick to adjust the height or float mechanism.

Others factors include

  • Water-saving
  • Less noisy

Wrapping up

So, this is all you need to know about how a toilet fill valve works. This guide will help you replace your toilet fill valve. Universal toilet fill valves mean that you don’t have to purchase one from each manufacturer. Fluidmaster and Korky have the best universal toilet fill valves available.

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