Is Your Toilet Hissing? Why and What to Do


A toilet hissing sound typically occurs due to a leaking fill valve or a malfunctioning ballcock assembly. The hissing noise is a result of water continuously flowing into the toilet tank, even when it’s not flushing.

Here’s a brief summary of the causes and solutions:

Causes of Toilet Hissing

  • Leaking Fill Valve: The most common cause is a fill valve that doesn’t seal properly, allowing water to escape into the overflow tube continuously.
  • Faulty Ballcock Assembly: If the ballcock assembly is damaged or malfunctioning, it can also lead to a continuous flow of water into the tank, causing hissing.

Solutions to Toilet Hissing

  • Adjust the Fill Valve: Try adjusting the fill valve’s float arm or screw to ensure it shuts off the water at the correct level. This might stop the hissing if the issue is minor.
  • Inspect and Replace Flapper Valve: Check the flapper valve (flush valve) for leaks. If it’s worn or damaged, replace it to create a proper seal and stop the hissing.
  • Clean or Replace Fill Valve: If the fill valve is the culprit, clean it to remove any debris that might be affecting its operation. If cleaning doesn’t work, consider replacing the fill valve with a new one.
  • Ballcock Replacement: If the ballcock assembly is the problem, replace it entirely with a new one to ensure a proper shut-off of water flow.
  • Call a Professional: If you’re not comfortable with these DIY fixes or the problem persists after attempting them, it’s advisable to call a plumber for a thorough inspection and repair.

How a Toilet Fill Valve Works


A toilet fill valve is the toilet tank part connected to the water supply line from underneath the tank. Its function is to automatically refill the tank after every flush.

The fill valve is also connected to the toilet float. A toilet float will move up and down relative to the level of water inside the tank.

When the tank is full of water, the float will be at the top of the tank as well. However, when the toilet is flushed and the tank is empty, the float moves to the bottom of the tank.

That move is responsible for opening the fill valve, and water starts to enter the tank. The float keeps moving up as the water level increases until its set height, when it shuts off the fill valve.

Whenever you have a hissing toilet, it means that the fill valve is not working as designed. Water is struggling to pass through the fill valve to the tank.

The hissing gets worse if not fixed and you will also note that the tank takes way too long to refill.

After years of usage, mineral present in the water like calcium and iron will be deposited inside the fill valve. As more minerals and debris get deposited, the flow of water through the valve to the tank is restricted.

Water is therefore forced to pass through a very narrow opening which increases its pressure. The increase in water pressure is what causes the hissing sound.

How Do I Get My Toilet to Stop Hissing?


So, how do you fix a hissing toilet? There are 2 proven ways that I know which will stop your hissing toilet.

You can start by trying to flush out the debris from the valve or you can replace the valve altogether. In some instances, you may need to only replace the valve seal but there is usually no guarantee that the toilet will stop hissing.

Given how inexpensive toilet fill valves are (about $20), replacing the entire valve is always the best and long-term solution. It is easy as well therefore you don’t need to hire a plumber.

Let us look at the 2 methods of fixing a hissing toilet.

1. Flush Debris Out of the Fill Valve

The water in your pipes is usually under pressure. What we want to do here is to use that water pressure (and the volume as well) to flush the debris out of the fill valve.

That way, the valve waterway will be clear and water can pass through it without the hissing sound. Or so we hope.

This is how to flush debris out of a toilet fill valve:

  • Start by turning off water supply to the toilet. The shut off valve is on the wall behind the toilet. Turn the valve knob all the clockwise.
  • Flush the toilet. It is hard to troubleshoot the problem with a tank full of water.
  • Lift off the tank lid and place it away in a safe place where it cannot fall off and crack.
  • Locate the flush valve. It will most likely have a cap that is of a different color from the rest of the body.
  • Hold the shank of the fill valve at the bottom with one hand, then hold the top assembly (including the float connection) with the other hand then turn it counterclockwise by a ¼ of a turn.
  • Inspect the valve seat carefully. Can you see debris at the top that you can easily remove with a sharp object like a wire or tweezers? Do It.
  • Hold a large plastic cup over the fill valve and turn on the toilet shut off valve completely. The cup will prevent the water from splashing all over you and the bathroom.
  • As the water blasts through the valve, it will flush out any debris that will be clogging it.
  • Turn off the water after 30 seconds.
  • Put back the fill valve cap by turning it in the opposite direction as before.
  • Turn on water to the toilet and listen carefully if the hissing noise has stopped.

If the hissing noise has stopped, your problem has been successfully fixed. Replace the fill valve if the hissing noise persists.

2. Replace the Toilet Fill Valve

Replacing a toilet fill is both easy and inexpensive. You can decide to buy an exact replacement as your old one from your toilet manufacturer or you can buy the universal ones.

Fluidmaster and Korky fill valves are the best in the market. You can also adjust them in accordance with the height of your tank so that should not be a problem.

Once you have bought the replacement fill valve, here is how to remove the old one and install the new one:

  • Turn off water supply to the toilet.
  • Flush the toilet and hold the lever down to remove as much water as possible.
  • Remove the toilet tank lid.
  • Use a sponge to soak up the water still left at the bottom of the tank.
  • Disconnect the water supply line from the tank. Use your hand only to prevent damaging the coupling. Gently use a wrench if the connection is too tight. Don’t forget to have a bowl to drain the water already inside the supply line.
  • Loosen the mounting/locknut. This is the big nut underneath the tank that holds the fill valve firmly.
  • With the locknut out, you will only need to disconnect the refill tube (small tube connected to the overflow tube) and take out the fill valve.
  • Adjust the new fill valve. Ideally, the fill valve cap should be about 3 inches higher than the overflow tube. Insert the valve in its hole and see how far you need to adjust it. Adjust it by holding the shank and then turning the upper assembly until you get the right height.
  • Push the fill valve through the hole at the bottom of the tank and secure it using the lock nut.
  • Connect the water supply line to the tank. You don’t need to use a wrench but if you must be very gentle.
  • Connect the refill tube to the overflow tube
  • Turn on water supply to the toilet.

As the tank fills with water, listen if the hissing sound has stopped. This repair should however be enough to completely get rid of it.

One thing you will need to do is adjust the level of water in the tank. Ideally, the water level should be just ½ inch below the overflow tube.

To do that, turn the big screw next to the float clockwise to increase the water level and counterclockwise to decrease the water level.

If you somehow still have the old ballcock/float ball fill valve, turn the screw where the float arm is connected to the fill valve.

And basically that is how to fix a hissing toilet. I hope that this guide was of help to you.

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