Why Won’t My Toilet Stop Running? Causes, Tests, and Solutions for Slow Toilet Leaks

Last Revised: 11/15/24 Does your toilet sound like it’s constantly running, even when you’re not using it? Have you noticed a sudden spike in your water bill, even though your water use hasn’t changed?  This guide will walk you through diagnosing and fixing common causes of a running toilet – saving water, money, and your sanity. 

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A running toilet might seem like a minor annoyance, but it could be wasting upwards of hundreds of gallons of water each week and leaving you to foot the bill.  Fortunately, fixing a leaking toilet is oftentimes a straightforward DIY job – if you know what you’re doing.  That’s why we at Mattioni put together this guide, drawing from over 76 years of experience helping homeowners fix plumbing issues on their own. In this article, we’ll cover the main causes of toilet leaks, how to diagnose them, and the exact steps to fix each kind.   After reading this, you’ll be prepared to stop any type of toilet leak. First, let’s look at how a toilet is supposed to function.   

How Does a Toilet Function Normally? 

When a toilet flushes properly, there are two main components that work together: the fill valve, and the flapper. Together, they release the right amount of water into the toilet bowl and refill the storage tank (the upper part of the toilet) to the proper level.  The flapper is a rubber seal at the bottom of the storage tank which lifts and allows water to flow from the storage tank into the bowl when you press the flush handle. Once the tank empties, the flapper then falls back into place to seal off the opening and allow the tank to refill.  The fill valve activates once a flush is completed to refill the tank with water from your home’s water supply. As the tank fills, a float connected to the fill valve rises and triggers the fill valve to shut off once the water reaches the set level.  Together, the flapper and fill valve ensure the toilet refills properly and stops running until the next flush.   

What Causes Water to Keep Running in a Toilet? 

Internal toilet leaks are often the result of a malfunction or wear and tear of one or both of the two main components. Here is what could be causing your toilet to run nonstop:   

How a Faulty Fill Valve Causes Constant Running Water 

If the fill valve malfunctions and doesn’t shut off completely (or the water level is calibrated too high), it won’t be able to do its job of controlling the water level in the storage tank. Rather than letting the water overflow over the top of the storage tank, the water will flow into the overflow tube. When this happens, water will leak into the bowl slowly.   

How a Worn-out Flapper Causes Constant Running Water 

If a toilet’s flapper is worn out, warped or not positioned directly over the opening, it won’t be able to create a tight seal between the storage tank and bowl. This small gap can allow water to trickle into the bowl continuously.   

How Hard Water Shortens the Lifespan of Toilet Components 

It’s possible that the root cause of these components malfunctioning is an abundance of minerals in your home’s water supply, also known as hard water.  Hard water can cause mineral deposits to build up on both the flapper and valve. Over time, this buildup can affect how well the components fit, possibly preventing a tight seal and leading to leaks.  If you notice water stains on the inside of your toilet’s storage tank, you may want to learn about hard water and solutions to this wider problem, like installing a water softener.   

Why Slow, but Steady, Leaks Matter 

While a slow leak might seem minor, it can have a significant impact on water usage and subsequently drive up household bills. A toilet leak can waste up to 180 gallons of water per week, according to the Environmental Protection Agency, leading to unexpected increases in your water bill. In many cases, a toilet leak can cause household costs to rise by several hundred dollars per year.  Between environmental factors and the impact on your finances, addressing even a minor leak is an important step to take.   

How to Test for Flapper or Fill Valve Failure 

Sometimes it can be easy to tell why your toilet is leaking. For instance, if your flapper is warped, the issue can be spotted quickly.  Other times it can be tough to tell if the leak is due to your flapper or fill valve, and it’s possible to end up wasting time and money fixing the wrong component. Simply hearing and seeing running water doesn’t mean you’ll always know what’s causing it.  If you’re having a hard time determining the cause of your toilet leak, here’s how to test each component:     

Testing the Flapper Inside Your Toilet’s Storage Tank 

There are a couple of easy ways to see if your flapper is the culprit behind your toilet leak. First, check the chain that’s attached to the flapper, as it may be too tight or tangled. If the chain can’t move freely, it could prevent the flapper from closing fully after each flush.  Food coloring can also be used in a simple test to check if your flapper is causing your leak:   

Testing Your Flapper Using Food Coloring 

1. Add a few drops of food coloring to the tank water and wait 10 – 15 minutes without flushing.
2. Check the bowl after 10 – 15 minutes. If you see colored water in the bowl, this indicates the flapper is leaking water from the tank to the bowl.

If your flapper can move freely, isn’t warped or damaged, and the food coloring test fails, it’s time to take a closer look at your fill valve.   

Testing Your Fill Valve by Marking the Water Level 

The reason that food coloring won’t make it to the toilet bowl if the flapper isn’t the issue is because the fill valve has a built-in mechanism to bypass the storage tank and overflow directly into the toilet bowl (instead of overflowing over the top of the tank.)  If the food coloring test fails, it indicates that the fill valve isn’t shutting off properly.  To confirm this, observe the water level in the tank. Ideally, the water level should be about one inch below the top of the overflow tube. If it’s too high, water may be flowing into the overflow tube, suggesting the fill valve isn’t shutting off correctly.   

The Last (and Best) Toilet Leak Test: Shut off the Water Supply 

If all else fails, here’s what you can do to see once and for all if either your fill valve or flapper is malfunctioning.  First, shut off the water supply to the toilet and then mark the water level inside the tank with a pencil or piece of tape. After marking the water level, wait 15–20 minutes. If the water level drops, it points to a flapper leak. If it remains the same, the fill valve is likely not shutting off correctly.  Once you’re clear on what exactly the problem is, here’s what to do next.   

What to Do if Your Fill Valve is the Problem

 

1. Adjust the Float

Your toilet may have either a ball float or a cylinder float.   A ball float is connected with a bendable arm that sets the water level. Gently bend the arm to lower the water level so the valve shuts off sooner.   A cylinder float is attached to the fill valve and can be adjusted using the exposed screw. With a screwdriver, gently turn the screw to move the float down slightly so it shuts off sooner.   

 2. Replace the Fill Valve

If adjusting your float doesn’t stop the overflow, you’ll need to replace the fill valve. Be sure to follow manufacturer instructions of how to safely install the new fill valve.  Once installed, test the new fill valve by allowing the tank to refill and observing to make sure it’s shutting off at the right level without excess water flowing into the overflow tube.   

What to Do if Your Flapper is the Problem 

 

1. Replace the Flapper

Once you’ve identified your flapper is either warped, damaged, or no longer providing a tight seal, you’ll need to replace it.  First, turn off the water supply to prevent any overflow. Next, flush the toilet to drain the tank (you might need to use a sponge to absorb any remaining water). Then, unhook and remove the old flapper before attaching and hooking the new one into place.  You can now turn the water back on and flush the toilet to ensure the new flapper works properly and no water trickles into the bowl.   

How to Prevent Future Leaks 

We might not think about the condition of our toilet’s fill valve and flapper often (and understandably so), but if you’ve already run into a leaking toilet once, the problem may resurface.   To avoid silent toilet leaks in the future, check your toilet periodically for signs of wear and tear, especially on these two main components. Regularly untangling the chain, inspecting the seal your flapper makes, and cleaning the tank components to prevent mineral buildup (if you have hard water) can extend the life of the parts and prevent costly leaks.   

Stop Flushing Your Money Down the Drain 

The constant running of a leaking toilet can be an annoyance and a hidden drain on your water usage. Fixing the problem early can keep your water bill low and restore your peace of mind.   It might seem like a small fix, but knowing exactly what steps are needed to get the job done can be confusing.   Most running toilets are caused by issues within the fill valve or flapper, both of which can be tested and fixed with straightforward steps like adjusting the water level or replacing a component. Following these steps can help you quickly identify and resolve most leaks.   If you’ve exhausted all testing and replacement options and still have a running toilet, a professional plumber can get to the root of the issue. The team at Mattioni has handled plumbing problems of all kinds for over 76 years.   Call us at (610) 400-8510 or book an appointment directly online to restore your toilet to full working order.  Don’t wait – take action today to stop the endless running water in your toilet and start saving money.   

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