Why Is My Toilet Running? A Simple Troubleshooting Guide
- The Best Life Awaits
- 2 days ago
- 3 min read
Updated: 1 day ago
Do you hear water running every time you walk past your bathroom?
Have you noticed the water in your toilet bowl moving — even though no one has flushed?
Chances are you have a leaking flush valve or a fill issue inside your tank.
Before replacing parts, it helps to understand how your toilet works.
Main Parts Inside a Toilet Tank
Most toilets contain these core components:
Fill Valve
Refills the tank after a flush.Located on the left side of the tank and connected to your home’s water supply.
Flush Valve
The central assembly that releases water into the bowl when you flush.
Flapper (or Canister)
The mechanical seal at the bottom of the tank. It holds water in the tank until you flush.(Some models, including ours, use a canister instead of a traditional rubber flapper.)
Float
Controls how much water fills the tank by signaling the fill valve when to shut off.
Overflow Tube
Prevents flooding.If water rises too high, it drains safely into the bowl instead of onto your floor.
Handle / Lever
Activates the flush mechanism.
The Two Most Common Toilet Problems
Toilet runs continuously
Toilet doesn’t flush properly
If your toilet doesn’t flush well, the issue is usually mechanical — a disconnected chain, broken lever, or flush valve linkage.
But if your toilet runs constantly, the problem is usually one of three things:
Incorrect water level (float adjustment)
Worn flapper or canister gasket
Faulty fill valve
Step 1: Check the Water Level
Remove the tank lid and look inside.
Is the water level too high and flowing into the overflow tube?
If yes, the issue is likely:
Float adjustment
Failing fill valve
If the water level looks correct but the toilet still runs, you likely have:
A leaking flapper
A bad canister gasket
Step 2: The One-Hour Test
This is the easiest diagnostic step.
Turn off the water supply to the toilet.
Do not flush.
Leave it alone for one hour.
Now check the tank:
If the water level is unchanged
The issue is likely float or fill valve related.
If the tank has drained significantly
Water is leaking into the bowl.
The problem is almost certainly your flapper, canister, or gasket.
There are only two main exit points for water inside the tank:
Fill valve opening
Flush valve opening (to the bowl)
If the fill valve were leaking externally, you would see water on the floor.
No water on the floor?
The leak is internal — almost always the flapper or canister seal.
Fixing a Water Level (Float) Issue
If water is overflowing into the overflow tube: Start by adjusting the float.
Adjustment mechanisms vary by model:
Modern toilets often use a vertical float integrated into the fill valve.
Older toilets may have a ball float on a rod.
On ours, the fill valve must be pressed down and rotated to adjust water level.
After adjusting:
Flush partially to lower water level.
Let the tank refill.
Repeat until the water stops about ¼ inch below the overflow tube.
If adjustment does not solve the issue, the fill valve itself is likely failing and should be replaced.
(We have a full post and video showing how to replace a fill valve.)
Fixing a Flapper or Canister Leak
If your water level looks correct but the tank still drains slowly over time, your seal is failing.
In our case, the toilet uses a canister-style flush valve.
The gasket between the canister and tank had deteriorated.
Water was slipping past the seal and into the bowl — even though the water level was below the overflow tube.
We confirmed this by turning off the water and returning later to find the tank empty.
That tells you the leak is at the bottom of the tank.
For this repair, we replaced the canister gasket.(See our step-by-step video here.)
Quick Diagnostic Summary
Water overflowing into overflow tube → Float or Fill Valve
Water level drops when supply is off → Flapper or Canister
Water on floor → External gasket or supply line issue
Final Thoughts
Toilet tank problems look complicated, but they’re usually one of three inexpensive parts.
Take five minutes to diagnose properly before replacing anything.
A systematic approach saves time, money, and frustration.
And once you fix one toilet, you’ll never be intimidated by one again.
In this video I'll show you how to replace this. You don't need any special tools. Just some confidence and a little bit of time.
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