If your toilet runs for a few seconds, stops, and then starts running again later without being flushed, it can be confusing and frustrating. This intermittent behavior often makes homeowners wonder if something serious is wrong.
In most cases, the cause is a slow leak inside the toilet tank that triggers the fill valve to turn on periodically.
Quick Answer:
A toilet that runs intermittently is usually losing small amounts of water from the tank due to a leaking flapper, improper water level, or worn internal parts. When the water level drops, the fill valve turns on briefly to refill the tank.
Why This Happens
Unlike a toilet that runs constantly, an intermittently running toilet usually seals almost correctly — but not completely.
Common causes include:
- A flapper that doesn’t seal perfectly every time
- Mineral buildup on the flush valve seat
- A water level set slightly too high
- A worn or sensitive fill valve
- Small leaks allowing water to slowly drain into the bowl
As water slowly escapes from the tank, the fill valve activates to replace it, causing the toilet to run briefly at random times.
This issue is closely related to a toilet that won’t stop running, but the leak is slower and less obvious.
Step-by-Step: How to Fix a Toilet That Runs Intermittently
Step 1: Perform the Dye Test
Add a few drops of food coloring to the toilet tank.
- Do not flush
- Wait 10–15 minutes
If colored water appears in the bowl, water is leaking from the tank even if you don’t see it.
Step 2: Inspect the Flapper
Check the rubber flapper at the bottom of the tank.
Look for:
- Warping or stiffness
- Cracks or surface damage
- Poor alignment over the flush valve
Even small imperfections can allow slow leaks that cause intermittent running.
Step 3: Clean the Flush Valve Seat
Mineral buildup where the flapper seals can prevent a tight seal.
- Turn off the toilet supply valve
- Flush to empty the tank
- Gently clean the sealing surface with a cloth or sponge
This often fixes slow leaks without replacing parts.
Step 4: Check the Water Level
If the tank water level is too high, water can slowly drain into the overflow tube.
The correct level is usually:
- About one inch below the top of the overflow tube
Lower the float slightly if needed.
Step 5: Evaluate the Fill Valve
If the flapper seals properly but the toilet still runs intermittently, the fill valve may be overly sensitive or worn.
Fill valves can activate too easily when tank levels drop slightly, even from minor evaporation or vibration.
What Not to Do
- Don’t ignore intermittent running
- Don’t assume the toilet is “just old”
- Don’t overtighten internal parts
- Don’t add chemical tablets to the tank
Small leaks tend to worsen over time and increase water bills.
When to Call a Professional
Call a plumber if:
- Intermittent running continues after replacing the flapper
- You can’t identify the source of the leak
- Water level changes unpredictably
- Multiple toilets show similar behavior
This may indicate pressure or supply-related issues.
Safety Notes
- Turn off the toilet supply before working inside the tank
- Handle porcelain and plastic parts gently
- Dry the tank base to detect leaks
- Replace brittle or cracked components promptly
Common Homeowner Questions
Is intermittent toilet running expensive?
Yes. Even small leaks can waste a significant amount of water over time.
Why does it only happen at night?
At night, water usage elsewhere is low, making small tank leaks more noticeable.
Will replacing the flapper usually fix this?
Yes. A new flapper solves most intermittent running problems.