One of the most useful ways to gather information about a swimming pool leak is by performing a Pump On / Pump Off Test.
This simple test helps determine whether your pool is losing water when the system is running or when it is turned off. While it does not pinpoint the exact location of a leak, it can provide valuable clues about whether the problem is related to underground plumbing or the pool structure itself.
At Atlantic Pool Leak Detection, we often recommend this test as part of the diagnostic process because it can help narrow down potential leak sources before more advanced testing is performed. If you’re new to leak detection, our Pool Leak Guide provides a complete overview of how pool leaks are diagnosed and located.
Understanding the Three Main Types of Pool Leaks
Most swimming pool leaks fall into one of three categories:
Pressure-Side Plumbing Leaks
These leaks occur in the return plumbing lines that carry water back to the pool after it passes through the equipment.
Suction-Side Plumbing Leaks
These leaks occur in plumbing lines that pull water from the pool to the equipment, including skimmer and main drain lines.
Structural or Non-Plumbing Leaks
These leaks occur within the pool itself and may involve:
- Vinyl liner holes or tears
- Pool light leaks
- Skimmer mouth cracks
- Return fitting leaks
- Structural cracks
- Leaks around penetrations and fittings
Some homeowners assume pool leaks are caused by plumbing, but structural leaks are also very common. Learn more about the most common signs your pool may have a leak if you’re unsure whether your pool is actually losing water.
Before You Begin
To ensure accurate results:
- Do not perform the test on rainy days
- Fill the pool to its normal operating level
- Mark the water level accurately using tape or another visible marker
- Do not allow the water level to drop below the skimmer while the pump is running
- Perform each test for the same amount of time, ideally 24 hours
Consistency is important. The more controlled the testing conditions are, the more useful the results will be.
How to Perform the Pump On / Pump Off Test
Step 1: Fill the Pool
Fill the pool to its normal operating level, typically halfway up the skimmer opening or tile line.
Step 2: Mark the Water Level
Use duct tape, painter’s tape, or another visible marker to clearly indicate the current water level.
Taking a photo at this stage can help document your starting point.
Step 3: Perform the Pump Off Test
Turn the pool pump completely OFF.
Allow the pool to sit for 24 hours (do not use or run the pool).
After 24 hours:
- Check the water level
- Compare it to your original mark
- Take photos of the tape to document any water loss
Step 4: Refill to the Original Level
If water loss occurred, refill the pool back to the original marked level before beginning the next test.
Step 5: Perform the Pump On Test
Turn the pool pump ON and allow it to operate normally for the same amount of time used during the Pump Off Test.
Ideally, using identical testing periods provides the most accurate comparison (i.e. 7pm > 7 pm).
Step 6: Compare Results
After the second 24-hour period:
- Check the water level
- Compare it to your original mark
- Take photos of the tape to document any water loss
- Compare loss when pump is off to when pump was on
What the Results May Indicate
Pool Loses More Water When the Pump Is ON
If the pool leaks faster (or only leaks) when the pump is running, this often indicates a pressure-side plumbing leak.
Common examples include:
- Return line leaks
- Polaris or vacuum line leaks
- Waterfall or other water feature leaks
- Cracked return fittings
- Underground plumbing failures
- Pressure-side plumbing connections
These types of leaks often require pool pressure testing to accurately confirm and diagnose.
Pool Loses More Water When the Pump Is OFF
If the pool leaks faster (or only leaks) when the pump is turned off, this can indicate a suction-side plumbing leak.
Common examples include:
- Skimmer line leaks
- Main drain line leaks
- Underground suction plumbing failures
In some situations, suction-side leaks may also cause air to enter the system, resulting in air bubbles at return jets or difficulty maintaining pump prime.
Pool Loses the Same Amount of Water Both ON and OFF
If water loss remains relatively consistent regardless of whether the pump is operating, the leak is often structural or non-plumbing related.
Common examples include:
- Vinyl liner leaks
- Pool light conduit leaks
- Skimmer cracks
- Return fitting leaks
- Concrete or gunite cracks
Many of these issues require specialized testing methods to accurately identify.
Important Limitations of the Pump On / Pump Off Test
While this test is extremely useful, it is not a definitive leak detection method.
The test can help narrow down possibilities, but it cannot:
- Confirm the exact source of the leak
- Pinpoint the location of a plumbing break
- Determine the severity of the leak
- Replace professional leak detection
In some cases, multiple leak sources may exist simultaneously, making the results much more difficult to interpret.
This is why professional leak detection often combines multiple testing methods, including pressure testing, electronic leak detection, and structural evaluations.
When Professional Leak Detection Is Recommended
You should consider professional leak detection if:
- Water loss exceeds normal evaporation
- The source of water loss is unclear
- You’ve already attempted repairs
- The Pump On / Pump Off Test suggests a plumbing issue
- Your pool continues losing water despite previous repairs
Many of the underground plumbing failures we identify cannot be confirmed without specialized equipment and testing procedures. Learn more about underground pool plumbing leaks and how they are professionally diagnosed.
Need Help Finding a Pool Leak?
The Pump On / Pump Off Test is an excellent starting point, but it is only one piece of the puzzle.
If your pool is losing water and you want an accurate diagnosis before spending money on repairs, Atlantic Pool Leak Detection can help.
We specialize in pressure testing, underground plumbing leak detection, structural leak detection, and advanced diagnostic services throughout New Jersey and Staten Island.
Call 732-333-3304 or contact us to schedule professional pool leak detection today.
FAQ
What does a Pump On / Pump Off Pool test tell you?
The test helps determine whether water loss occurs primarily when the system is running or when it is turned off, which can help narrow down potential leak sources.
Does losing more water with the pump on always mean a plumbing leak?
Not always, but it often suggests a pressure-side plumbing issue that may require pressure testing for confirmation.
Can a pool leak only when the pump is off?
Yes. Suction-side plumbing leaks often become more noticeable when the system is not operating.
What if my pool loses the same amount of water regardless of whether the pump is on or off?
This often suggests a structural leak, such as a liner leak, pool light leak, skimmer crack, or fitting leak.
How long should I perform the Pump On / Pump Off Test?
For best results, each test should be performed for 24 hours under similar weather conditions.
Is the Pump On / Pump Off Test enough to find a leak?
No. The test helps narrow down possibilities, but professional leak detection is often required to confirm and locate the exact source of water loss.
What is the next step after performing this test?
If the results suggest a plumbing leak or if water loss remains unexplained, professional leak detection and pressure testing are typically recommended.
