If you’ve noticed your pool losing water while the pump is running, you’re not alone. This is a common concern among pool owners, often sparking confusion or panic — especially if the leak isn’t immediately visible. Water loss can be caused by a variety of issues, ranging from mechanical failures and evaporation to underground leaks or simple oversight. In this article, we’ll explore the most likely causes behind why your pool is losing water when the pump is running and provide actionable advice on how to diagnose and resolve the issue efficiently.
Understanding Normal vs. Abnormal Water Loss
Before jumping to conclusions, it’s important to understand that all swimming pools experience some degree of water loss over time. Evaporation is a natural and expected phenomenon, especially in regions with high temperatures and low humidity. On average, a pool can lose approximately a quarter of an inch of water per day, which equals about 1.5 inches per week.
However, when your pool is losing more than an inch of water per day and the pump is operating normally, there could be an underlying issue. This becomes even more suspect when water levels drop significantly while the pump is running, suggesting that water may be escaping through the pool’s circulation system rather than simply evaporating.
Top Causes of Pool Water Loss When the Pump Is Running
Let’s break down the common reasons that might be causing your pool to lose water when the pump is on, and how to distinguish between each possible cause.
1. Evaporation: The Usual Suspect
What to Look For:
- Water levels drop gradually and consistently.
- The loss seems to correlate with weather patterns.
- Water loss occurs whether the pump is on or off.
While it might seem counterintuitive that evaporation could cause noticeable loss with the pump running, it’s still relevant. The circulation system doesn’t necessarily prevent evaporation. If water levels drop evenly whether the pump is on or off, evaporation is likely the primary factor. However, if the drop is more noticeable or accelerated when the pump is on, it’s time to look at other possibilities.
2. Leaks in the Pool Plumbing System
When your pool loses water specifically during pump operation, the problem might lie in the plumbing system behind your pool equipment. The pump generates pressure that can force water out through cracks, loose fittings, or deteriorated valves, especially during operation.
Signs of a Plumbing Leak:
- Wet spots near the pool pump, filter, or chlorinator.
- A hissing or spraying sound while the pump is running.
- Pool water level remains steady when the pump is off.
To diagnose a plumbing leak:
- Visually inspect all visible plumbing connections.
- Run the pump and observe for unusual water movement or wet zones.
- Use food coloring around pipe joints; if the color gets sucked into a joint, you likely have a leak.
3. Faulty Pool Pump or Motor Housing
The pool pump itself could be the culprit. If its seals, gaskets, or motor housing are worn or damaged, water can escape when the pump is on.
What to Check:
- Look for cracks or wear on the pump housing.
- Inspect the shaft seal, especially if your pump is older.
- Check the pressure side of the pump for leaks.
If water sprays or drips from the motor or pump basket while it’s running, this points directly to a mechanical failure in the pump system.
4. Leaks Around the Skimmer or Return Jets
Leaks can also form around the skimmer intake or return jets, especially if these were not sealed properly during installation or have shifted over time.
How to Test:
- Place a bucket filled with water next to the pool and mark both water levels.
- Turn off the pump and let it sit 24–48 hours.
- If the pool water drops faster than the bucket water, the leak is likely in the structure or plumbing.
Another method: shut off the pump and use food coloring around skimmer edges and jet fittings. If the color gets sucked into a spot, you’ve identified the leak.
5. Return Line Leak Below the Water Line
A leak in the return line under the water level might cause noticeable water loss only when the pump is on. This is because water is returned to the pool through the jets, and if there’s a leak here, the escaping water would not be visible from the pool’s surface.
How to Test:
- Turn off the pump and wait for the water level to stabilize.
- Mark the water level.
- Turn the pump on and observe if the water drops only when the pump is running.
This indicates a leak in the return line or another pipe that loses water during operation.
6. Leaks in the Filter System
Leakage can also occur in the pool’s filtration unit, particularly through cracked tanks or faulty multiport valves. These components can allow water to escape when the system is under pressure.
What to Look For:
- Water collecting around or spraying from the filter tank.
- A drop in water level only when the pump is on.
- Poor filter performance or pressure changes in the system.
Effective Strategies to Diagnose and Locate the Leak
Now that we’ve covered the main causes, let’s look at some practical steps pool owners can take to identify where water is escaping during pump operation.
1. The Bucket Test
The “bucket test” is a simple way to determine whether your water loss is due to evaporation or a leak.
How to Do It:
- Fill a standard 5-gallon bucket with pool water to about one inch below the rim.
- Place the bucket on a pool step or ledge, so it sits level with the pool’s water level.
- Mark both the water level in the bucket and the pool water line.
- Run the pool pump for 24–48 hours.
- Compare the evaporation between the two containers.
If the pool water level drops significantly more than the water in the bucket, you likely have a leak.
2. Visual Inspections
Start by inspecting for any visible signs of water loss, such as damp spots, puddles, or dripping from the pump and filter area. Pay special attention to the pump housing, filter tank, valve boxes, and underground plumbing access points.
If your pool pump is housed in a concrete or plastic equipment pad, look for wet areas, mold growth, or softened soil near connections.
3. Dye Test for Underwater Leaks
If you suspect a leak in the pool structure, a dye test (using food coloring or a pool leak detection dye) can be highly effective.
How It Works:
- Place the nozzle of the dye bottle close to suspected areas — skimmer, light niche, return fittings, etc.
- If the dye gets sucked into a joint or surface, that’s your leak point.
This method is most effective when the pump is on because the movement of water can highlight specific suction points.
4. Shut-Off Method to Identify Plumbing Leaks
This method helps isolate whether the leak is coming from plumbing under the water line or from the pool structure itself.
Steps:
- Turn off the pump and wait 24 hours. Note the water level.
- Turn the pump back on, observing changes in water level.
- Record whether the pool loses water more quickly or only when the pump is on.
If the drop in water is only noticeable when the pump is running, the leak is probably in the return line or another part of the pressurized plumbing system.
What to Do After Identifying the Leak
Once you’ve identified the source of the leak, your next steps will depend on its location, severity, and whether or not you are comfortable making the repairs yourself or whether a professional pool technician is needed.
1. Fixing Minor Leaks Yourself
For above-ground or surface leaks, such as a minor crack in the pump housing or a loose fitting, you may be able to repair it with:
- Pool epoxy or sealant for small cracks.
- Tightening or replacing unions, O-rings, or gaskets.
- Replacing shaft seals in the pump (often a DIY fixable issue).
2. Dealing with Underground Leaks
Underground leaks are more complex and often require professional intervention. Attempting to fix these yourself can lead to further complications, such as digging in the wrong location.
Professionals may use tools like:
- Electronic listening devices to locate underground breaks.
- Dye in pressurized lines for pinpointing leaks.
- Pressure testing to isolate problematic zones in the plumbing system.
3. Consulting with a Pool Leak Detection Specialist
If you cannot locate the leak after trying multiple methods, it may be time to hire a professional specializing in pool leak detection. These services use advanced tools, including thermal imaging, pressure gauges, and ground microphones, to precisely locate underground or hidden leaks.
They can also offer written reports and digital evidence of the leak location, helping ensure repairs are accurate and minimizing landscape or structural disruption.
Preventing Future Water Loss
Once the leak has been addressed, there are several proactive steps pool owners can take to minimize evaporation and prevent future leaks:
1. Cover the Pool
Using a pool cover, especially a solar or solid cover, can reduce evaporation by up to 95%. It also keeps debris out, reducing the load on your pump and filtration system.
2. Regular Maintenance and Inspections
Schedule regular inspections of:
– The pump, filter, and plumbing system.
– Pool structure for cracks or wear.
– Chemical levels to avoid corrosion or scale buildup that can damage components.
3. Monitoring Water Usage
Track how much water your pool uses monthly, both in terms of evaporation and refill needs. Significant changes can help you catch issues early.
4. Optimize Water Chemistry
Poor water chemistry not only affects swimmer comfort but can also damage equipment and pool surfaces, leading to future leaks. Keep pH, alkalinity, and chlorine levels balanced, and test regularly.
Conclusion: Don’t Ignore Water Loss in Your Pool
Noticing that your pool is losing water when the pump is running can be concerning, but with a methodical approach, most issues can be diagnosed and resolved. Whether it’s a simple issue like increased evaporation or a complex underground leak, early detection is key to avoiding more costly repairs.
Taking time to understand the root cause — whether through a bucket test, dye test, or professional inspection — can save thousands of gallons of water and potentially thousands of dollars in property and equipment damage.
As a pool owner, being proactive with maintenance, using proper diagnostics, and knowing when to call in professionals can help you keep your pool safe, efficient, and enjoyable for years to come.
Final Thoughts
Monitoring your pool’s behavior, keeping up with maintenance, and being mindful of changes in water levels—especially when the pump is running—are crucial for long-term pool health. If your pool is losing water unexpectedly, there’s almost always a reason. The earlier you address it, the better off you’ll be.
Don’t let a leak drain your resources — act fast, repair effectively, and enjoy your pool worry-free.
Why does my pool lose water specifically when the pump is running?
When your pool loses water only when the pump is running, it’s typically an indication of a leak in the pool’s plumbing or structure that becomes active under pressure. The pump circulates water through the filter, heater, and sanitation systems, and any weakness in these components can cause water to escape. Common sources include cracked pipes, damaged fittings, or a faulty return jet that allows water to seep out while the system is operating.
Another possibility is a problem with the pool’s hydrostatic valve or a deteriorating seal around the pump itself. When the pump is off, the pressure is reduced, and leaks may not be noticeable. To confirm whether it’s a leak, perform a simple bucket test and observe the water loss in relation to the pump’s on/off cycles. If the loss is significant only when the pump is active, it’s time to inspect the plumbing system and possibly call a professional.
Can evaporation cause noticeable water loss when the pump is running?
Evaporation can contribute to some degree of water loss, especially in dry, hot climates or when the pool features water features like fountains or waterfalls that increase surface exposure. However, evaporation affects the pool’s overall water level irrespective of whether the pump is on or off, so if you’re only noticing the loss when the pump is running, it’s unlikely to be due solely to evaporation.
To determine if evaporation is a factor, conduct a bucket test by placing a bucket filled with pool water on a pool step and marking both the inside of the bucket and the outside pool water level. Run the pump for a few days and compare the rate of water loss. If the pool loses significantly more water than the bucket when the pump is active, it strongly suggests a leak rather than evaporation.
What are common leak locations that cause water loss when the pump is on?
Leak locations vary but typically appear around the plumbing lines that carry water under pressure when the pump is running. These can be under the pool deck or in the equipment pad. Common spots include connections at the pump and filter, cracked PVC pipes, or faulty gaskets on valves and fittings. Pressure-side leaks can cause water to escape unnoticed, especially if the water seeps into the ground.
Other frequent sources include return jets in the pool walls, which may develop cracks or poor seals over time, and the main drain assembly if there’s a problem with the hydrostatic valve. Identifying the exact source of the leak may require pressure testing the lines or using dye testing around suspected areas while the pump is running. If the leak is not visible, hiring a professional leak detection service is the best course of action.
How can a faulty pool pump cause water loss?
A faulty pool pump itself can potentially be the source of water loss. If the pump’s seals or gaskets—especially within the lid or motor housing—are worn or damaged, water can escape when the pump is operating. These types of leaks are often noticeable around the pump housing as a steady drip or pool, especially during or immediately after the pump runs.
Additionally, improper installation or loose connections can also lead to water loss. The pump must be properly sealed to all incoming and outgoing pipes to prevent leakage. If you notice water dripping from the pump itself or pooling around the equipment when it’s on, it’s advisable to turn the system off immediately and inspect all seals and connections. Replacing seals, gaskets, or tightening fittings can help, but for complex issues, consult a licensed pool professional.
Is a leak in the filter or heater causing the water loss?
If your pool loses water when the pump is running, the problem could lie within the filter or heater unit. Both components are under pressure during operation, and any cracks, broken seals, or loose connections can lead to water escaping. Leaks in the filter can occur due to a cracked tank body, O-ring failure around the lid, or issues with the multiport valve.
A heater can lose water through problems in the heat exchanger or plumbing connections, especially if the heater has been improperly winterized or physically damaged. Look for signs like wet spots around the heater, unusual sounds during operation, or visible cracks. You can sometimes locate these leaks by isolating the heater and filter and running the system without them. If the leak goes away, consult a pool technician for further inspection and repairs.
Could backflow or improper valve settings be the reason for water loss?
Backflow or improperly set valves might contribute to pool water loss, particularly if the system’s multiport valve is faulty or partially misaligned. When the multiport valve is not in the correct position, water may be directed out through the waste line instead of returning to the pool. This can cause a noticeable drop in water level during pump operation.
Additionally, check for broken or loose internal parts within the valve that might allow for water backflow. Common signs include water spilling from the backwash line, loss of prime, or a significant reduction in filter pressure. Ensure your multiport valve settings are properly aligned for routine filtration and routinely inspect the valve for wear or damage. If issues persist, replacing the valve or consulting with a specialist may be necessary.
What should I do if I suspect my pool is leaking when the pump is on?
First, shut off the pump and inspect the equipment area for visible signs of water leakage. This includes checking the pump, filter, heater, and all plumbing connections. Even small drips or wet spots can indicate a larger issue. If no obvious source is found, conduct a bucket test and observe water loss over 24–48 hours with the pump on and off to confirm if it’s a leak rather than evaporation.
Next, consider professional leak detection methods. Companies use dye tests, pressure tests, and even electronic listening devices to pinpoint the exact location of a leak. Repair could involve patching a crack, replacing worn seals, or excavating underground pipes if a buried leak is identified. Addressing the problem early can prevent bigger issues and reduce your water waste, utility costs, and potential structural damage to your pool system.