If you’re a homeowner with an in-ground swimming pool, few things are more frustrating than watching your water level drop — especially when it happens faster while the pump is running. You might be thinking, “Isn’t the pump supposed to circulate water, not waste it?”
It’s a fair question, and the answer is both complex and straightforward.
Yes, your pool pump circulates water for filtration, heating, and sanitation. But when the system is running, it creates pressure in one half of your pool’s plumbing and vacuum in the other. This shift can also expose hidden leaks that only become active under these specific conditions.
If your swimming pool only loses water when the pump is on, or if you notice water loss increasing during pump operation, this is a sign that something is wrong — most likely a leak on the pressure side of the plumbing. In this guide, we’ll break down:
Why pressure affects water loss
Standard plumbing and equipment failure points
How to diagnose the issue
What to do next to stop the water loss for good
Let’s get into it.
Understanding Pool Plumbing: Suction vs. Pressure Sides
To grasp why a pool loses more water with the pump running, you first need to understand the layout of a typical pool’s plumbing system.
Every pool has two hydraulic zones:
- The suction side (from the pool to the pump): This includes the skimmers, main drains, and any vacuum lines, all of which draw water into the pump.
- The pressure side (from the pump back to the pool): This includes the filter, heater, chlorinator, return lines, and return jets.
When the pump is off, there’s little to no pressure or vacuum in the system. This is called a static state. When the pump is on, it creates positive pressure on the return side and a vacuum on the suction side, causing water and air to behave very differently.
That difference is crucial because:
- A leak in a return line (pressure side) may not be noticeable at all when the pump is off, but it can spray gallons per hour when it’s on.
- A leak in a suction line, such as the skimmer line, may draw air into the system when the pump is running, but it will not visibly cause water loss.
- Some leaks only “open up” under pressure or vacuum.
This is why some pools lose more water when the pump is running: the pressure is forcing water out through compromised fittings or plumbing.
Why the Pump Makes Leaks Worse
When your pool pump runs, it exerts force on every part of the return plumbing system. Think of it like increasing the water pressure in a garden hose — if there’s a weak spot, it will spray.
This “pressurization” forces water through:
Return jets
Valves and fittings
The filter housing
Heater manifolds
Chlorinator fittings
Underground return lines
If any of these components have a crack, pinhole, bad seal, or loose joint, they may leak more (or only) when the pump is on. These are called dynamic leaks — meaning they only occur under system pressure or vacuum.
Conversely, static leaks occur regardless of whether the pump is on or off, such as a cracked skimmer throat or shell crack.
Common Causes of Water Loss When the Pump Is Running
Here are the most frequent culprits of this type of leak:
1. Cracked or Broken Return Lines (Underground)
This is the most common cause of increased water loss with the pump on.
Your pool’s return lines — the pipes that carry filtered water back to the pool — are under positive pressure. If one of these lines is cracked or separated (often due to ground movement or aging polyvinyl chloride, or PVC), it will leak water into the soil every time the pump runs.
Warning signs include:
Soggy or sunken areas in your yard
Excessive water loss only when the pump is on
A sudden increase in your water bill
2. Leaking Return Jet Fittings
Return jets are threaded fittings in the pool wall. If the fitting is cracked or not sealed properly, it may leak behind the pool wall.
Sometimes, the pipe joint just behind the jet fitting will have a hairline fracture. This leak will usually not show on the surface but can cause significant water loss into the soil, especially when pressurized.
3. Leaking Equipment Pad (Valves, Unions, Filter)
The equipment pad contains several pressure-side components:
Filter (sand, DE, or cartridge)
Pump fittings
Backwash valve or multiport valve
Heater and chlorinator unions
Leaks often occur at:
Cracked filter tanks or lids
Loose union fittings
Worn valve O-rings
Damaged pump shaft seals
Broken pressure gauges
These leaks may be visible during inspection. If your filter is dripping, hissing, or spraying during operation, you may have found the source.
4. Backwash Line or Waste Line Leaks
Many pools have a sand filter with a multiport valve that includes a “waste” or “backwash” setting. If the valve is faulty or the gasket is worn, water may be discharged from the waste line, even when the filter setting is engaged.
This is often a silent leak — water flows directly to a drain or the yard, and you never notice it.
Suction-Side Leaks (Less Common But Still Possible)
While most dynamic leaks occur on the pressure side, suction-side issues can also cause abnormal water behavior when the pump is running.
Suction leaks draw air into the system. Signs of a suction-side leak include:
Air bubbles in the pump basket
Loss of prime or surging water flow
Excess air coming out of return jets
In severe cases, a suction leak may rupture the siphon, causing the pump to drain its system and resulting in slow water loss.
Areas to check:
Skimmer plumbing lines
Main drain lines
Pump lid O-ring
Pump housing cracks
How to Test for a Pressure-Side Leak
If you suspect your pool is losing more water while the pump is running, here’s how to test it.
The 24-Hour Pump Test
- Mark the water level on the tile or skimmer face with a piece of tape.
- Turn off the pump for 24 hours.
- Measure the drop in water level.
- Then, run the pump continuously for 24 hours.
- Measure again.
If the pool loses significantly more water when the pump is on, you almost certainly have a pressure-side plumbing leak.
Other Diagnostic Methods
Dye testing (for skimmer throats, light conduits, cracks)
Listening devices (professional leak detection)
Pressure testing the return lines (done by a pro)
Pipe camera inspection for visual confirmation of broken underground plumbing lines
What Happens If You Ignore the Leak?
Leaks that only occur while the pump runs can go undetected for weeks or months. But ignoring them causes serious problems:
Structural damage: Leaking water erodes soil, which can lead to deck cracks or pool shifting.
Wasted water: Gallons lost per day = higher bills.
Chemical imbalance: Leaks throw off chlorine, pH, and salt levels.
Pump and filter stress: Equipment may run dry, overheat, or wear out prematurely, resulting in reduced efficiency and increased maintenance costs.
How to Fix It
Some minor leaks, such as pump lid O-rings or unions, are DIY-friendly.
Homeowner Fixes Might Include:
Replacing pump lid O-rings
Tightening union fittings
Applying Teflon tape or pool lube to leaky joints
Replacing leaking Jandy Valves
But Most Pressure-Side Leaks Require a Pro
If the leak is underground or behind the wall, you need specialized tools like:
Pressure testing plugs
Pressure testing rigs
Electronic listening devices (hydrophones)
Helium tracer gas
Ultrasonic microphones
Swimming Pool Leak Detection Professionals, like those at Precision Leak Detection, can pinpoint the exact location of the leak without digging blindly and then advise on the best path forward — whether it’s an equipment leak, plumbing repairs, or rerouting plumbing lines.
Call Precision Leak Detection for Help
At Precision Leak Detection, we specialize in identifying and locating even the hardest-to-find leaks in in-ground swimming pools. If your pool loses more water when the pump is on, you’re likely dealing with a pressure-side plumbing issue — and that’s exactly what we’re trained to solve.
If you live in West Palm Beach or anywhere in Palm Beach County and are seeking expert swimming pool leak detection services, please get in touch with us — we’re here to help.
We use professional-level:
Fluorescent Dyes
Microphones
Pressure Testing Equipment
Cameras
We’ll locate your leak quickly, explain the issue clearly, and connect you with trustworthy repair options to get your pool watertight again — no guesswork, no wasted water, and no endless frustration.
Don’t let your pool leak another day.
Contact Precision Leak Detection.
