Pool Losing Water Only When Equipment Is On
If your pool holds water fine overnight but drops every time the pump runs — that's not a mystery. That's a pressure-side leak, and it has a specific set of causes.
Call For a Diagnosis — 214-972-3330When a pool only loses water while the pump is running, the leak is on the pressure side of the plumbing — the pipes that carry water under positive pressure from the pump back to the pool. The pump is literally pushing water through the break and into the surrounding soil every hour it runs. Shut the pump off and the leak stops. Turn it on and water loss resumes.
The Pump Creates the Pressure That Creates the Leak
Your pool plumbing operates in two zones. Understanding which zone your leak is in tells us exactly where to look — and this symptom pattern makes the diagnosis unusually precise.
The pressure side runs from the pump outlet through the filter and heater, then underground through return lines back to the pool. When the pump is running, this entire section operates under positive pressure — typically 10 to 25 PSI. Any crack, failed joint, or loose fitting in this zone allows the pump to actively push water out with every revolution of the impeller.
When the pump shuts off, the pressure drops to zero and the leak stops — which is exactly why you only see water loss during pump run cycles.
Every Location Where a Pump-On Leak Can Be Coming From
Most pump-on leaks are on the pressure side — downstream of the pump. But a suction-side line can also lose water when the pump is running if the equipment sits higher than the pool. Here's every source to check.
Return Line Underground
HiddenPipe crack, failed joint, or soil movement separation beneath the deck. The pump pushes water directly into the surrounding soil with no visible sign above ground. Requires pressure testing and acoustic detection to locate.
Return Fittings in Pool Wall
Below WaterlineEyeball fitting or directional return gasket fails. Under pump pressure, water is pushed behind the fitting and through the shell into the soil. Dye testing with the pump running confirms this source in minutes.
Equipment Pad Plumbing
VisibleUnion fittings, valve bodies, or plumbing connections at the filter, heater, or chlorinator can leak under operating pressure. Check for wet concrete, dripping, or white mineral deposits at the pad while the pump is running.
Backwash / Waste Line
VisibleA worn multiport valve spider gasket bleeds water to the backwash line every time the pump runs — even in filter mode. Check if water is flowing out the backwash port while the valve is set to filter. Common and inexpensive to fix.
Heater or Heat Pump Connections
VisiblePVC unions at heater inlet and outlet connections fail with age and heat cycling. The leak only occurs under operating pressure. Look for water staining on the heater housing or wet concrete beneath the unit while running.
Cleaner Line or Booster Pump
VariesDedicated pressure cleaner lines and booster pump connections can develop leaks that only appear during pump operation. If you have an in-floor cleaning system or booster pump, those lines are included in a full pressure-side inspection.
Spa Spillway
VisibleWhen the pump is running and the spa is active, water flows over the spillway into the pool. A cracked or poorly sealed spillway lip can bleed water behind the spa wall and into the surrounding soil — only while the system is running and flow is passing over it. Check for wet areas around the spa exterior wall during pump operation.
Waterfall or Water Feature
VariesWaterfalls and decorative water features only carry water when the pump is running. A crack in the waterfall basin, a failed seal at the base, or water sheeting off the back side of the feature can drain significant volume — entirely invisible when the pump is off. Isolating the water feature line during testing confirms or eliminates this source.
Suction Line Leak (Elevated Equipment)
HiddenIf your equipment pad sits higher than the pool, the pump actively pulls water uphill through the suction lines when running. Any crack or failed joint on that suction line will lose water under the negative pressure created by the pump — a pump-on leak that looks like a pressure-side problem but is actually upstream of the pump. This is specific to installations where the pad is elevated above pool level.
Before calling anyone, walk your equipment pad while the pump is running and look for wet concrete, dripping connections, or water pooling around the filter, heater, and valve. Also check the backwash line — if water is flowing out of it while in filter mode, your multiport valve spider gasket has failed. These are visible fixes. If the equipment pad is dry and the pool is still losing water, the leak is underground.
If your pool loses water whether the pump is running or not, you definitely have a structural leak unrelated to equipment — most likely a shell crack, failed fitting, or underground pipe break that drains continuously regardless of pump state. This pattern eliminates evaporation and equipment issues from the equation entirely. Call for a diagnosis.
The Two-Night Test — Confirm Pump-On Water Loss in 48 Hours
Before calling, run this test. It confirms the pattern and gives us the diagnostic information we need to bring the right equipment to your job.
Night One — Pump On
Mark the water level at the skimmer with tape before bed. Leave the pump running all night on its normal timer. In the morning, mark the new water level and measure the drop.
Night Two — Pump Off
The following night, mark the water level again at the same time. Turn the pump completely off. Also turn off any auto-fill valve. In the morning, measure the drop again.
Compare the Results
If Night One (pump on) showed significantly more water loss than Night Two (pump off) — you have a confirmed pressure-side leak. The bigger the gap between the two measurements, the larger the pressure-side leak.
Inspect the Equipment Pad During the Day
While the pump is running in daylight, inspect every visible connection at the equipment pad. Look for wet spots, dripping, or white mineral deposits that indicate a slow leak under pressure. Run the pump, then walk the perimeter — sometimes you can hear the hiss of a pressurized leak before you see it.
Call Us With Both Numbers
When you call, tell us how much the pool dropped on each night. That single data point tells us the approximate rate of loss and whether we're likely dealing with a visible equipment pad leak or a buried return line break — and we'll come prepared for both.
If your pool is dropping more than half an inch per pump cycle, every hour the pump runs is pushing dozens of gallons into the soil beneath your pool. This erodes the foundation support beneath your shell and deck. Limit pump run time to the minimum needed to keep water sanitized until the leak is found and repaired.
The Underground Return Line Break — Most Damaging, Easiest to Miss
Of all the pump-on leak sources, an underground return line break is the one that causes the most long-term damage and the one homeowners are least likely to find on their own.
Return lines run from your equipment pad underground beneath your pool deck and back up through the pool wall to the return jets. They can be 20 to 60 feet long in a typical DFW pool — buried under concrete, pavers, or soil the entire way. When a joint fails or the pipe develops a crack from soil movement or freeze damage, the pump sends hundreds of gallons into the ground every day.
No Visible Signs Above Ground
Underground return line leaks often leave no surface evidence for months. The water disperses through the soil before reaching the surface. By the time soil erosion becomes visible, significant damage has already occurred beneath the deck.
Wet or Soft Spots in the Yard
In some cases — particularly after heavy rain has saturated the soil — you may notice an unusually wet area in your yard or near the pool equipment in a location that doesn't make sense. This can indicate the approximate direction of the underground leak.
Sinking or Cracked Pool Deck
A return line leaking under the deck for months or years erodes the soil beneath the concrete slab. Sections of the deck begin to sink, crack, or tilt. By this point the leak has been active long enough to cause structural concerns.
Pressure Test Confirms It in Minutes
The return line is isolated and pressurized. If it won't hold pressure, there's a break underground. Electronic acoustic listening equipment then locates the exact position of the break so we know precisely where to excavate — minimizing disruption to your deck.
Why Pressure-Side Leaks Are So Common in DFW Pools
North Texas conditions create specific stress on the pressure side of pool plumbing that most homeowners don't realize until the water bill arrives.
Clay Soil Moves Return Lines
The same expansive clay that cracks pool shells also moves underground pipes. As soil swells and contracts through seasonal moisture changes, PVC return lines get pushed, pulled, and stressed at joints and elbows over years — until a joint finally gives way.
Freeze Events Split Pressurized Lines
The 2021 winter storm left ice in return lines across DFW. Water expands 9% when it freezes, and that expansion inside a pressurized pipe is enough to split joints or fracture PVC. Many return line breaks in DFW pools trace directly back to that event.
Pump Timers Run for Long Cycles
Many DFW homeowners run pumps 8 to 12 hours a day in summer to maintain water quality and operate solar or gas heaters. A return line break active during a 10-hour pump cycle loses far more water per day than a shell or fitting leak of the same size.
High Operating Pressures in Aging Systems
Older DFW pools with clogged or undersized plumbing run at higher-than-normal system pressures. A line already stressed by soil movement is more likely to fail under elevated operating pressure — and will lose water faster when it does.
How We Find a Pressure-Side Leak
Pump-on leak diagnosis follows a specific sequence that moves from visible to hidden, and from inexpensive to find to requires equipment. We never skip to the expensive step before ruling out the simple ones.
Equipment Pad Inspection Under Live Pressure
We inspect every fitting, union, valve, and connection at the equipment pad while the pump is running. Visible leaks at the pad — filter connections, heater unions, valve bodies — are found immediately and repaired without pressure testing.
Multiport Valve Backwash Test
We check whether water is flowing to the backwash line while the valve is in filter mode. A worn spider gasket is a common, low-cost fix that causes significant pump-on water loss.
Return Fitting Dye Test
With the pump running, we dye test all return fittings inside the pool. Under operating pressure, any fitting with a failed gasket will show dye being pulled through or pushed behind the fitting — confirming it as the leak source before any excavation.
Return Line Pressure Testing
If visible checks pass, we isolate and pressure test the return lines. A line that won't hold pressure has a break underground. This step tells us which specific line is failing so we know exactly where to focus the acoustic search.
Acoustic Detection — Finding the Exact Break Point
Once we know which line is failing, electronic listening equipment locates the exact position of the break beneath the deck. We mark the spot precisely — minimizing how much concrete or paving needs to be removed to access the pipe.
What the Repair Looks Like Once We Find the Source
Pool Leak Detection
Full pressure-side diagnostic inspection including equipment pad, return fittings, and underground line testing.
Dye Test
Confirms whether return fittings inside the pool wall are leaking under live pump pressure.
Return Line Repair
Locate and repair underground return line breaks — with or without deck access depending on pipe depth.
Concrete Pool Deck
Repair and restore deck sections affected by soil erosion from an active underground pressure-side leak.
Pool Foundation Repair
Address structural damage caused by soil washout from a long-running underground return line leak.
Pool Skimmer Repair
If both pump-on and pump-off loss is present, skimmer leaks are tested alongside pressure-side sources.
Learn More About Pool Water Loss
Pool Losing Water When Pump Is On — Common Questions
The pump creates positive pressure in the return lines. Any crack, failed joint, or loose fitting on the pressure side of the system allows water to be actively pushed out while the pump is running. When the pump shuts off, pressure drops to zero and the leak stops. This is called a pressure-side leak, and it is always downstream of the pump.
In DFW, the most common cause is an underground return line break or failed pipe joint. Clay soil movement and freeze events both stress the buried return lines over time until a joint separates or a pipe cracks. The leak is invisible above ground and can push hundreds of gallons a day into the surrounding soil.
Yes — return lines only carry water under pressure when the pump is running. When the pump is off, these lines depressurize and the leak stops. This is exactly what makes pump-on water loss such a reliable indicator of a return line problem rather than a shell or skimmer issue.
Yes. Every pump cycle actively pushes water through the break into the soil beneath your pool, eroding the support beneath your shell and deck. Limit pump runtime to the minimum needed for sanitation until the leak is repaired. Do not ignore a confirmed pump-on leak — the structural damage from ongoing soil erosion is cumulative and expensive to fix.
First, pressure testing confirms which return line is failing. Then electronic acoustic listening equipment is used to pinpoint the exact break location beneath the deck without excavation. Once we mark the spot, the excavation footprint is as small as possible — often just a single core or cut rather than tearing up entire deck sections.
Losing water in both scenarios means you likely have leaks on both sides of the system — a pressure-side leak active when the pump is on, and a shell or fitting leak that continues when the pump is off. This is common in older DFW pools that have been affected by soil movement. A full diagnostic inspection will identify and prioritize all active leak points.
Pool Holds Fine Overnight But Drops Every Time the Pump Runs?
That's a pressure-side leak — and every hour the pump runs, it's pushing water into the soil beneath your pool. Let's find it before it becomes a structural problem.
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