If your pool only seems to lose water while the pump is running—but not when it’s off—you’re not alone. Many pool owners are puzzled by this exact scenario, especially those with an attached in-ground spa. What’s happening behind the scenes often comes down to a pressure-side leak in the spa’s plumbing, damage to the spa structure, or a leak in the spa light conduit, which only becomes active when the system is circulating water.
These types of leaks aren’t just annoying—they can waste hundreds of gallons of water per day, increase your utility bills, and slowly cause underground damage. Fortunately, there are clear signs to look for and practical steps you can take to figure out where the problem is coming from.
In this article, we’ll break down:
Why do leaks worsen when the pump is running
Why attached spas are often the hidden culprit
The most common spa-related leak locations
DIY diagnostic steps you can take
When it’s time to call a professional leak detection company
Let’s explore why your spa might be draining your pool—and your wallet.
Why the Leak Only Happens When the Pump Runs
Your pool and spa’s circulation system has two sides:
The suction side, where water is pulled from drains and skimmers into the pump
The pressure side, where water is pushed through return lines, jets, and other features
When the pump is off, all of the water is static—there’s no pressure in the system. If a leak is structural (like a crack in the shell or a faulty fitting), it may still leak, but very slowly.
When the pump is on, pressure builds in the return lines and spa plumbing. If there’s any weakness in the system—like a cracked return pipe, a broken jet fitting, or a leaking spa light conduit—the water is pushed out with force. This is why you may lose far more water during pump operation.
If your pool leaks only when the system is running, especially if it has an attached spa, it is likely that the issue is actually located within your spa itself and has nothing to do with the pool.
3 Common Spa-Related Leaks That Worsen With the Pump Running
1. Cracked Spa Return Lines or Jets
The spa return lines carry pressurized water to the jets. These lines are buried underground or behind the spa wall, and they’re under constant pressure when the pump is running.
If any of the following are damaged, they can leak significantly when the system is on:
A cracked PVC elbow or tee
A leaking jet body or gasket
Because these components are under pressure, leaks often stop completely once the pump stops.
Signs of a leaking spa return line:
Water loss only during spa circulation
Bubbling or weak spa jets
Puddles or dampness around the spa or equipment
Sinking decking or erosion of the yard near your spa
2. Spa Light Niche or Conduit Leaks
Most in-ground spas have an underwater light set into a niche or housing. The electrical wiring runs through a flexible conduit out the back of the niche and underground. If this conduit isn’t sealed properly—or becomes cracked—it becomes a perfect path for water to escape.
Spa light leaks often behave in a very specific way:
Water loss stops at the height of the light
The niche may fill with water
Conduit pipes act like a drain
When the pump is on, turbulence or slight suction can pull more water through the gap, making the leak worse.
Signs of a spa light leak:
Water loss down to the light level, then stops
Water or condensation behind the lens
Electrical problems or tripped GFCI breakers
Gurgling or bubbles around the light housing
DIY tip: With the pump off, apply pool dye near the light fixture. If the dye gets pulled in, you likely have a leak. You can also lower the water below the light and monitor to see if the leak stops.
3. Cracks in the Spa Shell or Finish
Structural cracks in the spa shell, finish, or around fittings can also leak—and if they’re located near a return fitting or jet, they may leak worse under pressure.
Sometimes the crack is behind the plaster or tile, near where plumbing enters the spa. The force of returning water can open up these cracks and allow water to push through.
Signs of a shell crack:
Continuous water loss in your spa, even with the pump off
Cracking or hollow spots in plaster or tile
Moisture around spa walls
DIY tip: Dye test visible cracks with the pump off. If the dye gets sucked in, there’s a structural leak. Be sure to inspect closely around jets, lights, and drain fittings.
How to Tell If the Spa Is Causing the Leak
If your pool has an attached spa and you’re losing water only during pump operation, follow these steps to pinpoint the spa as the cause:
Step 1: Do a Bucket Test
Fill a bucket with pool water and place it on a spa step
Mark the water level inside and outside the bucket
Leave your pump turned off for 24 hours
Result:
If the spa drops faster than the bucket, you’ve confirmed a leak
If the pool stays steady and the spa drops, your spa is the problem
Step 2: Close the Spa Jandy Valve
If you have a dedicated spa return line or Jandy valve, close it and run the pool in pool-only mode.
If the water loss stops, you’ve confirmed the problem is with the spa
If it continues, the issue may be in shared plumbing or the pool itself
Step 3: Run the System in Spa-Only Mode
Switch your valve configuration so that water circulates only through the spa. Let it run for several hours.
If the spa loses water quickly, you’ve confirmed a leak within your spa
If not, you can rule out a leak within your spa system
Step 4: Inspect While Pump Is Running
With the system running in spa mode, inspect:
Around the spa structure
Around jet bodies and fittings
At the equipment pad—especially valves and unions
Look for:
Spraying or dripping water
Bubbling at jets
Moisture or erosion in the soil
Other Components to Inspect
Even if it’s not one of the three big issues above, several other components can leak when the pump is running, including:
Spa Suction Lines
Cracks on the suction side draw air in, leading to:
Bubbling at return jets
Pump losing prime
Cavitation noise in the pump
While they don’t usually spray water out, they can reduce circulation and worsen other leaks.
Spa Check Valve
If this fails, it may:
Allow spa water to siphon into the pool
Cause the spa to drain quickly after the pump shuts off
Contribute to weird pressure fluctuations
Valves and Unions
A loose or cracked union or failing gasket can leak only when pressurized.
DIY tip: Dry the area, run the pump, and feel for moisture. Hand-tighten any unions and replace worn O-rings as needed.
What Happens If You Don’t Fix It?
Ignoring a spa-side leak can lead to:
Hundreds of dollars in lost water and chemicals
Elevated utility bills from running the autofill or topping off manually
Long-term structural damage to decking, soil, or foundations
Electrical hazards if water enters the light conduit or equipment
The sooner you diagnose the leak, the better. Many small leaks can be resolved affordably before they grow into major repairs.
When to Call a Professional
If your spa is losing water during circulation and you can’t pinpoint the cause, it’s time to call in a professional leak detection company.
Pros like Precision Leak Detection use:
Pressure testing
Electronic listening equipment
Thermal imaging
Plumbing line locators
Dye injection tools
They’ll locate the leak accurately and non-destructively, saving you time, water, and frustration.
Don’t Let Your Spa Drain Your Wallet
If your swimming pool is only losing water when the pump is running, and you have an attached spa, the culprit is likely a leak in your spa’s pressurized plumbing, fittings, or light conduit.
These leaks often hide in plain sight—until the system is circulating, and pressure forces the water out. DIY tests can help narrow it down, but many spa leaks are buried behind walls or under decks, making them difficult to reach without professional tools.
That’s where Precision Leak Detection comes in.
We’re a professional swimming pool leak detection company based in Jupiter, Florida, and we proudly serve all of Palm Beach County, including:
West Palm Beach
Palm Beach Gardens
Lake Worth Beach
Wellington
Boynton Beach
Delray Beach
Royal Palm Beach
Palm Springs, and more.
If you’re dealing with spa-related water loss or any mystery leak, we’ll locate it quickly and help you fix it permanently.
Stop wasting time, money, and water. Call Precision Leak Detection today—and let us help you protect your pool, spa, and peace of mind.
