Pool losing water
Why does my pool only lose water when the pump is running?
A pool that loses water only while the pump runs is almost always leaking from the pressure side of the plumbing, because the return pipework is the only part of the system carrying pressure when the pump is on and sitting idle when it is off. It is the single most useful thing a pool owner can notice.
What it usually is
Pressure is what makes the difference
Water loss that appears only when the equipment runs usually means the leak is in the return plumbing, downstream of the pump.
A pool has two halves of buried plumbing. The suction side draws water out through the skimmer and main drain and is under vacuum while the pump runs. The return side is pushed back into the pool through the wall fittings, and that is the half under pressure.
A small split behaves completely differently depending on the switch. With the pump off the pipe is full but static, and the fault barely weeps. Turn the pump on and the same fault is being pushed at, so the loss jumps. The rate changes with the schedule rather than with the weather.
Be honest about it
What it could be instead
Two innocent explanations produce the same appearance:
- A water feature or spa spillway on the pump schedule. It throws spray, and wind removes a share of it from the system. Nothing is broken.
- A multiport valve not seated on its detent. Part of the flow goes to waste whenever the pump runs. That is a valve position, not a leak.
- Air on the suction side. Bubbles in the pump basket move the surface and throw off a casual reading.
Narrowing it down
Which half of the plumbing the timing points at
| When the loss happens | Likely half of the system | What to check first |
|---|---|---|
| Only while the pump runs | The return, or pressure, side | Multiport valve position, then the pad-to-pool run |
| Only while the pump is off | The suction side, or the shell | Whether the level settles at one height and stops |
| The same either way | The shell and its fittings | Tile line, skimmer throat, light niche and returns |
Ten minutes, on your own
What you can settle yourself in two days
The answer you get is what we would otherwise spend the first part of a visit establishing.
Close the auto-fill first
Nothing below means anything while an auto-fill quietly replaces the water.
Mark the level, run the normal schedule, read it a day later
Grease pencil on the tile, the time written beside it, one reading at the same hour.
Re-mark, leave the equipment off for a day, read it again
Same mark, same hour, no pump. The difference between the two days is the whole test.
Walk the pad-to-pool run with the pump going
Damp granite, a soft spot or deck that stays darker. A pressure-side leak often shows above itself.
How urgent is it
Not an emergency, but it will not heal
A pressure-side leak rarely needs somebody tonight. It is also the kind that grows, because the pressure that reveals it works on it every time the pump starts.
Act sooner if the ground close to the house is saturated, or a deck has begun to move. Short of that, a shorter pump schedule while you arrange the visit is a fair compromise: the pool still turns over and the leak gets fewer hours.
If you call us
How the pressure side gets tested
Each line is plugged at the pool wall and pressurised on its own, so a return that holds and a return that does not are separated instead of averaged. From there it is acoustic listening along the run, because pressurised water escaping underground makes a sound that carries, and line tracing to put the pipe on the surface in chalk.
Where deck or soil confuses the sound, thermal imaging reads it another way. The repair then goes where the leak is rather than where the pipe is easiest to reach, which usually means one small opening: pool leak repair.
Straight answers
Pressure-side questions
Does losing water only when the pump runs definitely mean a pressure-side leak?
It is the most likely explanation, not the only one. A water feature that runs with the pump, or a multiport valve off its detent, lose water on the same schedule with no pipe at fault. Both are quick to eliminate.
Should I stop running the pump until it is fixed?
Running it less means the leak runs less, so a shorter schedule is sensible. Stopping altogether for long in Valley heat brings water-quality problems of its own.
Is a pressure-side leak worse than a leak in the shell?
Not worse, different. A shell leak loses water steadily. A pressure-side leak loses it in bursts tied to the schedule and puts that water under a deck, so it is the one that tends to show up as sunken paving.
Can the leak be found without breaking up the deck?
Finding it, yes, and that is the whole purpose of pressure testing, listening and line tracing. Reaching it usually needs one opening, in the right place rather than a hopeful one.
Tell us what the water is doing
Two readings a day apart, one with the pump on its schedule and one with it off.
Tell us which day was worse
Pump on or pump off, and by how much. It decides how we arrive set up.
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What customers in the Phoenix valley have written. The words are theirs, on listings we cannot edit.
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