So you ran the bucket test and confirmed it: your pool is leaking, not evaporating. Before you call anyone, there is one more free test worth running — and it can narrow a leak down from "somewhere in the pool" to one of three specific places.
All it takes is your tape mark, your pump timer, and 48 hours.
The test: measure water loss twice
You are going to measure how much water the pool loses over two back-to-back periods — one with the pump running, one with the pump off.
- Fill the pool to the middle of the waterline tile and mark the level with tape.
- Run the pump normally for 24 hours. Autofill off, water features off, no swimming. Measure the drop from your tape mark and write it down.
- Refill to the same mark. Now shut the pump off completely for the next 24 hours — same conditions otherwise.
- Measure the drop again and compare the two numbers.
Try to run both halves in similar weather. A calm, mild stretch gives you the cleanest comparison — a windy 100° day versus a rainy one will muddy the numbers.
Loses more with the pump ON: pressure-side plumbing
When the pump runs, everything downstream of it — the return lines carrying filtered water back to the pool — is under pressure. If one of those lines has a crack or a failed fitting, the pump is actively forcing water out through the breach. Shut the pump off and the pressure disappears, so the loss slows.
More loss with the pump running points to a return-line leak. Other clues that fit this picture:
- A soft, soggy, or unusually green patch of lawn near the pool
- Water seeping up between deck joints or pavers
- Loss that clearly tracks your pump schedule
Loses more with the pump OFF: suction-side plumbing
This one surprises people. The lines feeding the pump — from the skimmer and main drain — are under vacuum while the pump runs. A breach in a suction line often pulls air in rather than letting water out, so the leak is partially masked while the equipment is on. Shut the pump off, the vacuum disappears, and water drains freely out of the same breach.
More loss with the pump off points to a skimmer or main-drain line leak. The telltale companions:
- Air bubbles blowing out of the return jets
- Air churning in the pump basket
- A pump that struggles to hold prime
Loses the same either way: the pool shell
If the loss rate barely changes, the plumbing probably is not your problem — the water is escaping through the structure itself. The usual suspects are the skimmer throat (where plastic meets gunite), a light niche, return fittings, the main drain body, or a crack in the plaster.
Shell leaks come with their own bonus clue: where the water stops. A structural leak can only drain the pool down to its own depth, so if the level keeps dropping with everything off, let it (within reason — do not let a plaster pool sit empty) and note where it stabilizes:
- Stops at the bottom of the skimmer mouth — skimmer throat is the prime suspect
- Stops at the light — likely the light niche or its conduit
- Keeps dropping past the returns — main drain or a floor-level crack
What this test can and can't do
The pump on/off comparison is a genuinely useful screen — it is one of the first questions we ask when you call, and your answer shapes where we look first. But it narrows the search; it does not finish it. Houston pools regularly have more than one leak (shifting clay soil sees to that), and some leaks bleed water in both modes.
That is where the professional visit comes in. We pressure test each plumbing line in isolation to confirm exactly which pipe is losing water and pinpoint the failure underground — no exploratory digging. For the shell, dye testing and acoustic equipment locate the exact fitting or crack. You get the leak marked, explained, and quoted in writing, and every repair is guaranteed.
Run the test, note your numbers, and book a leak detection appointment or call (281) 252-4233. Telling us "it loses twice as much with the pump running" on the phone genuinely saves time on the deck — and gets your pool holding water sooner.