—Leaks and diagnostics
When a pool loses water with the pump off
If the level keeps falling with the pump stopped, the water is leaving through something that holds water all the time, not just when it is pumped.
With the pump off, the pressure side of the plumbing is not pressurised, so losses from return lines and the plant usually reduce or stop. Continuing loss therefore often points towards the shell, fittings set into it — skimmers, light niches, returns and drains — or suction lines that remain full of pool water.
Diagnostic guides describe what an owner can observe safely. They do not locate a fault; testing on site does that.
Observing where the level stops falling, if it does, is a useful clue: stopping at the bottom of the skimmer mouth suggests the skimmer or its line; stopping at a light suggests the niche. Recording levels and these observations before calling for detection speeds up the work.
01Summary
What matters most here
Pressure side ruled down
Returns are not pressurised.
Shell and fittings
Common sources when the pump is off.
Where it stops
Points towards a fitting.
02Detail
Possible sources
Skimmers
Cracks or failed seals where the skimmer meets the shell.
Light niches
Gaps around the fitting or conduit.
Returns and drains
Seals where fittings pass through the shell.
Shell cracks
Through the structure and waterproofing.
Suction lines
Leaks in pipes that stay full of water.
03Before you start
What to record
Daily level marks
With the pump off.
Where loss stops
If it stabilises.
Damp areas
Around the pool or plant.
—In practice
How to use this
With the pump off and any top-up disabled, let the level fall and note exactly where it stops. Check around skimmers, light niches and fittings at that level for clues. Share the stopping level and your observations with whoever carries out detection, so testing can start at the most likely point rather than everywhere at once.
Mistakes worth avoiding
- Assuming it must be the pipework
- Loss that continues with the pump off often points to the shell, fittings or suction lines, not the pressurised returns.
- Not noting where it stops
- The level at which loss stops is one of the most useful clues available. Record it before refilling the pool.
04FAQ
Common questions about this
- Does pump-off loss mean a shell leak?
- It often points to the shell or fittings, but suction lines can also leak.
- Should I let the level drop to see where it stops?
- Only a little and with advice, to avoid groundwater risks.
- Can a light niche leak?
- Yes, commonly around the conduit or seal.
- How is a shell leak confirmed?
- Dye testing near suspect points.
05Read next
More pool guides on this
- When a pool loses water only while the pump runsAnother way faults and leaks are traced.
- Persistently high filter pressureAnother way faults and leaks are traced.
- Planning a villa poolAnother stage of planning and building a pool.
- The stages of building a poolAnother stage of planning and building a pool.
Filed under technical diagnostics & leak detection. Every pool guide is listed on the pool guides page.
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