—Leaks and diagnostics
When a pool loses water only while the pump runs
Loss that happens with the pump on but not off usually lives where the water is under pressure: after the pump.
When the pump runs, the return side of the system — from the pump through the filter, heater and chlorinator back to the pool — is under pressure. Leaks there may only lose water while running. Common places include joints and unions in the plant, filter and valve seals, and buried return lines. A multiport valve that lets water pass to waste can also drain the pool through the backwash line.
Diagnostic guides describe what an owner can observe safely. They do not locate a fault; testing on site does that.
Checking the plant for drips and damp, and checking the backwash outlet for water flowing during normal filtration, are simple first steps. Pressure testing return lines confirms underground leaks. Recording loss with the pump on and off makes the pattern clear.
01Summary
What matters most here
Pressure side
Leaks after the pump.
Check the plant
Drips, damp and seals.
Check the backwash line
Water passing to waste.
02Detail
Where to look
Plant joints
Unions, valves and fittings.
Filter and valve
Seals and multiport gaskets.
Backwash outlet
Flow during normal filtering.
Return lines
Buried pipes under pressure.
03Before you start
Simple checks
Inspect the plant
With the pump running.
Check the waste outlet
For flowing water.
Compare loss
Pump on versus off.
—In practice
How to use this
Compare the loss with the pump running and with it stopped. If loss happens mainly while running, inspect the plant’s joints, unions and valves, check whether the backwash line runs water while it should not, and arrange pressure testing of the return lines to find any leak buried under the deck.
Mistakes worth avoiding
- Ignoring the backwash line
- A multiport valve that lets water pass to waste is a common cause of loss while running, and easily overlooked.
- Only checking the shell
- Loss that happens only while the pump runs usually lives on the pressure side after the pump, not in the shell.
04FAQ
Common questions about this
- Why does loss only happen when running?
- The return side is only pressurised when the pump runs.
- Can the filter valve leak to waste?
- Yes, worn multiport valves can pass water to the backwash line.
- How are buried return lines tested?
- By isolating and pressure testing them.
- Is a drip in the plant important?
- Even small drips add up over days.
05Read next
More pool guides on this
- Persistently high filter pressureAnother way faults and leaks are traced.
- Persistently low system 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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Loss only while running?
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