DIY Pool Tips • ⏱️ 6 Min Read • 📅 Updated September 2026 • 🛡️ Reviewed by C-36 Master Plumber

Pool Losing Water: Pump ON vs. Pump OFF

Direct Service Resource: Need immediate professional assistance for this issue? Visit our dedicated Pool Leak Diagnostics page.

One of the quickest ways to diagnose where your swimming pool is leaking is by tracking water loss under two different operating conditions: with the filtration pump running versus completely turned off.

Case 1: Pool Loses More Water When the Pump Is RUNNING

This indicates a leak on the pressure side of your system (return lines, chlorinator, pool cleaner dedicated lines, or multiport valve backwash line). When the pump is active, pressure forces water out of cracks faster.

Case 2: Pool Loses More Water When the Pump Is OFF

This points to a suction side leak (skimmer pipes or main drain lines) or a crack in the pool shell, vinyl liner, or light niche. When the pump is off, water seeks its natural level without suction pulling air.

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