Pool Loses One Inch Per Week - Evaporation vs Leak Decision Tree

Why this matters

The most expensive misdiagnosis in pool service is selling a leak detection on a pool that is simply evaporating. The second most expensive is dismissing a real leak as evaporation and watching a slab undermine over six months. The tree below converts a vague homeowner complaint ("I'm adding water all the time") into a defendable yes-or-no on whether to deploy dye, plug-and-pressure, or hydrophone. Knowing the field probability ordering keeps the diagnosis to one visit instead of three, and protects the relationship when the answer is "your pool is fine, this is July."

Symptom presentation

Homeowner reports water dropping more than the usual auto-fill cycle. Useful numerical thresholds for a typical resi 15K-25K gal pool, no cover:

• Under 0.25 inch per day in summer high heat plus low humidity: within normal evaporation.
• 0.25 to 0.5 inch per day in mild conditions: suspect leak.
• 0.5 to 2 inches per day: high probability leak, prioritize.
• Greater than 2 inches per day: structural or main drain failure - investigate immediately.

Pattern A: equal loss with pump on and off. Pattern B: loss only when pump is running (pressure side). Pattern C: loss only when pump is off (suction-side or hydrostatic relief). Pattern D: loss stops at a specific water line and holds.

Pattern D is the gift - the water has found its own level at the leak elevation, and the leak is at or just above that line.

Quick checks - 10 minute pass

1. Weather check. Pull last 7 days from a local weather station: high temp, low humidity, wind speed. 95 F + 25 percent humidity + 15 mph wind can evaporate 0.4 inch per day from an unshaded resi pool.
2. Cover status. Solar cover or liquid cover suppresses 70-95 percent of evaporation. No cover plus desert summer is the bucket-test prerequisite.
3. Recent service changes. New heater, new pump, fresh backwash, new fill line. A heater on with the cover off accelerates evap.
4. Visible wet spots. Walk the equipment pad, the deck around skimmers, the deck edge. A persistent wet stripe of decking in dry weather is not evaporation.
5. Auto-fill state. Confirm it is actually working - half the "losing water" calls are a stuck auto-fill float or a closed isolation valve.

Isolation tree

Step 1 - The bucket test

The bucket test isolates evaporation from leakage. Set a 5-gallon bucket on the second step, fill to within 1 inch of the rim, fill it with pool water (not tap - same temperature). Mark the water level inside the bucket and the pool level outside with grease pencil. Run pool normally for 24 hours. Re-measure.

Interpretation:

• Pool drop equals bucket drop within 1/8 inch: pure evaporation. Stop. Educate customer.
• Pool drop exceeds bucket drop by more than 1/8 inch per 24 hours: confirmed leak. Continue tree.
• Bucket drop exceeds pool drop: customer adding water during the test or rain occurred - retest.

Step 2 - Pump on vs pump off split

Run the pool 24 hours pump on, mark, then 24 hours pump off, mark. Two buckets, one for each window.

• Equal loss in both windows: leak is on the shell, fittings below static line, or skimmer throat - the leak is gravity driven and runs whether the pump is moving water or not.
• More loss with pump on: pressure-side leak. Plumbing between pump discharge and the return jets. Often a glued joint at the equipment pad, a heater, or a return eyeball.
• More loss with pump off: suction-side at a fitting that drains when not under vacuum, OR hydrostatic relief valve in the main drain stuck open and shell is leaking to the ground until water level reaches the relief, OR a vacuum-side line break that draws air when running but loses water when off.

Step 3 - Find the static line

Let the pool drop with pump off until it stops dropping. The leak is at or just above that line. Common static-line landings:

• Just below skimmer throat: skimmer body crack, skimmer-to-pipe joint, skimmer faceplate gasket on vinyl liner.
• Just below tile band: tile-grout line behind expansion joint, or a return fitting gasket.
• 6-12 inches below water line: return fitting eyeball gasket, light niche conduit.
• All the way to step or main drain: drain plug, main drain pot fitting, or structural shell crack on the floor.

Step 4 - Pressure decay (when accessible)

For pressure-side suspicions on equipment pad: isolate the suspect leg, plug the returns or skimmer with winterization plugs, pressurize the line with a hand pump to 15-20 psi, watch the gauge for 15 minutes. Holding pressure equals no leak on that leg. Decay equals leak somewhere on that leg, listen with hydrophone or dye-trace from there.

Step 5 - Visual dye check

For losses localized to the bowl: red phenol or food-grade tracing dye, pump off, water still. Pencil-thin trickle of dye at suspect cracks, light niches, return fittings, skimmer faceplate. Dye is pulled into a real leak in seconds.

Confirming the diagnosis

Hydrophone or geophone for buried plumbing on suspected pressure-side losses. Dye for shell and surface fittings. Pressure test for plumbing isolation. Document the bucket test result with photos of both marks at 24 hours - this is the artifact that closes out an "is it really leaking" dispute with the customer or their insurer.

Remediation routing

• Pure evaporation: educate, sell a solar cover or liquid cover if appropriate, document the recommendation. Do not sell leak detection.
• Skimmer body crack on plaster pool: epoxy putty rated for underwater cure, or full skimmer replacement on a vinyl pool.
• Glued PVC joint: cut out 4 inches each side, slip union, dry fit, glue with primer plus medium-body cement, 30-min cure before pressure.
• Light niche conduit leak: putty seal at the niche, or pull-and-replace cord and reseal at the deck box.
• Shell crack: structural assessment first, then epoxy injection or refinish.
• Main drain hydrostatic stuck: pump drain enough to access, replace valve, refill.

References

• ANSI/APSP/ICC-15 Residential Swimming Pool and Spa Energy Efficiency, evaporation modeling reference.
• IAPMO USPSHTC Uniform Swimming Pool, Spa and Hot Tub Code, leak testing section.
• ASTM E96 Standard Test Methods for Water Vapor Transmission, used by APSP for cover-effectiveness comparisons.
• Pentair Pool Builders Reference, Section 12 Leak Detection.
• National Plasterers Council Technical Bulletin on Hydrostatic Relief Valves.