Pool losing water only
when the pump is running?

The inverse diagnostic pattern from a suction-side leak: if your pool loses water only when the pump is running, the leak is on the pressure side of the system. This narrows diagnosis significantly because the pressure-side plumbing path is a distinct set of components — and the repair scopes are very different from shell or suction-side work.

What "pressure side" actually means.

Water in a pool circulates on a loop. From the pool, through the skimmer and main drain, to the pump (suction side). Then from the pump forward through the filter, optional heater, optional chlorinator, and back to the pool via return jets (pressure side). Everything between the pump's outlet and the return jets is under positive pressure whenever the pump runs.

A leak on the pressure side behaves predictably: water is forced out of the leak path by pump pressure. When the pump stops, pressure equalizes and water stops exiting. This creates the distinctive "loss only when pump is running" pattern.

The pattern is useful because it excludes large categories of leak sources. The pool shell itself, the skimmer throat, the main drain, and the suction plumbing are all on the other side of the pump — none of them respond to pump-state this way.

Common pressure-side leak sources.

Return jet fittings (about 30% of cases)

Return jets are the fittings where water flows back into the pool, typically one or two on each side. The fitting itself can fail at the threaded joint, the eyeball can crack, or the wall penetration can develop a gap. All three create pressure-side leaks.

This is often the easiest pressure-side repair — the fitting is accessible, replacement parts are inexpensive, and the work typically runs $350-$800.

Underground return plumbing (about 25%)

The pipe runs from the pump back to the return jets are underground. When they fail — typically at a joint or fitting — the leak pushes water into the deck substrate during pump operation. Often accompanied by visible wet spots on the pool deck or dying grass near the return side of the pool.

Repair depends on pinpointing. Electronic listening equipment can locate the failure within a few inches along a 30-foot run. Targeted excavation replaces the failed section. If a contractor is quoting full re-route, that's usually a sign they didn't pinpoint.

Heater connections (about 15%)

Pool heaters — especially gas heaters — have multiple high-pressure connections that develop leaks with age. Header manifolds crack, input/output unions fail, and bypass valves can split. Often visible as water weeping or spraying on the equipment pad.

Repair costs vary by heater type. Simple union replacements run $200-$400. Header repairs can run $450-$1,200. Full heater bypass or replacement is rare but sometimes indicated for older units.

Filter connections and pressure valves (about 10%)

The filter is pressurized during pump operation. Failed O-rings on filter lids, cracked pressure gauges, and multiport valve leaks all create pressure-side leak patterns. These are usually visible on the equipment pad.

Usually inexpensive repairs. O-rings run $15-$40 in parts. Full valve replacements $150-$400.

Chlorinator and saltcell connections (about 10%)

Inline chlorinators and saltcell housings are pressurized. Cracks in the housing or failed gaskets create pressure-side leaks. Common in older saltcells approaching end-of-life.

Return-side feature plumbing (about 10%)

If the pool has water features (waterfalls, deck jets, spa spillovers), their plumbing is typically pressure-side. Feature system leaks show the same pump-on pattern.

Visible clues that help narrow it.

Pressure-side leaks often have visible evidence the homeowner can observe before the specialist arrives:

Diagnostic process for pressure-side leaks.

A specialist's typical sequence:

Step 1: Visual inspection of the equipment pad

Most pressure-side leaks at the pad are visible — water weeping from unions, wet concrete under the heater, water pooling near the filter. This finds 40-50% of pressure-side leaks immediately.

Step 2: Return jet inspection

Each return jet is inspected above and below water. Fitting integrity, eyeball condition, and wall penetration are checked. Dye testing at each return confirms which, if any, are leaking.

Step 3: Isolation testing

If the leak isn't at an obvious visible location, each return circuit is isolated and pressure-tested independently. This tells the specialist which underground run contains the failure.

Step 4: Electronic pinpointing

For underground return leaks, electronic listening equipment (or sometimes ground-penetrating sensors) identifies the specific failure point. This is what enables targeted excavation vs. full re-route.

Typical repair costs.

Equipment damage risk from ignoring it.

Unlike suction-side leaks (which can damage the pump through air ingestion), pressure-side leaks don't typically damage the pool's equipment directly. The main consequences of ignoring a pressure-side leak are:

Water waste. Even a small leak running 8 hours a day (standard pump schedule) adds up. A 50-gallon-per-hour leak wastes 400 gallons per day, 12,000 per month, 144,000 per year. At Florida water rates, that's $400-900 annually depending on municipality.

Chemistry cost. Constant makeup water dilutes chemistry and requires continuous rebalancing. Chemical costs can run $50-150 per month higher than baseline during an active leak.

Substrate erosion (underground leaks only). Same as suction-side — water eroding soil around the pool can cause deck settlement over time. Typically a 1-2 year timeline before noticeable effects.

Efflorescence on retaining walls. Persistent underground moisture shows up as white mineral staining on pool decks and nearby walls. Cosmetic issue but hard to reverse once it sets in.