PoolLeakFix • Leak Pattern Diagnosis
Pool Loses Water Only When the Pump Is Running – What It Usually Means
If your pool seems to hold level when the pump is off but drops when the pump is running, that pattern is a big
clue. Evaporation doesn’t suddenly start and stop with the pump. A loss that shows up mainly when water is
circulating often points toward a plumbing or feature-line leak, not just weather.
Why the “Pump On” Pattern Matters
When the pump runs, it puts the circulation system under pressure and pulls water through every line, fitting,
and feature that’s tied into the equipment pad. If something is cracked, loose, or damaged, it may only show
itself when:
- The system is under pressure (return lines, feature lines).
- The system is under suction (skimmers, main drain, vacuum lines).
- Additional features are activated (spa spillover, fountains, bubblers, deck jets).
That’s why a pool that mostly holds its level with the pump off but drops when it’s on is taken seriously in
leak detection.
First Step: Confirm the Pattern
Before you read too much into it, make sure the pattern is real:
- Turn off any autofill so it can’t hide the loss.
- Bring the pool to your normal water level.
- Mark the level on the tile or inside the skimmer.
- Run the pump on its normal schedule for 24 hours and measure how far it drops.
- Refill to the mark, then leave the pump off for 24 hours and measure again.
If the pool loses noticeably more water in the “pump on” period than the “pump off” period, that pattern is
worth treating as a plumbing-related leak until proven otherwise.
Use a Bucket Test with Pump On vs Pump Off
A bucket test makes the comparison clearer:
-
Place a bucket of pool water on a step and mark:
- The water level inside the bucket,
- The pool water level on the tile or in the skimmer.
- Run the pump for 24 hours as normal and compare the drops.
- Refill, repeat the test with the pump off for 24 hours.
What you’re looking for:
- Pump on: Pool drops more than the bucket.
- Pump off: Pool and bucket drops are similar.
If that’s your result, it’s a classic sign that the leak is tied into the circulation system rather than the
shell alone.
Common Places That Leak When the Pump Runs
Some of the usual suspects when loss shows up mainly with the pump running:
- Return lines: Cracked or damaged pipes on the pressure side pushing water into the ground.
-
Feature lines: Spa spillovers, fountains, bubblers, and deck jets that only see full flow
when the pump or specific valves are on. -
Equipment pad plumbing: Loose unions, fittings, or valves that only drip or spray when the
system is under pressure.
Leaks on the suction side (skimmers, main drain, vacuum line) are usually more about air being pulled
in and pump performance, but a suction-side pipe that’s cracked below waterline can still move water
into surrounding soil.
What You Can Check Yourself
Without special tools, you can still do a few quick checks:
-
Walk the equipment pad with the pump running and look for damp soil, dripping fittings, or
small sprays of water. -
Listen for changes when turning features on and off. If loss jumps when a spa spillover or
fountain comes on, that line deserves a closer look. -
Use a simple dye kit around visible fittings, returns, and the skimmer throat to see if dye
gets pulled into a crack or gap.
View basic pool leak detection dye kits
These checks won’t tell you everything, but they can help you narrow down where to focus a professional leak
detection visit.
When to Call a Leak Detection Pro
It’s time to bring in a leak detection specialist when:
- The pool consistently loses more water with the pump on than with it off.
- Bucket tests back up that pattern.
- You see damp areas, settling, or other signs near plumbing runs or the deck.
- You’re adding significant water each week just to keep up.
A good leak detection visit will pressure-test lines, listen for leaks, and pinpoint the issue so you’re fixing
the right section instead of guessing and digging blindly.