Help! Leak detection in a fiberglass pool.

[Flipside12]

I have a leak somewhere in my plumbing. Plugged returns and no water loss. Unplug returns and fill over returns and I’m not losing water.
Unplug returns and run the pump I start losing water. Water loss stops just below returns and skimmer. Can anyone give me any additional insight. Any chance it’s a main drain issue or most certainly the returns? Thanks in advance!

Notenly have one pressure side line coming out of my filter it must be split underground to the two return jets.

 

[Texas Splash]

Welcome to TFP! I would suspect the leak is still along that pressure side somewhere after the filter. Perhaps the leak isn't quite large enough to make a difference with the system off, but once you apply pressure with the pump running, it springs the leak. If it were on the suction side, you would expect water water loss with the system off and/or air in the lines (bubbles).

So if your return lines (one main that splits to the two return jets) are buried underground and perhaps under concrete or decking, you may need to contact a leak company. Many of them have equipment that can pinpoint a leak from above ground to avoid unnecessary digging.

The only other possibility I can think of is if you have a multiport valve (sand or DE flter), perhaps you are losing some water to waste (i.e. bad spider gasket), but I don't know if you have that. Don't forget to update your signature for us.