Q: When my pool pump is running, I don't lose any water. When the pool pump is OFF, I lose about a 1/2 inch of water a day. Does this rule out that the leak is in the pool pump? Why is my pool only leaking when the pool pump is OFF?
A: Two possibilities. 1/ If this is a sand filtered pool, and you have a buried backwash line, have a look at where that line ends when the pump is running. If you see water running out when you have the filter set on "filter", there's an issue with the key stem gasket in the multiport valve ( the valve on top of your filter with "backwash, filter, rinse etc) or a problem with the multiport housing in general, like distortion from heat. 2/ If your backwash line isn't leaking water out, then the only other possible issue would be a break in your skimmer or main drain suction plumbing line. A break there won't necessarily cause water loss when the pump is on but it sure as heck will when the pump is off. A dead giveaway is the presence of a lot of air in the pump when it's running. The most common spot for one of these breaks is directly under the skimmer itself, where the plumbing line makes a 90 degree turn to your equipment. It won't be a leak in the pool itself.You can rule that out. As for the pump itself, if it was a leaky shaft seal on your pump or a crack in the plumbing at the equipment, you'd notice it. It'd be pissing water out under it. Once you've ruled out that it's not the mutiport valve to blame, your best bet is to hire a pro in to pressure test the suction lines and locate the leak. Once it's located, you have the option of repairing it yourself or have the pro do it. It may cost anywhere from 60 to 300 bucks depending on the leak location and how easy it is to get to.