Suction Line Leaks
As condition 3 indicates if your pool is leaking more when the pump is turned off then this is a red flag to be sure. What is strange about this situation is that there is very little reason to make a pool leak more with the pump off. A pressurized plumbing system can obviously cause a leak to leak more but why would turning on the pump cause the pool to leak less?
In most cases the answer to this question will lay somewhere in the suction lines, either the skimmer or the main drains. It is possible with certain suction line leaks to experience a leak that is minimized by the suction of the pump and allowed to leak profusely when not under pressure.
Air In The Pump
If you suspect that you have a suction line leak then you may have noticed your pump running with air trapped in it. In a perfect installation a pool pump will purge all the air from the system and no bubbles at all should be visible in the pump housing.
The presence of air indicates the system is no longer closed loop (or that the pump is not functioning properly) and may also cause the pump to be difficult to prime if it is located above grade.
If your pump runs with zero air in it then it is unlikely that you have a suction line leak however there is an exception to this rule also. If the suction line leak is itself submerged in water, even if only temporarly, can prevent the leak from allowing air into the system. This leak would still increase when the pump is turned off.
Isolating The Main Drain
If your pool has a main drain then you need to determine if it is a seperate line all the way to the pump or if it is connected to the underside of the skimmer. It can actually be both of these as sometimes there is a dedicated suction line but also a non pressurized pipe connected to the skimmer designed to prevent the pool pump from gluping air and losing prime if the water level in the pool were to drop below the skimmer mouth.
By looking at your pump you can determine if you have one or two suction lines. If you have two lines that get connected together in front of the pump then you can isolate your suction lines at least somewhat. If you have a single pipe entering into the pump room and into the suction side of the pump then you either do not have a main drain or the main drain runs from the bottom of hte pool up to the skimmer and connects in with the skimmer suction line.
If you happen to have a leak in your main drain line and your pool is built in such a way that you can not isolate the main drain line then you will have some trouble in identifying the leak location without enlisting the help of a professional. That or being willing and able to swim down to the main drain, remove the cover and install plugs into the ports to allow you to test the system.
You can now proceed to the leak detection water level test which is a very strong indicator of where your pool is leaking.
The menu on the right is a chronological step by step process to determine where a swimming pool is leaking. This website is written as a readthrough technical assistance guide for locating leaks. When you believe you have identified where the pool is leaking you need to go to our sister website to see pictures and learn more about the actual swimming pool leak repair process.