Diagnosing main drain leak

[sartor]

Hello,

I have a relatively large pool (30' x 50', 50K gallons), with the pump/cartridge filter located about 2-3 feet *above* the height of the pool. There are four separate main drain pipes coming up out of the ground at the pump, with separate Jandy valves for each. These correspond to the three skimmers, plus one for the main drain (there are two main drains in the bottom of the pool, but they presumably join together before coming back to the pump).

I'm pretty sure that I've got a main drain leak, but wanted to get confirmation of my interpretation. If I keep all four Jandy valves open, then I don't really get any air bubbles coming in/visible through the clear plastic top of the pump input strainer basket. But if I shut off the three skimmers, keeping *just* the main drain valve open, then I'm seeing a decent amount of air bubbles coming into the pump input strainer gasket.

So my interpretation of this is that with the pump sucking *only* through the main drain (skimmers off), the pressure force is causing it to forcefully draw in air somewhere in that drain line from the drain to the pump, whereas when the skimmers are also open, there is less suction force operating on the main drain line and hence it isn't pulling air in from that line (or at least less so).

Does this make sense, e.g., is my interpretation correct? Does the presence of air bubbles coming into the pump strainer basket from the main drain line but not from the skimmers signify that there must be an air intake leak somewhere in the main drain line?

Thanks for the help!

Tom

[Teapot]

Hi Tom, That was the best description of an issue, detailed and well thought out.
Is there a water loss?
With a 2HP pump drawing through just one pipe it is quite likely as you are really starving the hydraulics so a sucking in air leak is possible even and quite often the most common the pump pre filter lid. Why would you only want to run the main drains, as most if not all the dirt enters the pool from the top?

[Denniswiseman]

Hi Tom, That was the best description of an issue, detailed and well thought out.
Is there a water loss?
With a 2HP pump drawing through just one pipe it is quite likely as you are really starving the hydraulics so a sucking in air leak is possible even and quite often the most common the pump pre filter lid. Why would you only want to run the main drains, as most if not all the dirt enters the pool from the top?

Perfect answerto a perfect description
My only comment is try the bucket test if you think you have a leak

[sartor]

Thanks Teapot and Denniswiseman,

The reason for my running with just the main drains open was as part of my attempt at diagnosis as to where I might have a leak. My thinking was to run 24 hours with just skimmer 1, then just skimmer 2, then just skimmer 3, and then just the mains, and see if there is a difference in water loss for each, figuring that if more loss with one of the lines then that's the one with the leak. But then I suppose that even with a line turned off at the Jandy valve, but *not* plugged off back at the skimmer/main drain, water could/would still leak out in that line, at least at any point in that line that is lower than the pool water height (remember that my pump/filter are a couple feet *higher* than the pool, so I presume that any line that I shut *off* at the Jandy will naturally flow back down towards the pool, even if the pump is running, although I suppose that if the line truly is airtight from the Jandy back towards the pool that could "hold" the water in/up, just like a straw with the end plugged off). Does all this sound right/make sense?

In any event, I had been seeing maybe an inch of water loss per day a few weeks ago when I was monitoring, and I'd been trying to do the diagnostics to see if anything changed based on pumps on/off and which lines open/closed. But this has been complicated by below freezing weather, which forces my pump to turn on, and I've yet to do a true/legitimate bucket test (the pool is also covered at the moment and we've had off and on rain). So I really need to wait to get some more consistent and convenient conditions. The whole reason I began investigating is because I discovered I'd lost maybe 5" or so and the skimmers were sucking air, and I couldn't force the pump *off* due to the freezing weather, and I tried running on *just* the main drain (so the skimmers wouldn't suck air) and I then noticed the air bubbles in the basket, but as you say that could just be due to under-capacity of just having one feed line). Since then I've filled and seen the loss and filled again (and again, a few times). So now I'm in the midst of trying to do some more precise diagnostics to see where the problem might be.

Thanks for your help!

Tom

[Denniswiseman]

If you have lost 1" in this cold weather you definately have a leak and a bucket test wouldn't be necessary
The only way to test the lines is to block off both ends which would be difficult in cold weather. Turning the Jandy valves off will hold water in the pipe until you introduce air then it will fall to the pool level
When you can block them all off you will seeif the leak is in the pool or the pipes

[Teapot]

But then I suppose that even with a line turned off at the Jandy valve, but *not* plugged off back at the skimmer/main drain, water could/would still leak out in that line, at least at any point in that line that is lower than the pool water height (remember that my pump/filter are a couple feet *higher* than the pool, so I presume that any line that I shut *off* at the Jandy will naturally flow back down towards the pool, even if the pump is running, although I suppose that if the line truly is airtight from the Jandy back towards the pool that could "hold" the water in/up, just like a straw with the end plugged off). Does all this sound right/make sense?


Tom

Hi Tom, Yes the pump being higher would do as you say but normally only to the point of equilibrium with the pool water level (a couple of feet). The weather temperature certainly won't help but it could be time for the leak testing guys. They can listen for leaks with hydrophones and also test your lines etc, whether they want to play in the cold is another matter.