Lots of air bubbles from main drain, but none from skimmers

[et]

We have a 30,000 gal in ground pool with a 1 hp hayward super pump and a sand filter.

Drawing only from the 2 skimmers we get no air visible in the pump housing during operation, but when we open the main drain inlet to the pump, we get a flood of air bubbles - really a torrent.

1. I know it seems obvious that there may be a break or a crack in the main drain line between the bottom of the pool and that diverter valve just before the pump intake, but is there any other possible explanation? There are no known water leaks or unexplained water loss.

2. If you can keep the pool clean just drawing from the skimmer baskets, do you really need to open the main drain during normal operation?

Thanks for any expert tips!

 

[Bama Rambler]

There's not many other possibilities than a leak. Since you say that there's no water leaking I'd start looking at the piping above the water line. Maybe just under the ground or even right at the valve.

There are a lot of pools that have had the main drain blocked off completely because of a leak, so in reality you can get by without it.

 

[frustratedpoolmom]

If you can't locate the source - angling your returns if possible so some go towards the bottom to improve circulation should help if you don't use the bottom drain.

Any damp or wet ground visible?

 

[duraleigh]

Welcome to the forum.

Check the above ground portion of the line first....especially the diverter valve for cracks, etc. That's the most likely.

If your pool has a deep end, I would really try to bring the main drain into play. If not, you will probably be fine without it.

Did you try it just one time or does it do it always. If there is any air in that line at all, it will take a while for it to purge.

 

[JohnT]

Welcome to TFP.

Are you saying that with the drain valve open in addition to the skimmers that you get bubbles?

If that is the case, I'd check the valve that controls the drain before going too far looking for a problem.

Some bubbles at first may be normal if the water in the line has dropped to the pool level, leaving air in the line up to the pump.

 

[et]

Yes, you get tons of air bubbles when you open the main drain valve, while at the same time drawing from the skimmers. You also get a very low flow when pulling only from the main drain, and I'm not sure it would even prime this way. I first assumed this was due to the depth we are pulling from - however with a full pool, one would think that the static pressure of a full 6' pool would be enough to push water in the main drain right up to the top of the pool level. If so, it should not be hard to pull. That then takes me back to suspecting a line or fitting break, again in the main drain line. But again with no known water leaks that doesn't add up either. So it's perplexing.

 

[Grape Ape]

I suggest you put a wet rag or the stream for the garden hose on the any above ground sections of the pipe in question. With any luck, you’ll find a spot that stops the bubbles and you’ll be able to keep your main drain with spending much cash. It sounds like you leak is above the water level.

 

[waste]

Welcome to TFP!!

Just to summarize the advice so far -- check the connections above pool water level on the MD line, between the pool and the valve. Use the hose or wet rag to isolate the source (I've also heard that shaving cream can be used to identify where the leak is). I wouldn't be surprised if you found a bad o-ring on the valve for the MD line

 

[mas985]

Yes, you get tons of air bubbles when you open the main drain valve, while at the same time drawing from the skimmers. You also get a very low flow when pulling only from the main drain, and I'm not sure it would even prime this way. I first assumed this was due to the depth we are pulling from - however with a full pool, one would think that the static pressure of a full 6' pool would be enough to push water in the main drain right up to the top of the pool level. If so, it should not be hard to pull. That then takes me back to suspecting a line or fitting break, again in the main drain line. But again with no known water leaks that doesn't add up either. So it's perplexing.

There is no static head difference between a skimmer and a main drain as the water level is the same for both. Also, it doesn't matter where you pull the water from, the static head is the same.

However, it does sound like you might have a leak in the the main drain suction line and it doesn't mean that you would necessarily have a water leak. Air and water have much different density so depending what the leak might be, it could very well leak air with the pump on but not leak water with the pump off. If shuting off the main drain eliminates the air, then the air is comming in through that line.

 

[DD Urchin]

I know this is an old thread, but I'm surprised no one has mentioned the cavitation effect. It is possible that your main drain line is harder to suck water through (because of extra length etc.) and you are getting bubbles only in this line and not the skimmer because of cavitation. I found this out when I had the exact same phenomenon you describe. I also noticed that my pool does not appear to be losing any water. I would normally get around 55gpm flow from the skimmer and main drain line combined with both valves full open. When I closed the main drain line partially (ball valve turned 45 degrees), it reduced the cavitation effect and now I get 65gpm flow. Your bubbles might also be due to cavitation, not a leak.

 

[mas985]

Cavitation does not work that way. Cavitation is vaporization of water and that only occurs at very low pressure near a pure vacuum which does not occur anywhere in pool plumbing except in the impeller.

What you are experiencing is an air leak. Reducing the flow in the main drain reduces the suction pressure in that line so the leak just does not leak as much but it doesn't mean that it isn't there.