Closing pool - drain question

[PoolCloser]

My husband and I are closing our pool by ourselves for the first time. It has gone pretty smoothly. We've blown out the skimmers, returns, fountains etc. We blew out the main drain successfully, we think, but we have four drains at the bottom of our pool that are all identical. We have gotten air to blow out 3 of the 4 but the last one hasn't had any bubbles come out. We know we've got the main drain and two drains that are intakes for the fountains... But what is the fourth drain? Is it the backup of the main drain? Should we see air come out?

Thanks for the help!

 

[Texas Splash]

Welcome to TFP! Post some pics of the drains and the equipment pad if you can please. Drains are plumbed in pairs (for safety), so it's possible two of the items are just one main drain unit for suction, while the other two are for something else. Or perhaps you have two sets of drains (pairs). Not sure. But the pics could help.

 

[PoolCloser]

Thanks for the reply! Here are images of the drains and all of the equipment in the back. The main pump is obviously the drain and skimmers, the second pump is two sheer descents and the third pump is one sheer descent and five fountain spouts.

 

[Newdude]

Those are likely 2 runs, with with 2 ports each, on a 'T'. Here's a picture of mine before it was installed.



If it's unlevel or asymmetrical in the least, air will shoot out one but not the other, taking the path of least resistance. It's fine.