Can I operate pool with main drain closed?

[CallMeIshmael]

I have a 40K inground pool that is losing maybe an inch or two of water every three or four days. It is heated to 82 and it does seem when the heater operates more I lose water faster. I wanted to close the main drain to see if maybe I was losing water there. When I did so I also noticed that my two pool skimmer/filters worked better. Any thoughts would be appreciated.

Hi! If it helps, our 50 y.o. inground pool (20k gal) has been plumbed to work with the main drain closed normally, for the same reason mshumac mentioned- better skimmer action. Since we have a lot of trees around, it has worked much better that way. (I say "has" cause we're having it replumbed for other reasons)

As for the leak, I'd put top priority on tracking that problem down, since the water loss is likely running underground somewhere, which can cause more involved problems! Have you read about the Bucket & Tape test here on the forum? I'd try that while isolating your suspected conditions (heater on/off, main drain open/closed, individual skimmers at a time) to see if you can track down at least the line in which the leak occurs. From your post it sounds like you're already on track, but just wanted to make sure, since the bucket will help keep your results consistent, especially if placed on the step to keep water temps consistent.
Good luck!