If there is air in the pump, it's either a suction side or the pump is cavitating, that is unable to suck enough water and the water you do see is boiling under a vacuum.

When this is happening, check to see if it goes away when you shut the skimmer or drain, ensuring there is always a water source to feed the pump.

If that doesn't narrow it to a suction line, then it's likely to be the lid gasket, the connection to the pump, or the winter drain plug.

Scott

DH did not check the winter drain plug, so he will. I'm not sure if it went away when he closed the center drain. If it does go away when he closes the center drain, does that mean something is wrong with the center drain? Why didn't the air come in right away?

If the drain has an issue deep, it's possible it was filling a void. When the suction started, it was pulling the water from the void until the void only had air.

Is this a liner pool or concrete?

Scott

This is a liner pool, 20 X 40, 33,000gal, eight feet deep where drain is.

Scott, DH checked the winter drain and it looks good. When he closed the center drain the air coming into the pump and out of the returns stopped. What does this mean and what do we have to do?

Put a winter plug in the center drain and valve it off.

Scott

Husband doesn't understand exactly what you mean. We never use winter plug in center drain of vinyl pool. We blow out lines and plug skimmers and returns and disconnect the equipment when closing pool.
Do you mean to keep the center drain closed now? How will the water be filtered? Please explain. Thanks!

Turn off the pump. Dive down with a Phillips head screw driver and remove the TWO screws holding the cover. Plug the drain.

Run the pool off the skimmer only for suction. Run like this until it's time to replace the liner. Then, when the pool is empty, you can fix the drain line.

Scott

Scott, we just replaced the liner about 3 years ago so it will hopefully be a long time before doing it again.
Is it OK to keep the center drain closed? Will the water be clean? Anything else we can do?
Once again thank you so much!

Skimmers normally do about 80% of the work anyway. Yes, you'll be fine. Many pools are built without drains.

Scott

Thanks again Scott. We truly appreciate your input!

Scott, I am happy to say that you lead me in the right direction and I think I have found where the air is coming from. When closing/opening the main drain I noticed a drop of water by the valve. I mentioned it to DH. Then we realized that when he opened the drain he did not turn the valve as open (tight) as I did. When I opened it, no air, when he did, air. I am so happy it is not underground. He plans on changing the valve soon but in the meantime, no air and chlorinator is working. Thanks so much for your help!