According to the previous posts about the spa water level going down when the pump is off, I need to install a check valve on the spa side. In my case, blocking the pool/spa bypass will cause the spa water level to remain as is (NOT go down). I could not find any reading that explains why this happens exactly. I understand that check valves are installed so the water at the elevated spa will not drain down to the swimming pool. My valves are above the spa level, so I don't understand how the spa water will make it to the pool just due to gravity. Is there another pipe connection other than the visible manual valves? Could someone help explaining it?
Before I install a check valve, I'd like to understand why it will solve my problem. BTW, can someone confirm if installing a check valve on the spa return line will correct this problem?