I have a partially above ground Spa and an in-ground pool. When the spa is filled to spillover with pumps off, water siphons through the spa pump and into the pool until the spa water level drops about 13 inches.
What confuses me is that the Spa drain is about 12 inches below the pool returns and the highest pipe in this circuit is about 5 inches above the highest spa water level.
I cannot figure out why siphoning is occurring or how to stop this. Instinctively I'm guessing I should raise the highest point 13" (since the water drops 13"), but don't really want to make the change only to find out it doesn't work. Suggestions how to analyze and solve this?