I have an overflow spa and it works fine. The only issue is that when I shut my filter off, the water level drains to about the pool level. My system is set up so that it has 4 intakes into a manifold (spa drain, skimmer, vacuum, and main drain) and four outputs from a manifold (spa Jets, stair jets, pool return and water feature. All the plumbing after the pump and after the filter are new. The manifolds are original and so is the pump (installed 2003). The valves have a ball inside to shut off water by turning. Could the spa drain one have a crack in it allowing water to go through thus going back into the pool?