Please help me understand why this is true.
To the best of my knowledge, the spa jet pump and booster pump are plumbed separately from the returns and drain. I have 2 drains in my spa: one on the floor and one on the wall. I've concluded that the drain on the wall goes to the circulation pump / filter, and the other goes to the spa jet / booster pumps.
With the valves set to spillover mode (drain from pool, return to spa), my filter shows 11 PSI (10 PSI baseline when I cleaned the filter last month). When I flip on the spa jets and booster pumps, my filter pressure climbs to 16 PSI.
How can this be? The only explanation is that the two systems must be connected somewhere, right?