Hello All,
I recently got myself into a pool mystery that I cannot figure out for the life of me
. Hopefully the SMEs on this forum could help out 
.
At the bottom of the post is the layout of my pool/spa equipment pad: suction on the right, heater and returns on the left. The spa is an overspill spa with the water level higher than pool. The overspill will be on when pool pump is on. I recently changed out the old DE filter with the new cartridge filter shown in the pic.
Big thanks in advance to any thoughts anyone would share! Right now, I'm having a still pool in the backyard in the hottest time of the summer 
I recently got myself into a pool mystery that I cannot figure out for the life of me
. Hopefully the SMEs on this forum could help out . At the bottom of the post is the layout of my pool/spa equipment pad: suction on the right, heater and returns on the left. The spa is an overspill spa with the water level higher than pool. The overspill will be on when pool pump is on. I recently changed out the old DE filter with the new cartridge filter shown in the pic.
- The old DE filter had a multiport valve connected to DE filter on the outlet side with backwash function. There is a pipe - the "What is this?!" pipe in the pic - connected to the valve port with a small clear glass bottle cap where you can see how clean/dirty the water is. When I backwashed the pool, I turned the valve to backwash/waste position. The water would then flow through the port with clear glass and towards this pipe. You can probably see that it has a spigot on it if you enlarge the pic. I would have a hose hooked to the spigot and turn it on. Backwash water would drain that way.
- When taking out the DE filter, i thought we no longer needed the backwash line so I cut it off from the valve and capped that pipe. Hooked up inlet and outlet of the new filter and thought all is well. NO! When the pool pump turns on, the water in the spa quickly receded to bottom and then the pump shut off. It looks like the water got all suctioned into the pool. If I release the pressure in filter, water will slowly rise in the spa again but only to about pool level or maybe a little lower.
- I don't understand what is the cause of this fiasco and whether or not this pipe is playing any role in it. I searched up a lot of plumbing diagrams but failed to understand what this pipe does. I even tried to pour food dye in the pipe and then fed water into it to see if it'll come into the pool somehow but it doesn't look like it feeds into the pool or spa at all. So it feels like a real drain pipe. But if it is not part of the pool circulation and bears no impact, why was I seeing the spa drain problem? This pipe is the only thing that was disconnected from the filter connection comparing to before setting.
