Looking to close my pool myself this year and before I blow out lines I'd like to figure out what the 4th pipe going into the ground is for. I had a guy close it last year and recall him saying that we had an unusual system. Attached are some photos for orientation. I have 2 pipes (one from skimmer, one from drain) entering a valve before the pump/multiport/filter. I have another 2 pipes (one of which is the return) also connecting into the pump/multiport/filter, however the associated valve is in the off position. I can open the valve to the return for the purposes of blowing out the lines. Does anyone have any idea what the extra pipe could be for? We don't have a vacuum system connected to pump.