I've got a Nautilus DE filter system & an old Pentair push/pull type sliding valve that services a 20,000 gallon pool. Lately, whenever I turn on the pump, the water pressure in the filter starts off at its norm of 15 psi and then, after a couple of minutes, a loud bang can be heard in the vicinity of the return line/ valve and the pressures jumps to 40 psi. The water in the pump skimmer stops circulating as if blocked. This happens whether or not the filter grid is in the filter, so a dirty grid is not the problem.
I took the valve stem out and it appeared to be OK. I took it to the local Leslie's Pool Shop and the manager there said that, although the seals on the valve were badly worn, he did not think the valve was causing the problem.
I stuck my shop vac hose in the return line from inside the filter housing to see if I could vacuum out the problem. Surprisingly to me, the suction from the shop vac could be felt in the intake line at the bottom of the filter housing. This made no sense to me, as I assumed that in normal recirculating mode the intake & return lines would be closed to each other. When I blocked the intake line at the bottom of the filter housing, the shop vac started pulling out a lot of water and quite a few leaves. After sucking out about 10 gallons of water, I turned on the pump and continued to have the same high water pressure problem.
Does anyone have any ideas as to the problem & the fix? Thanks.