I have a Pentair SD35 sand filter which I replaced the sand in. I used 100lbs of #20 silica sand as recommended. I also removed the standpipe and laterals and inspected, everything was fine. After putting back together and running a backwash cycle a heavy amount of sand was coming out of the waste valve. I have read common causes are cracked filter parts (were none), and too much sand (I used 100 lbs). Is sand coming out on the first backwash common? I did not try the filter position as I was afraid of putting sand into the pool.