When I backwash my sandfilter the psi either doesn't drop or drops only 2-3psi.

New IG pool completed last August. Has a Pentair TR140 Sand Filter (like 36" diameter, 950 lbs of sand) and a 2hp Pentair pump.

At initial setup last year the pump ran at approx 10psi and it took a long time before I needed to backwash (at 22psi). When that finally happened it only dropped pressure to the 15-16psi range. Only had to backwash 2 more times that season and each time dropped down to 15-16.

I opened pool myself and at start the pressure was 16psi but upon backwashing at 22psi it only dropped to 20psi. I have backwashed many times this season but the water never seems to be that dirty, just the high psi readings.

I called Pentair since everything is still under warranty and the rep told me that my sand has gone bad and that the only thing that will fix this problem is to replace all 950 lbs of sand.

One other weird thing - the multivalve (and yes the pump is always off whenever I switch positions). It used to be that when I pushed down on the handle and held it before turning it to a new setting it would release the pressure in th pump pot. Now the pump pot stays full unless I physically remove the top.

Please help!