I have a 36,000 gallon in ground pool that sits around 9 feet below the level of my pump, and the pump, filter, etc are around 30 feet from the pool. I have just fitted a new Pentair IntellifloXF pump to replace a Hayward EcoStar that was killed by a surge from the power company. I run a Hayward DE filter and a Auto Pilot SWG and have 2 skimmers in the pool both running 1 1/2 inch pipe to a 2 inch Jandy 3 way valve and through a 2 inch Jandy non-return valve to the pump. All plumbing around the pump and filter is 2 inch schedule 80 that I installed at the beginning of last season and there are 2 returns to the pool, both 1 1/2 inch. It all ran fine last year with the Hayward pump.
My issue is that the pump will prime up fine and when run at speed 3 (2350 rpm) runs around 10 PSI on my filter gauge with new DE added, but over a period of around 2+ hours the flow will slowly diminish and the pressure on the filter gauge will rise to around 17 PSI and flow will be very weak back to the pool. Obviously I would expect this behavior as the filter becomes clogged, but this is happening even though the water is now pretty clean.
I am late opening the pool so assumed as I said above that the pressure build up was just the filter getting full of Algae, etc and needing to be back-washed, but if I turn off the pump for a few minutes and then turn it back on, it goes through a quick prime at around 3450 rpm for around 10-15 seconds and then settles back to normal running with good flow.
I've tightened all of the screw fittings and regularly bled the DE filter, but there really is little to no air in the filter when I bleed it.
Does anyone have any ides of what is going on or what I can try next. I am really running out of ideas and my pool service center wants $250 call out for the first 1/2 hour and $150 per 1/2 hour thereafter to look at this. It would be cheaper to install a new pool!
Sorry for the long message, but any help anyone could offer would be greatly appreciated.