I also have this problem. Did any of you determine the cause? I'd like some advice on it. Of late, my starting pressure is unusually high, and I have been experiencing very short filter cycles - as in only 4-5 days before needing to backwash again due to reduced flow. This is maddening cause I've never experienced this before. I use biguanide and so understand the issues associated with it - the white water mold, etc. - I've dealt with all that, but this is a different problem the way it seems. When I take apart the filter after the flow is reduced, all I'm seeing is DE. I know what the mold and slime look like, and there is very little there.
Tonight, after backwashing, I turned on the pump before adding DE and, to my amazement, the pressure was only one pound less than before I started - already at 19 and it was only slightly above 20 before backwashing this morning. This makes no sense to me, and I can't believe that this is all attributable to biguanide chemistry-related issues as I'm being told on a well-known pool website.
My typical starting pressure over the 30 years I've been running the pool has usually been around 11. This season, however, it is always around 13-15 - is gradually getting higher though - and now tonight was 19. I suspect I'll be doing all this again in a few days, maybe even tomorrow? What I do want to do next time is to run the system without putting the grids in. I've done this whenever we've gone on vacations during the pool season. The pressure stays constant at 10-11. IF it is now suddenly higher, then I know there must be some sort of mechanical issue. If, however, the pressure is as low as usual and only is elevated once the grids go back in, then I suppose it COULD be the grids but I've been chemically cleaning them (I have two sets so as to alternate) and doing everything the same that I have been for 30 years.
Any thoughts? Could there be a build-up of organic matter in the lines? An issue with the pump? I recently bought a product called Baquacil Line Cleaner and plan to use that. My gauge is new - I bought one of those glycerin-filled gauges to start the season - it's calibrated in half-pound increments and has a larger face. I assume it's showing me the right pressure (?) Never used one of these before.
Thanks so much for any advice or suggestions.