My DE filter has a typical start up pressure of 18. Recently the start up pressure dropped to 15 or so which has never happened in the two years I have operated my pool. The flow in the returns is good and I see no air bubbles in the return lines and pump basket.

Any idea as to why the drop? A few months ago I switched to cellulose fiber filtering media instead of DE as it is a little safer and does a better filtering job. The start up pressure remained at 18 until recently though so I don't believe it is the filtering media?

Thanks for any thoughts,

Mike