@Carl: You need to go by the COMMERCIAL rated flow, not the residential flow. This is true on all three types of filters (sand, DE, cartridge). The commercial rates are based on actual performance data; the residential rates are -- as far as I can tell -- simply WAG's that the manufacturers were able to bully past the NSF.

Consider: As you know, the load (people per 10k gal) is GREATER on AG pools, than on IG pools. Thus, the filter loading with 8 hour turn-over flow is greater on AG pools, than on IG pools. Yet, the NSF rating for AG pools (gpm/sft) is HIGHER than it is for IG pools, even though it should be LOWER, given the greater load.

=> With sand, excess flow blows the dirt through the filter AND blows sand out of the filter when backwashing.

=> With cartridge filters, excess flow leads to excess pressure differential across the filter pleats, leading to pleat collapse, as shown in the picture below, where the pleats have collapsed against each other, blocking flow into the 'valley' between pleats.

=> DE filters are the least sensitive to overloading, but with some filters, excess flow leads to excess pressure differential and causes collapse of the underlying grid.



(photo shows both pleat collapse and damage from use of a pressure washer (fuzzy fibers)