What is the "residential" flow rate for swimming pool cartridge filters and how does it differ from sand or DE filtration?

Filter manufacturers size all three systems in accordance with the National Sanitation Foundation (NSF) Standard 50 "Circulation System Components for Swimming Pools."

High-rate sand -- Max flow rate 25 GPM per square foot of surface area, normally sized between 15 and 20 GPM per sq. ft. A minimum flow rate of 11 GPM per sq. ft. is required to utilize the torturous path filtration of depth type filter media. Flow rates over 25 GPM can lead to channeling.

Diatomaceous Earth (DE) -- Max flow rate 2.5 GPM per square foot of surface area, normally sized at 2 GPM per sq. ft. Proper flow rate and pressure are essential to assure a uniform cake formation on the support grids.

Cartridge -- Max flow rate 1 GPM per square foot of area, can be sized between 0.25 GPM and 1 GPM per sq. ft.

Why is there such a wide variation in flow for cartridge filters?
Unlike DE or sand filters where the flow rate needs to be specific to optimize the performance of the filter system, cartridge filters can work at any flow rate. In fact, the lower the flow per square foot, the more efficient the system works.

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