The Fleck 2510 uses “buttons” to set the backwashing flow rate up to a maximum of 17gpm. They are washer-like pieces, come in a wide selection of flow rates, and are inexpensive ($2.50). Ohio Pure Water, from whom I am buying my GAC filter, made a brilliant suggestion. Leave the valve wide-open and install a ball valve in the drain line. While in full backwashing mode, open the valve until carbon just starts to come out, then close it a bit. So kudos to Chuck at
www.ohiopurewater.com for solving that problem.
My plan is to take the ball valve suggestion and when it is properly set, measure the (backwash) flow rate. I will then install enough 4½” x 20” BB (Big Blue) 5µ sediment filters, in parallel, to supply either that flow rate or my household’s maximum SFR, whichever is greater (I suspect it will only take one BB but it might take two).
As an aside, I have some difficulty determining the flow rates of various filters. I know from experience that given enough pressure I can draw >10gpm through a 2”x10” 1 micron sediment filter but at that rate I doubt that it is filtering out very many 1µ particles. Axeon has some useful data.