Doesn't that work out to 36 GPM for a 21" filter - to backwash? And 27 GPM or less to filter? I guestimate that I have about 30' total head restriction in my system. The flow curves in this brochure

https://hayward-pool-assets.com/asse...f?fromCDN=true

Show that the SP1750 1/2 hp pump is making about 35 GPM at 30'. That pump might still need throttled to get down to 27 GPM on my system. It also shows that this pump makes about 40 GPM at 25'. Almost exactly what I read the 1 hp pump makes on low. Is 27 GPM enough flow for a manual vacuum head to be effective? I'm certain the 18" filter is plenty big enough for my pool, but it looks like there is no pump small enough to properly operate that filter. It looks like even the 21" filter needs to be throttled on my system - because there is so little piping restriction - to get down to a 12 GPM/sqft flow rate. A one half hp pump would be fine and dandy for a 21" above ground filter system, and Hayward is marketing a 1.5 hp 18" system. Makes me shake my head in confusion.