I'm not sure, but I'm curious when comparing the cartridge size I'm getting. If I did the Pool Math correctly to calculate your pool volume in gallons, I come up with about 24,000. With mine, I come up with about 26,000. I'm getting the next size up cartridge (I think considering Pentair vs Jandy) from your 420. So does my extra 2,000 gallons make it reasonable for me to go up a cartridge size, but overkill for you to do the same? I don't know. It seems to me that our 2,000 gallon difference puts us in the same range. This is what I'm curious about..... Thanks! Suz.